1. health and fitness
2. disease

18
Molecular Motors in Plant Cells
A. S. N. Reddy
18.1
Introduction
Molecular motors regulate diverse cellular functions including the organization
and dynamics of microtubule (MT) and actin cytoskeleton, cytoplasmic streaming,
cell polarity, cell growth, morphogenesis, chromosome segregation and transport
of vesicles, organelles and macromolecular complexes. In eukaryotes, there are
three broad families of molecular motors: the kinesins, the dyneins and the myosins. All three types of motors utilize energy from the hydrolysis of ATP to move
along filamentous structures. Kinesins and dyneins move on MTs whereas the
myosins translocate on actin filaments. Using biochemical, cell biological, molecular, and genetic approaches several molecular motors have been identified in
plants and functions of some of these are beginning to be understood. Recent completion of genome sequence of several eukaryotes ranging from yeast, a simple
unicellular eukaryote, to highly evolved multicellular organisms including humans
and flowering plants has permitted comparative analysis of three families of motors in various species. Such analysis across phylogenetically divergent species
has yielded some interesting insights into evolutionary and functional relationships among motor proteins from various organisms. Systematic analyses of the
recently completed Arabidopsis genome sequence with the conserved motor domain of kinesins, myosins and dyneins revealed the presence of 78 molecular motors (61 kinesin-like proteins and17 myosins) in this organism. Of the two families
of MT-based motors, dyneins are absent in flowering plants. Surprisingly, Arabidopsis has the largest number of kinesin-like proteins (KLPs) as compared to
other multicellular organisms including humans, suggesting that the kinesin
superfamily is expanded considerably in plants. Also, all plant myosins belong
to two novel classes. Although the identification of molecular motors in plants is
progressing rapidly due to genome and EST sequencing projects, only a few
plant motors have been characterized in any detail and the functions of many
these motors are not known. Nevertheless, it is becoming obvious that plants contain novel families of KLPs and myosins with novel functions and regulatory meMolecular Motors. Edited by Manfred Schliwa
Copyright c 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN 3-527-30594 -7
434
18.2 Microtubule-based Motors
chanisms. In this article I focused primarily on plant molecular motors, their function and regulation with emphasis on the model plant, Arabidopsis. For other aspects of plant motors that are not covered here the reader is directed to other reviews (Asada and Collings, 1997, Liu and Lee, 2001, Reddy, 2001b, Yamamoto
et al., 1999).
18.2
Microtubule-based Motors
The motors that move on MTs belong to two different families: kinesins and KLPs
that move either toward the plus-end or minus-end of the MTs, and the dyneins,
which move toward the minus-end of MTs.
18.2.1
Kinesin-like Proteins
Members of the kinesin superfamily, which consists of conventional kinesin and
kinesin-like proteins (KLPs), are known or implicated to play important roles in
many fundamental cellular and developmental processes including intracellular
transport of vesicles and organelles, mitotic and meiotic spindle formation and
elongation, chromosome segregation, germplasm aggregation, MT organization
and dynamics and intraflagellar transport (Endow, 1999, Goldstein and Philip,
199, Hirokawa, 1998, Reddy, 2001b). A large number of KLPs have been identified
in plants and animals (Berg et al., 2001, Lawrence et al., 2002, Reddy and Day,
2001a, 2001b, Siddiqui, 2002, Yamashita et al., 2000; Tab. 18.1). All members of
the kinesin superfamily have a highly conserved catalytic region of about 350
amino acid residues, known as the motor domain, with ATP- and MT-binding
sites. The motor domain in KLPs is located either in the N terminus, C terminus
or in the middle of the protein. In addition to the motor domain, most KLPs have a
stalk region that forms an alpha-helical coiled-coil region which aids in dimerization, and a highly variable tail, which is thought to interact with a specific cargo.
Outside the motor domain KLPs share no sequence similarity between the members of kinesin superfamily.
Although molecular motors have been implicated in a variety of cellular processes in plants including spindle function, cytokinesis, cell polarity, morphogenTable 18.1
The total number of KLPs and myosins in the completely sequenced organisms.
S. cerevisiase
S. pombe
C. elegans
D. melanogater
H. sapiens A. thalaiana
KLPs
6a
9a
21b
24a
45c
61a
Myosins
5d
5d
17d
13d
40d
17d,e
Total
11
36
37
a
14
b
c
85
d
78
e
Day and Reddy, 2001b; Siddiqui, 2002; Miki et al., 2001; Berg et al., 2001; Day and Reddy, 2001a
18 Molecular Motors in Plant Cells
esis, organelle and vesicle transport, and cytoplasmic streaming (Asada and Collings, 1997, Lloyd, 1991, Reddy, 2001b, Williamson, 1993), until recently very little
was known about their molecular identity. Using antibodies to animal kinesins,
KLPs have been identified in pollen tubes of tobacco and other plants (Cai et al.,
1993,; Liu et al., 1994, Tiezzi et al., 1992). Recently, Cai et al. isolated an MTbased motor (90 kD) from tobacco pollen tubes which induced MT gliding in motility assays (Cai et al., 2000). Immunolocalization studies have indicated that this
motor binds to organelles associated with MTs in the cortical regions of the pollen
tube, suggesting its involvement in organelle transport. Two KLPs (125 and
120 kDa) that showed plus-end motor activity in in vitro motility assays and MT-dependent ATPase and GTPase activity, were isolated from tobacco phragmoplasts
(Asada and Shibaoka, 1994, Asada et al., 1991).
Using various approaches cDNAs for several KLPs have been characterized
from plants. Ten of these are from Arabidopsis (KatA, KatB, KatC, KatD, AtKCBP,
AtPAKRP1, AtPAKRP2, AtMKRP1, AtMKRP2, AtNCAK1) (Itoh et al., 2001, Lee
and Liu, 2000, Lee et al., 2001, Mitsui et al., 1996, Mitsui et al., 1994, 1993; Nishihama et al., 2002, Reddy et al., 1996b, Tamura et al., 1999), 15 from tobacco
(TKRP125, KCBP, NCAK1, NCAK2, and TBK1 to TBK11) (Asada et al., 1997,
Matsui et al., 2001, Nishihama et al., 2002, Wang et al., 1996), two from carrot
(KRP120-1 and KRP120-2 (Barroso et al., 2000) and one from potato (KCBP). A
kat gene family (katA, katB, katC, katD, and katE) encoding KLPs in Arabidopsis
thaliana was characterized using primers corresponding to conserved regions of
the kinesin motor domain (Mitsui et al., 1993, 1994, 1996, Tamura et al., 1999).
Using a similar approach, 12 different KLPs that belong to seven subfamilies
have been isolated from tobacco BY-2 cells (Matsui et al., 2001). It is interesting
that in tobacco BY-2 cells alone, 15 different KLPs that belong to different subfamilies are expressed (Asada et al., 1997, Bowser and Reddy, 1997, Matsui et al., 2001,
Nishihama et al., 2002). Using an indirect assay, KatA (89 kD) protein has been
shown to be a minus-end-directed motor and a similar protein has been found
in carrot and tobacco (Liu et al., 1996). The central region of KatD shares sequence
similarity to the motor domain of kinesin and is followed by about 240 residues at
the C-terminus. Phylogenetic analysis with the KatD motor domain indicates that it
belongs to the C-terminal family despite the fact that it has a 240-amino acid
stretch following the motor domain (Reddy and Day, 2001b, Tamura et al., 1999).
The amino-terminal region of KatD showed sequence similarity to calponin homology (CH) domain. KatD is a flower-specific motor protein suggesting that it may
function in transport or cytoskeleton organization in pollen. Asada et al. (1997) isolated a cDNA encoding the 125-kDa polypeptide (TKRP125, tobacco kinesin-related
polypeptide of 125 kDa). It is an N-terminal KLP with strong similarity to members
of the BimC (blocked in mitosis) subfamily. Two other members of BimC class
KLPs (DcKRP120-1 and DcKRP120-2) were isolated from carrot suspension cells
(Barroso et al., 2000). DcKRP120-1 is a homolog of TKRP125. In Arabidopsis
there seems to be at least three TKRP-like proteins. Recently, two phragmoplast-associated kinesin-related proteins (AtPAKRP1 and AtPAKRP2) have been
isolated from Arabidopsis and characterized (Lee and Liu, 2000, Lee et al., 2001).
435
436
18.2 Microtubule-based Motors
AtPAKRP1 is an N-terminal KLP and is most closely related to XKlp2, an ungrouped KLP from Xenopus laevis. AtPAKRP2 is also an N-terminal motor with
no known homologs in non-plant systems. In phylogenetic analysis, AtPAKRP2
does not group with any of the existing families of KLPs. These motors have distinct functions in cytokinesis (Lee et al., 2001).
KCBP (kinesin-like calmodulin-binding protein), a novel KLP with a C-terminal
motor domain was isolated from a number of plants as a calmodulin-binding protein (Abdel-Ghany et al., 2000, Abdel-Ghany and Reddy, 2000, Reddy et al., 1996a,
1996b, Wang et al., 1996). Although KCBP, like other kinesin-like proteins, contains three distinct regions (a motor domain, a coiled-coil stalk and a tail), it has
two features that make it unique among members of the kinesin superfamily in
eukaryotes. These include (i) a calmodulin-binding domain adjacent to the
motor domain at the C-terminus, and (ii) a myosin tail homology and talin-like region in the N-terminal tail (Reddy and Reddy, 1999). KCBP binds calmodulin in a
calcium-dependent manner at physiological calcium concentrations (Reddy et al.,
1996b, 1999). KCBP is a minus-end directed MT motor (Song et al., 1997) and
has two MT binding domains, one located at the C terminus and the second
one located at the N terminus (Narasimhulu and Reddy, 1998). Unlike the MT
binding domain in the C terminus, the N-terminal region of KCBP binds MTs
both in the presence and absence of ATP, indicating that the MT binding domain
in the N terminus is insensitive to ATP. KCBP is highly conserved in phylogenetically divergent plant species including dicots, monocots, gymnosperms and algae
(S. E. Abdel-Ghany and A. S. N. Reddy, unpublished data). Recently, a calmodulin-binding C-terminal kinesin (kinesin C) was cloned from sea urchin (Rogers
et al., 1999). The calmodulin-binding domain of kinesin C shared 35 % sequence
identity with the calmodulin-binding domain in KCBP. The existence of calmodulin-binding KLPs in both plants and sea urchin suggests that the origin of this
group of KLPs pre-dates the divergence of plants and animals from a common ancestor which is believed to have occurred about 1.5 billion years ago (Wang et al.,
1999). If this was the case, there must have been insertion of some domains such
as MyTH4 and talin-like regions in the tail of KCBP (or deletion of these domains
in kinesin C) to acquire functional specialization of these KLPs. Alternatively, calmodulin-binding KLPs may have evolved independently in plant and animals after
they diverged from a common ancestor. The amino-terminal tail and stalk regions
of KCBPs from different plant systems are highly conserved and contain myosin
tail homology (MyTH4) and talin-like regions that are not present in kinesin C.
In phylogenetic analysis of KLPs KCBPs and kinesin C are grouped with other
known C-terminal KLPs. However, Arabidopsis KCBP together with its orthologs
from other plants constitute a distinct group within the C-terminal subfamily of
motors.
Recently, two Arabidopsis KLPs (AtMKRP1 and AtMKRP2) with mitochondrial
targeting signals were found to be targeted to mitochondria, suggesting that
these motors might function in this organelle (Itoh et al., 2001). AtMKRP1 and
AtMKRP2 belong to ungrouped KLPs (Reddy and Day, 2001b). What functions
these MKRPs perform in mitochondria remains to be seen. There are other Arabi-
18 Molecular Motors in Plant Cells
dopsis KLPs with targeting signals to other organelles such as chloroplasts. It
would be interesting to see if any other Arabidopsis KLPs are targeted to organelles. KLPs that are targeted to organelles have not been reported previously in
the literature.
Mutations in the HINKEL (HIK) gene in Arabidopsis result in defective cytokinesis. These mutants have high frequency of incomplete cell walls and multinucleate cells. The HIK gene encodes a plant-specific N-terminal KLP that plays an important role in cytokinesis (Strompen et al., 2002). In a recent report, Nishihama
et al. (2002) have shown that two tobacco KLPs (NACK1 and NACK2) interact
with NPK1, a mitogen-activated kinase kinase kinase, and stimulate the activity
of the kinase. Orthologs of NACK1 and NACK2 have been identified in Arabidopsis and AtNACK1 was found to be identical to HIK. These KLPs are highly diverged N-terminal motors and do not group with any of the known KLP subfamilies (Reddy and Day, 2001b). The closest non-plant KLP that shares significant similarity with the motor domain of NCAKs is CENP-E. At least eight Arabidopsis
KLPs are known to function in some aspect of cell division (see Section 18.4.1
on cell division).
Analysis of the recently completed Arabidopsis genome sequence with the conserved motor domain of KLPs has resulted in identification of 61 KLPs in the
model plant (Reddy and Day, 2001b). A corresponding cDNA or EST (Expressed
Sequence Tag) was found for 37 of these KLPs (Tab. 18.2), suggesting that most
KLPs in Arabidopsis are expressed and not likely to be pseudogenes. The genes encoding KLPs are distributed throughout the genome. Chromosome 3 has the highest number (18) of KLP genes. In comparison, S. cerevisiae, S. pombe, C. elegans,
Drosophila, and Homo sapiens have 6, 9, 21, 24 and 45 KLPs respectively
(Tab. 18.1). Surprisingly, the Arabidopsis genome contains the largest number of
KLPs among all eukaryote genomes that have been sequenced. Arabidopsis has
the highest percentage (0.24 %) of the total number of genes as compared to S. cerevisiae and S. pombe with 0.1 and 0.17 % respectively, C. elegans with 0.11 % and
Drosophila with 0.18 %. Based on the number of KLPs in Arabidopsis, the number
of cellular processes that are regulated by KLPs in plants is likely to match or exceed the number of processes controlled by animal KLPs.
Only 10 of the 61 Arabidopsis KLPs have been reported in the literature (Lee and
Liu, 2000, Lee et al., 2001, Itoh, 2001, Liu et al., 1996, Mitsui et al., 1993, 1994,
Nishihama et al., 2002, Reddy et al., 1996b, Tamura et al., 1999) whereas the
three AtKRP125 KLPs (AtKRP125a,b, and c; Tab. 18.2) show sequence similarity
to a tobacco kinesin (NtKRP125) that was isolated from phragmoplasts of tobacco
(Asada et al., 1997). AtKRP125b has 68 % identity with NtKRP125 over the 1000
residues they have in common.
The number of known and predicted introns in KLPs range from 3 to 34
(Tab. 18.2). In the Arabidopsis genome, the number of introns ranges from 0 to
77 with an average of about five (Reddy, 2001c). More than 85 % of Arabidopsis
genes have 10 or less introns while the Arabidopsis kinesin genes have an average
of 16.4 with only 10 genes having 10 or less introns. The number and locations of
the introns need to be verified experimentally. It is likely that the predicted sizes of
437
C-terminal
C-terminal
‡
‡
‡
‡
At2g36200 (AtKRP125b)
At2g28620 (AtKRP125c)
At3g45850
At4g05190
At4g21270 (AtKatA)
At4g27180 (AtKatB)
At5g54670 (AtKatC)
At5g27550
At2g22610
At1g72250
At3g10310
At1g18410
At1g73860
At1g63640
At5g41310
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
‡
Internal
‡
967
Internalc
1050
1162
*
897
1195
1068
1056
c
c
c
c
425
Internalc
Internal
Internal
Internal
Internal
‡
c
N-terminal
b
754*
745*
793
777
C-terminal
C-terminal
1058
1076
1056
1022
#aa
N-terminal
N-terminal
N-terminal
N-terminal
‡
At2g37420 (AtKRP125a)
1
Motor
location
EST/
cDNA
Kinesin-like proteins in Arabidopsis.
Gene code
(published name)
Table 18.2
19
19
17
17
16
17
18
6
15
15
8
17
21
18
22
17
# of
introns
a
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Minus
ND
ND
ND
ND
ND
Motility
CC
CC
CC
CC
CC
Other
Domains
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
CC, CH
CC, CH
CC
CC
CC, CH
CC
CC
CC
CC
CC
Mitotic MT arrays CC
ND
ND
ND
ND
ND
Cellular
localization
Mitsui et al, 1994.
Mitsui et al, 1994.
Mitsui et al., 1993; Liu et al., 1992.
Asada et al., 1997.
Asada et al., 1997.
Asada et al., 1997;
Barroso et al., 2000.
Ref
438
18.2 Microtubule-based Motors
N-terminal
C-terminal
N-terminal
N-terminal
‡
‡
‡
‡
At5g10470
At5g65460
At5g27950
At1g55550
At5g65930 (AtKCBP)
At1g20060
At3g10180
At1g59540
At5g06670
At3g12020
At1g21730 (MKRP1)
At4g39050 (MKRP2)
At2g21380
21
22
23
24
25
26
27
28
29
30
31
32
33
767
At3g44730
861
1264
640
887
b
b
‡
‡
‡
‡
1273
b
N-terminal
N-terminal
N-terminal
857
1121
(1055)
909*
(890)
956
997
823
459
N-terminal
N-terminal
21
923*
N-terminal
18
22
22
22
22
17
11
20
9
8
22
22
14
14
18
17
1259
N-terminal
N-terminal
N-terminal
b
Internal
Internal
c
20
‡
At2g47500
c
19
1032*
Internalc
At1g09170
987
Internalc
18
‡
At5g27000 (AtKatD)
17
ND
ND
ND
ND
ND
ND
ND
ND
Minus
ND
ND
ND
ND
ND
ND
ND
ND
ND
Mitochondria
Mitochondria
ND
ND
ND
ND
ND
PPB, spindle,
Spindle poles,
Phragmoplast
ND
ND
ND
ND
ND
ND
ND
ND
CC
CC
CC
CC
CC
CC
CC
CC, MyTH4
Talin-like,
CBD, PEST
CC
CC
CC
CC
CC
CC, CH
CC, CH
CC, CH
Itoh et al., 2001.
Itoh et al., 2001.
Reddy et al., 1996.
Tamura et al., 1999.
18 Molecular Motors in Plant Cells
439
At4g24170
At3g16630
At3g16060
At5g02370
At1g18550
At3g49650
At5g23910
At3g50240
At5g47820
41
42
43
44
45
46
47
48
49
N-terminal
N-terminal
N-terminal
N-terminal
‡
‡
‡
1032
1075
665
813
22
21
17
14
3
N-terminal
703
10
*
12
12
14
9
13
13
664
706
799
1263
968
9
11
13
11
# of
introns
N-terminal
Internal
Internal
‡
‡
‡
N-terminal
1087
N-terminal
At5g42490
40
1037
N-terminal
‡
At5g66310
39
N-terminal
At3g51150
38
581
‡
At2g21300
37
834
932
N-terminal
At4g38950
36
N-terminal
1003*
(974)
‡
At3g43210 (ATNACK2)
35
N-terminal
#aa
N-terminal
‡
At1g18370
(HINKEL/AtNACK1)
34
Motor
location
‡
EST/
cDNA
(continued).
Gene code
(published name)
Table 18.2
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Motility
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Interzonal MTs
Phragmoplast
Phragmoplast
equator
Cellular
localization
CC
CC
HhH1
CC
CC
CC
CC
CC
CC
CC
CC
CC
CC,
CC
Other
Domains
Nishihama et al., 2002.
Strompen et al., 2002;
Nishihama et al., 2002.
Ref
440
18.2 Microtubule-based Motors
N-terminal
‡
‡
At3g19050
At3g17360
At3g44050
At3g20150
At3g23670
At4g14150 (AtPAKRP1)
At4g14330 (AtPAKRP2)
55
56
57
58
59
60
61
‡
N-terminal
959
(869)
9
24
1292
(1294)
14
16
1103*
I
20
1229
N-terminal
1268*
30
2158
N-terminal
N-terminal
34
*
1070
20
16
895
2756
N-terminal
N-terminal
18
*
885
11
*
24
439
1335*
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
CC,
CC
CC
CC,
CC,
CC, ARM
CC, ARM
CC, ARM,
CC
CC
Punctate staining CC
of Phragmoplast
Spindle midzone, CC
Phragmoplast
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Lee et al., 2001
Lee and Liu, 2000
by indirect assay; balthough the motor is N-terminal, it groups into C-terminal family; calthough the motor is internal, it groups into C-terminal family; iNCBI predicted
protein has 1662 amino acids; *slight discrepancy in #aa between AtDB and MIPS predicitions; The number of amino acids in parenthesis are based on the deduced
number from cDNA. EST, expressed sequence tag, CC, coiled coil; CH, calponin homology domain; MyTH4, domain present in the tail region of some myosins; Talinlike, talin-like domain found in some myosins and band 4.1 superfamily; CBD, calmodulin binding domain; PEST, motif rich in proline, glutamine, serine and threonine
residues; ARM, Armadillo/beta-catenin-like repeats; HhH1, helix-hairpin-helix. (Modified from Reddy and Day, 2001a)
a
N-terminal
‡
At3g54870
54
N-terminal
‡
At1g12430
53
N-terminal
‡
At1g01950
52
N-terminal
‡
At3g63480
51
N-terminal
At5g60930
50
18 Molecular Motors in Plant Cells
441
442
18.2 Microtubule-based Motors
the proteins in Tab. 18.2 and 18.3 may change somewhat as corresponding cDNAs
are characterized. In a few cases where the cDNAs were isolated recently there are
some differences between the predicted and actual size of the protein due to inaccurate predictions of introns and exons (Itoh et al., 2001, Lee and Liu, 2000, Lee
et al., 2001).
All Arabidopsis KLPs except three have a coiled-coil region, suggesting that they
may function as homo or heterodimers (Fig. 18.1 and Tab. 18.2). Interestingly,
some Arabidopsis KLPs have predicted domains that are not present non-plant
Table 18.3
Myosins in Arabidopsis.
Name
# of aa
Gene code
EST/
cDNA
Class Domains
Reference
1. At ATM
1166
AT3g19960
(ATM1)a
‡
VIII
MD, CC, IQ
(Knight and
Kendric-Jone, 1993)
2. At ATM2
1111
1101b
AT5g54280
ATM2/AtMYOS1a
‡
VIII
MD, CC, IQ
(Kinkema et al.,
1994)
3. At VIIIA
1085
AT1g50360
‡
VIII
MD, CC, IQ
4. At VIIIB
1126
AT4g27370
VIII
MD, CC, IQ
5. At MYA1
1520
1599c
AT1g17580
(AtMYA1)a
XI
MD, CC, IQ
6. At MYA2
1505c
1515
AT5g43900
(AtMYA2)a
XI
MD, IQ
(Kinkema and
Schiefelbein, 1994)
7. At XIA
1730
AT1g04600
‡
XI
MD, CC, IQ
(Kinkema et al.,
1994)
8. At XIB
1519
AT1g04160
‡
XI
MD, IQ
9. At XIC
1572
AT1g08730
‡
XI
MD, CC, IQ
10. At XID
1611
AT2g33240
‡
XI
MD, CC, IQ
11. At XIE
1529
XI
MD, CC, IQ
XI
MD, IQ
XI
MD, CC, IQ
12. At XIF
1556
13. At XIG
1502
‡
AT1g54560
d
AT2g31900
‡
AT2g20290
AT4g28710
‡
XI
MD, CC, IQ
1374
AT4g33200
‡
XI
MD, CC, IQ
16. At XIJ
1242
963b
998b
AT3g58160
AtMYOS3)a
(AtMYA3)a
XI
MD, CC, IQ
17. At XIK
1544
AT5g20490
IX
MD, CC, IQ
14. At XIH
1452
15. At XI-I
a
d
b
(Kinkema et al.,
1994)
Name as reported in the literature; Number of amino acids previously reported for partial sequence;
Number of amino acids predicted by NCBI; dEdited for full-length sequence; MD, motor domain;
CC, Coiled-coil region; IQ, putative calmodulin-binding motif (Modified from Reddy and Day, 2001a)
c
U
Structural features of Arabidopsis
KLPs. The deduced amino acid sequence of
KLPs in the Arabidopsis database was analyzed
for the presence of various domains (Reddy and
Day, 2001b). The KLPs are arranged according
to their grouping in the phylogenetic tree in
Fig. 18.3. Characterized KLPs are indicated in
Figure 18.1.
MCAK/KIF2
Unc104/KIF1
Sp
Chromo/KIF4
Sc
BimC
Ce
KRP85/95
Dm
KHC
At
MKLP1
10
8
6
4
2
0
C. Term
26
24
22
20
18
Kip3
Number of KLPs
18 Molecular Motors in Plant Cells
bold. ARM, Armadillo/beta-catenin-like repeat;
CBD, calmodulin-binding domain; CC, coiledcoil region; CH, Calponin homology domain;
MD, motor domain; MyTH4, myosin tail
homology region; Talin-like, talin-like domain;
HhH1, helix-hairpin-helix domain. Bar 100 aa.
Modified from Reddy and Day, 2001b.
KLPs (Fig. 18.1, Tab. 18.2). Six of the Arabidopsis KLPs have a calponin homology
(CH) domain, which is an actin-binding domain present in the N-termini of spectrin-like proteins. The CH domain is a protein module of approximately 110 residues found in cytoskeletal and signal transduction proteins either as two domains
in tandem or as a single copy (Banuelos et al., 1998). Proteins with a tandem pair
of CH domains cross-link F-actin, bundle actin or connect intermediate filaments
to cytoskeleton. Proteins with a single copy are involved in signal transduction (Banuelos et al., 1998, Leinweber et al., 1999). Perhaps the KLPs containing a CH domain bind actin and are involved in signal transduction or linking of actin and
MTs. Some domains that are involved in protein protein or protein DNA interactions are also present in some KLPs (Fig. 18.1 and Tab. 18.2). These include armadillo repeats, a helix-hairpin-helix DNA-binding domain. KCBP has MyTH4 and
talin-like domains present in some myosins, suggesting that it has domains of
both MT- and actin-based motors. Such motors may be involved in cross talk between MT and actin cytoskeleton (Reddy, 2001b). KCBP also has been shown to
have a calmodulin-binding domain (Reddy et al., 1996b).
Phylogenetic analysis
In non-plant systems, nine subfamilies of KLPs with some ungrouped KLPs have
been identified by phylogenetic analysis using the conserved motor domain
(Kim and Endow, 2000). Three of the subfamilies (KHC, KRP85/95, and
Unc104/KIF1) are involved in transport (Goldstein and Philip, 1999). Members
18.2.1.1
443
444
18.2 Microtubule-based Motors
from the other subfamilies (C-terminal, Kip3, MKLP1, BimC, chromokinesin/
KIF4, and MCAK/KIF4) function in various processes associated with cell division
(Goldstein and Philip, 1999). A phylogenetic analysis of the 61 Arabidopsis KLPs
motor domain sequences with 113 other motor domain sequences from nonplant systems has revealed that seven of the nine families are represented in Arabidopsis (Figs 18.2 and 18.3). However, several Arabidopsis KLPs do not fall into
any of the nine subfamilies and are likely to represent additional subfamilies
that are unique to plants (Figs 18.2 and 18.3). Most of the Arabidopsis KLPs are
more closely related to another Arabidopsis or another plant kinesin than to any
other kinesin used in the comparison. The subfamilies that are involved in transport are under-represented in Arabidopsis (Fig. 18.2). There are no members of the
KRP85/95 or Unc104/KIF1 subfamilies in Arabidopsis (Figs 18.2 and 18.3). No
KHCs have been reported in plants. The phylogenetic tree indicates that there is
possibly one KHC-type kinesin in Arabidopsis. At3g63480 falls into the KHC
group with a closer relationship to KHCs found in fungi and, like fungal KHC,
lacks a binding site for kinesin light chain proteins (Diefenbach et al., 1998;
Reddy and Day, 2001b). One Arabidopsis kinesin groups with the MKLP1 subfamily (Fig. 18.3). Two internal motor Arabidopsis KLPs grouped with the MCAK/KIF2
subfamily (also called the internal family) whose members are involved in vesicle
transport, chromosome movement and MT catastrophe (Goldstein and Philip,
1999). Three Arabidopsis KLPs fall into a group with Kip3 subfamily members
in which ScKip3 is involved in nuclear movement (DeZwaan et al., 1997). Three
Arabidopsis KLPs form a branch off of the chromokinesin/KIF4 subfamily members, some of which are involved in vesicle transport (HsKIF) and spindle organization and chromosome positioning (Xlklp1) (Goldstein and Philip, 1999). Arabidopsis has many KLPs (24 out of 61) that do not fall into any of the known groups
(Figs 18.2 and 18.3). Members of BimC subfamily, which are N-terminal plus-end
motors, are present in all five eukaryotic organisms that have been sequenced. The
three NtKRP125-like Arabidopsis KLPs (AtKRP125a, b, and c) are grouped with the
tobacco homolog in the BimC subfamily which are involved in cross-linking and
anti-parallel sliding of MTs (Goldstein and Philip, 1999).
Twenty-one Arabidopsis KLPs were grouped into the C-terminal subfamily. This
is an unusually large number compared to the other organisms. It is also unusual
in that 11 KLPs in this group have internal motors and five have N-terminal motors. The internal KLPs have a motor domain that is closer to the C-terminus than
the N-terminus but each has some sequence C-terminal to the motor domain and
could be called internal depending on the parameters used to define an internal
motor. It will be interesting to find out the direction of movement of these KLPs
whose motor domains are either N-terminal or internal but are most closely related
to the C-terminal subfamily. KLPs in the C-terminal subfamily translocate toward
the minus-end of MTs (Woehlke and Schliwa, 2000) and have a conserved sequence
at the neck/motor core junction (Endow and Waligora, 1998). It was reported that
the residues G and N residues at the neck/motor core junction are necessary for
minus-end directed movement (Endow and Waligora, 1998). Analysis of the
neck/motor core junction of the 21 C-terminal class Arabidopsis KLPs showed con-
18 Molecular Motors in Plant Cells
Strict
BimC
C-Terminal
MKLP1
AnBIMC
ScCIN8
ScKIP1
SpoCUT7
BmKRP
DmKLP61F
CeF23B12.8
HsKSP
XlEg5
XlEg52
NTKRP125
AtKRP125b
At3g45850
AtKRP125a
AtKRP125c
AnKLPA
SpoKLP2
SpoPKL1
ScKAR3
DmNCD
CgCHO2
MmKIFC1
HsCHO2
XlCTK2
At4g05190
AtKATA
AtKATB
AtKATC
CeKLP
HsKIFC3
MsFKIF2
MmKIFC2
XlCTK1
At5g27550
At2g22610
At1g72250
At3g10310
At1g18410
At1g73860
At1g63640
At5g41310
AtKATD
At1g09170
At2g47500
At3g44730
CeC41G7.2
CeMO1E11
CeWO2B12
At5g10470
At5g65460
At5g27950
At1g55550
NtKCBP
StKCBP
AtKCBP
SpKinesinC
CeMO3D4.1
CgCHO1
HsMKLP1
DmPACKLP
MmKlp174
At1g20060
CrKLP1
ScKIP2
HsCENPE
At3g10180
At1g59540
UmKin1
At5g06670
At3g12020
AtMKRP1
AtMKRP2
At2g21380
AtHINKEL/NACK1
Figure 18.2. The number of KLPs in each family in different organisms. C.Term, C-Terminal
motor; Chromo/KIF4, chromokinesin/KIF4; U,
ungrouped. At, Arabidopsis thaliana, Dm, Dro-
KRP85/95
AtNACK2
At4g38950
At2g21300
At3g51150
At5g66310
At5g42490
At4g24170
CeC06G3.2
CeLC33H5
CeF20C5
HsKIF3B
MmKIF3B
XlKlp3
sophila melanogaster; Ce, Caenorhabditis elegans;
Sc, Saccharomyces cerevisiae; Sp, Schizosaccharomyces pombe. Adapted from Reddy and Day,
2001b.
servation of these residues in most of the C-terminal Arabidopsis KLPs (Reddy and
Day, 2001b).
Several Arabidopsis KLPs show some similarity to other ungrouped KLPs
(Figs 18.2 and 18.3). The ungrouped centromeric proteins (HsCENPE and
UmKin1) cluster with seven Arabidopsis KLPs. CENP-E binds to the kinetochore
throughout mitosis and to MTs of the spindle mid-zone during late stages of mitosis (Goldstein and Philip, 1999). One Arabidopsis kinesin is paired with HsKid,
445
446
18.2 Microtubule-based Motors
Unc104/KIF1
MCAK/KIF2
KIP3
Chromo/KIF4
KHC
Phylogenetic tree of all Arabidopsis KLPs along with113 non-plant KLPs. The
tree was generated using the motor domain of
KLPs as described (Reddy and Day 2001a).
Vertical dashes indicate ungrouped KLPs (light
dashes, those grouped with other KLPs; bold
dashes, exclusively Arabidopsis KLPs). The
Arabidopsis KLPs are in bold. KLPs from the
following organisms were used: An, Aspergillus
nidulans; Bm, Bombyx mori; Ce, Caenorhabditis
elegans; Cf, Cylindrotheca fusiformis; Cg, Cricetulus griseus; Cr, Chlamydomonas rheinhardtii; Dd,
Figure 18.3.
KRP85/95
HsKIF3C
MmKIF3C
RnKIF3C
SpKIN95
CrFLA10
HsKIF3
MmKIF3A
SpKIN2A
DmKlp68D
CeOSM3
CeF56E3
DmKlp73
CeUNC104
HsATSV
MmKIF1A
MmKIF1B
RnKIF1B
HsbKIF1C
RnKIF1D
CeLR144
Dm38B
HsCMKRP
CeK11d9.C
CgMCAK
RnKRP2
HsMCAK
XlKCM1
HsKin2
MmKIF2
MmKIF2C
XlKCM2
LmKIN
At3g16630
At3g16060
CfDSK1
HsKid
At5g02370
DmKlp67A
At1g18550
ScKIP3
ScVII607
SpoBC2F12.13
SpoBC649.01C
At3g49650
At5g23910
CeTO1G1
Ggchrkin
HsKIF4
MmKIF4
Xlklp1
At3g50240
At5g47820
At5g60930
Dmklp3A
CeY43F4B
CeKHC
DmKHC
LpKHC
HsKHC
MmKHCS
MmKHCx
HsnKHC
HsxKHC
MmKIF5c
MmKHC
SpKHC
NcKHC
NhKin1
UmKin2
SyKin1
At3g63480
DdK7
CeLF22F4
DmNOD
At1g01950
At1g12430
At3g54870
LcKIN
Xlklp2
At3g19050
At3g17360
At3g44050
At3g20150
At3g23670
AtPAKRP1
AtPAKRP2
ScSMY1
Dictyostelium discoideum; Dm, Drosophila melanogaster; Gg, Gallus gallus; Hs, Homo sapiens;
Lc, Leishmania chagasi; Lm, Leishmania major;
Lp, Loligo pealii; Mm, Mus musculus; Ms, Morone saxatilis; Nc, Neurospora crassa; Nh, Nectria
haematococca; N, Nicotiana tabacum; St, Solanum tuberosum; Rn, Rattus norvegicus; Sc, Saccharomyces cerevisiae; Sp, Strongylocentrotus
purpuratus; Spo, Schizosaccharomyces pombe; Sr,
Syncephalastrum racemosum; Um, Ustilago
maydis; Xl, Xenopus laevis. Modified from Reddy
and Day 2001b.
18 Molecular Motors in Plant Cells
an ungrouped kinesin. HsKid is a kinesin-like DNA-binding protein that is involved in spindle formation and the movements of chromosomes during mitosis
and meiosis (Goldstein and Philip, 1999). Arabidopsis PAKRP1 along with five
other Arabidopsis KLPs are grouped with XlKlp2. XlKlp2 is required for centrosome separation and maintenance of spindle bipolarity (Boleti et al., 1996) whereas
PAKRP1 associates with the phragmoplast (Lee and Liu, 2000) and is expected to
function in cytokinesis. Three Arabidopsis KLPs form a subgroup separate from
any other kinesin but share a branch with CeLF22F4 and DmNOD (Fig. 18.3).
Eight other Arabidopsis KLPs form a subgroup separate from KLPs of any other
organism. In many cases a group of Arabidopsis KLPs forms a separate branch
within the major subgroup in which they fall.
The large number of KLPs in plants is expected to control many diverse cellular
processes including some plant-specific functions. There are many MT-associated
processes that are unique to plants (Lloyd and Hussey, 2001, Reddy, 2001b). For
example, during cell division in plants several plant-specific MT arrays such as
the preprophase band and the phragmoplast are formed that are important in determining the future location of the cell wall and cell wall formation, respectively
(Lloyd and Hussey, 2001). These unique processes are likely to require additional
plant-specific motors. In addition, centrosomes play an important role in MT organization in animals whereas plants have no well-defined centrosomes. Hence, MT
organization and dynamics in plants may also require additional MT motors. The
presence of unique organelles such as chloroplasts and MT-dependent processes
such as cell wall synthesis may also warrant additional motors. Also, cell to cell
transport of macromolecules such as RNA and viruses through plasmodesmata
may require MT-based motors (Reddy, 2001b). It is very likely that plant KLPs
may participate in functions other than transport, and MT dynamics and organization. Recently, it has been shown that two plant KLPs function as activators of a
protein kinase (Nishihama et al., 2002). It appears that flowering plants do not
have dyneins. Hence, some KLPs may be performing functions that are carried
out by dyneins in other systems.
18.2.2
Dyneins
Dyneins move along MTs toward the minus ends. The motor domain in dyneins
(Z 1000 amino acids) is much longer than in kinesins (Z 340 amino acids) and
does not share sequence similarity with the kinesin motor domain. Cytoplasmic
dynein is a large multi-subunit complex consisting of two heavy chains
(Z 530 kDa each), three intermediate chains (74 kDa), and four light intermediate
chains (Z 55 kDa) (Hirokawa, 1998). The central and C-terminal regions of dynein
heavy chain, which are predicted to form a globular structure, interact with MTs
and contain motor activity whereas the N-terminal region is thought to bind
cargo. The intermediate and light intermediate chains of cytoplasmic dynein associate with a protein complex called dynactin which is composed of 10 subunits
(Hirokawa, 1998). Although there is some immunological and biochemical data in-
447
448
18.3 Actin-based Motors
dicating the presence of dynein heavy chains in plants (Moscatelli et al., 1995), sequences that are similar to dynein and dynactin subunits have not been found in
the Arabidopsis genome or plant EST databases (Lawrence et al., 2001), suggesting
that they may be absent in flowering plants. The functions performed by dyneins
in animals may be carried out by the expanded family members of the kinesin
superfamily. Alternatively, it is possible that plant dyneins have diverged considerably, hence are not recognizable by the current search programs.
18.3
Actin-based Motors
18.3.1
Myosins
Myosins, a diverse group of actin-based molecular motors, perform a broad array of
cellular functions. A large number of myosins have been described in many eukaryotes (Berg et al., 2001, Reddy and Day, 2001a). Most of these are identified based on
sequence similarity with the myosin motor domain. All myosin heavy chains have
three common domains: a highly conserved motor domain (Z 850 amino acids)
located at the amino-terminus in almost all myosins, which is followed by a
neck region that binds to calmodulin or calmodulin-related proteins and a C-terminal
tail domain. The calmodulin-binding domain ‘IQ motif’ in these myosins has a consensus sequence ‘IQXXXRGXXXR’ (I, isoleucine; Q, glutamine; R, arginine; G, glycine) (Wolenski, 1995). This motif is present in one to seven repeats depending on the
type of myosin (Mermall et al., 1998). The only myosin that does not contain an ‘IQ
motif’ is myosin XIV. The tail region of different myosins varies considerably in
length and structure. The tail is characteristic to each myosin with interesting domains that are also found in non-motor proteins. The number of myosins in an
organism varies among organisms. However, not all types of myosins have been
identified in a single organism (Berg et al., 2001, Reddy and Day, 2001a).
Although there is compelling indirect evidence for the role of actin and actinbased motors in various transport processes in plants, little is known about specific
plant myosins that perform these functions. Myosins have been identified in plants
both biochemically (Heslop-Harrison and Heslop-Harrison, 1989, Miller et al.,
1995, Yokota et al., 1995, 1999b, Yokota and Shimmen, 1994, 1999) and at the molecular level (Kinkema and Schiefelbein, 1994, Kinkema et al., 1994, Knight and
Kendrick-Jones, 1993). Immunological detection of myosins using animal myosin
antibodies identified proteins of various sizes from different plants (Parke et al.,
1996, Qiao et al., 1989, Tang et al., 1989). Immunofluorescence studies localized
myosin to the surface of organelles, and the vegetative nuclei and generative
cells in pollen grains and tubes (Heslop-Harrison and Heslop-Harrison, 1989), to
the active streaming lanes and cortical surface in pollen tubes (Miller et al.,
1995) and more recently to plasmodesmata in root tissues (Radford and White,
1998, Reichelt et al., 1999). Motility assays (Kohno et al., 1991) and ATPase assays
18 Molecular Motors in Plant Cells
(Kohno et al., 1991, Ma and Yen, 1989; Vahey et al., 1982) using myosin-like proteins isolated from plants have also demonstrated the presence of myosins in
plants. Using PCR-based approaches, a few myosins have been cloned from Arabidopsis and other plants (Kashiyama et al., 2000, Kinkema and Schiefelbein, 1994,
Kinkema et al., 1994, Knight and Kendrick-Jones, 1993, Liu et al., 2001, Moepps
et al., 1993).
A 170-kDa myosin has been purified from lily pollen and the antibodies against
this myosin stained particles of various sizes in the apical cytoplasm of lily and
tobacco pollen tubes (Kohno et al., 1990, 1991, 1992, Yokota and Shimmen,
1994). These reports indicate that several myosins are likely to be present in pollen
tubes. The 170- and 175-kDa myosins that translocate F-actin at a velocity of about
9 and 3 4 mm s 1, respectively, were found to be associated with calmodulin (Yokota et al., 1999a, 1999b). A myosin II-like protein was identified in Nitella by Kato
et al. (Kato and Tonomura, 1977) and a myosin was purified from Chara (Yamamoto et al., 1994, 1995). In in vitro motility assays, the characean myosin translocated actin at 50 mm s 1. Rotary-shadowed images of the purified myosin showed
two heads that are comparable to heads of myosin II in shape and size (Yamamoto
et al., 1995). In Arabidopsis, the expression of several myosins in a given cell type
implies that actin-based motors are involved in a wide range of cellular functions.
By analyzing the Arabidopsis genome sequence with the conserved motor domain of myosins, a total of 17 myosins were identified in Arabidopsis (Reddy
and Day, 2001b). Table 18.3 lists the myosins by name as given in the phylogenetic
tree by Hodge and Cope (2000) and as assigned by Reddy and Day (2001a). In comparison, S. cerevisiae, Schizosaacharomyces pombe, C. elegans, D. melanogaster and
H. sapiens have 5, 5, 17,13 and 40 myosins respectively. Five of the 17 Arabidopsis
myosins have been reported in the literature (Kashiyama et al., 2000, Kinkema and
Schiefelbein, 1994, Kinkema et al., 1994, Knight and Kendrick-Jones, 1993). All
Arabidopsis myosins have three to six putative calmodulin-binding ‘IQ’ motifs
(Fig. 18.4). The IQ domains usually follow right after the motor domain but are
separated slightly from the motor domain in At XID, At XI-I, and At XIK. There
are three or four IQ domains in class VIII myosins and five or six in class XI except
for At XIK which has only four (Fig. 18.4). There are coiled-coil domains in all the
myosins that differ in length and number. They often follow directly after the IQ
domains but in some cases there is intervening sequence. Based on the presence of
the coiled-coil domains, it is likely that the Arabidopsis myosins either form dimers
or interact with other proteins (Cope et al., 2000). The class XI myosins are much
longer than the class VIII myosins with the difference being in the length of the Cterminal region following the conserved domains found in myosins. Myosins containing IQ domains are typically calmodulin-sensitive. However, the interaction
with, or regulation by calmodulin has not yet been demonstrated for Arabidopsis
myosins. The information on myosin light chains is limited in plants. Recently, Yokota and his colleagues (Yokota et al., 1999a, Yokota and Shimmen, 1994, 1999)
have purified two myosins from plants and demonstrated that calmodulin associates with the purified myosin and regulates its motor activity. Since the genes encoding these proteins are not cloned the structural features of these myosins are
449
450
18.3 Actin-based Motors
1. At ATM
3. At VIIIA
2. At ATM2
4. At VIIIB
13. At XIG
14. At XIH
6. At MYA2
8. At XIB
10. At XID
7. At XIA
12. At XIF
9. At XIC
11. At XIE
16. At XIJ
5. At MYA2
15. At XI-I
17. At XIK
Motor Domain
IQ Motif
Schematic diagram showing various predicted domains in Arabidopsis myosins.
The Arabidopsis genome database was searched
with the conserved motor domain of a myosin to
identify Arabidopsis myosins. The deduced
amino acid sequence of Arabidopsis myosins
Figure 18.4.
Coiled-coil Domain
was analyzed for the presence of various domains (Reddy and Day, 2001a). The numbers
refer to the numbers in Tab. 18.1. The first four
myosins are in class VIII and the following 13 are
in class XI. The bar represents 100 amino acids.
Adapted from Reddy and Day, 2001a.
not known. Besides the motor, IQ and coiled-coil domains, several other domains
have been identified in myosins from classes other than the plant classes VIII and
XI. These include SH3 domains (Src homology 3 domains), ankyrin repeats,
MYTH4 (domain of unknown function found in a few classes of myosins), zincbinding domain, plecstrin homology, FERM/talin (band 4.1/ezrin/radixin/moesin),
GPA-rich domains and a protein kinase domain (Berg et al., 2001). Plant myosins
do not have any of these other domains.
18 Molecular Motors in Plant Cells
18.3.2
Phylogenetic analysis
Phylogenetic analysis using the conserved motor domain grouped all known myosins into 18 distinct classes (Burge and Rogers, 2000). Phylogenetic analysis of the
Arabidopsis myosins’ motor domain with non-plant and plant myosins revealed
that all the Arabidopsis myosins and other plant myosins fall into only two groups,
class VIII and class XI (Fig. 18.5). Four Arabidopsis myosins are in class VIII and
13 in class XI. These groups contain exclusively plant or algal myosins with no animal or fungal myosins, suggesting that plants have a unique set of myosins
(Fig. 18.5). The motor domain in all cases is in the N-terminal region (Fig. 18.4).
The motor domain starts at about 50 55 residues for the class XI myosins while
the class VIII myosins have a longer N-terminal region prior to the motor domain
(99 159 residues). An algal (Chara corallina) myosin, Cc ccm, does group with the
plant class XI myosins but is on a separate branch from any other vlass XI myosin
(Fig. 18.5). A phylogenetic tree that was constructed using the full-length sequences of Arabidopsis and other plant myosins showed similar grouping of
plant myosins (Reddy and Day, 2001a). Among the class XI myosins the similarity
ranges from 40 85 % (full length) and 61 91 %(motor domain). The similarity
between the class VIII myosins ranges from 50 83 % (full length) and 64 92 %
(motor domain). When class VIII myosins are compared to class XI myosins the
similarity only ranges from 22 29 % (full-length) and 35 42 % (motor domain).
Myosins have 131 highly conserved residues spread throughout the motor domain
that define a core consensus sequence (Cope et al., 2000). Comparison of an alignment of Arabidopsis myosin motor domains to these conserved sequences shows a
great deal of conservation among them (Reddy and Day, 2001a), suggesting that
they are capable of motor function.
18.4
Cellular Roles of Motors
18.4.1
Cell Division
Although many of the dynamic processes of cell division such as formation of the
spindle, congression of chromosomes at the equatorial region and migration of
chromatids during cell division are common to both animals and plants, there
are significant differences between plant and animal cell division. For example,
the dividing plant cells organize their cytoskeletal elements into unique structures
that are not found in non-plant systems. Just prior to prophase, plant cortical MTs
rearrange to form a band of MTs called the pre-prophase band (PPB) (Goddard
et al., 1994, Gunning and Wick, 1985), which accurately predicts the future location
of the cell plate. Another distinctive feature of plant mitosis is the formation of a
bipolar spindle in the absence of centrosomes (Franklin and Cande, 1999, Smir-
451
452
18.4 Cellular Roles of Motors
100
100
82
64
100
100
100
74
100
100
100
69
65
82
90
90
87
100
100
74
100
100
96
100
90
100
55
64
100
100
100
100
100
100
99
100
50
99
100
56
54
100
96
100
100
Phylogenetic analysis of Arabidopsis myosins. A phylogenetic tree was generated as described earlier (Reddy and Day,
2001a). The myosin groups, as defined by Hodge
and Cope (2000) and Yamashita et al (2000), are
identified on the left in roman numerals. The
Arabidopsis myosins are in bold. Myosins
from the following organisms were used: Ac,
Acanthamoeba castellani; Acl, Acetabularia cliftoni; Cc, Chara corallina, Ha, Helianthus annuus;
Figure 18.5.
At ATM
At VIIIA
Ha hamy1
Zm ZMM3
At ATM2
At VIIIB
Ha hamy3
At MYA1
At XIC
At XIE
At XIJ
AT XIK
Ha hamy2
Ha hamy5
Zm MYO1
At MYA2
At XIB
Ha hamy4
At XIG
At XIH
At XIA
At XID
At XIF
At XI-I
Cc ccm
Acl myo1
Acl myo2
Ac HMWMI
Bt X
Mm X
Ce HUM-2
Mm Dilute
Rn myr6
Sc Myo2
Ce HUM-3
Dm 95F
Mm Waltzer
Ce HUM4
En csmA
Pg csm1
Ce HUM-6
Dm 35B
Hs UsherIb
Ce HUM-7
Hs IXa
Mm IXb
Rn myrV
Ce IA
Dm IB
Hs Ib
Sc MYO3
Dd IA
Ce IIB
Dm IIB
Dd II
Hs nmIIA
Sc MYOI
Dm PDZA
Hs MysPDZ
Dd MyoI
Dd MyoJ
Dd myoM
Dm NinaC
Lp III
Hs XV
Mm XV
Pf PfM-A
Tg myoA
Rn XVI
VIII
XI
XIII
IV
X
V
VI
XII
XVII
VII
IX
I
II
XVIII
III
XV
XIV
XVI
Zm, Zea mays; Bt, Bos taurus; Mm, Mus musculus;
Ce, Caenorhabditis elegans; Dm, Drosophila melanogaster; Rn, Rattus norvegicus; Sc, Saccharomyces cerevisiae; Hs, Homo sapiens; Dd, Dictyostelium discoideum; Lp, Limulus polyphemus;
En, Emericella nidulans; Pg, Pyricularia grisea;
Pf, Plasmodium falciparum; and Tg, Toxoplasma
gondii. The numbers at the branches indicate the
number of times the dichotomy was supported
out of 100 bootstrap tries.
18 Molecular Motors in Plant Cells
nova and Bajer, 1992). The structures involved in the organization of MTs are the
centrosomes in animal cells and the spindle pole body (SPB) in fungal cells, which
are lacking in plant cells (Smirnova and Bajer, 1992). Finally, cytokinesis, the process that produces two daughter cells following the completion of nuclear division,
in plants is accomplished quite differently from animals. Cytokinesis in plant cells
occurs via the formation of a polysaccharide cell plate, which expands from the
center to the periphery (Staehelin and Hepler, 1996, Sylvester, 2000). The phragmoplast, a structure composed of two disks of parallel MTs with their plus ends toward the equatorial zone and actin filaments (Euteneur et al., 1982, Hepler and
Jackson, 1968, Staehelin and Hepler, 1996, Wick, 1991), is involved in cell plate formation. Vesicles carrying the cell plate materials are transported to the equatorial
zone of the phragmoplast (Samuels et al., 1995, Staehelin and Hepler, 1996, Yasuhara et al., 1993). The association of vesicles with phragmoplast MTs indicate that
MT-based motors are likely to be involved in transporting the vesicles to the cell
plate (Otegui and Staehelin, 2000, Otegui et al., 2001, Samuels et al., 1995). The
fusion of vesicles in the cell plate is mediated by phragmoplastin, a dynamin
homolog, in plants (Gu and Verma, 1996, 1997, Samuels et al., 1995). The plant
spindle, which is formed in the absence of a well-defined centrosome, and several
unique plant-specific MT arrays such as the pre-prophase band and the phragmoplast (Lloyd and Hussey, 2001) suggest that plants are likely to contain additional
plant-specific KLPs that are not present in animal cells.
In animals, six subfamilies of KLPs have been shown to be involved in some aspect of cell division (Goldstein and Philip, 1999, Reddy, 2001b). Several recent reports on KLPs expression and immuunolocalization indicate that at least seven
plant KLPs (Kat A, KatB/C, TKRP125, AtKCBP, AtPAKRP1, AtPAKRP2,
DcKRP120-2, HINKEL/AtNACK1) that belong to different families have mitotic
function (Asada, 1996, Barroso et al., 2000, Bowser and Reddy, 1997, Lee and
Liu, 2000, Lee et al., 2001, Liu et al., 1996, Nishihama et al., 2002, Smirnova
et al., 1998, Strompen et al., 2002). Some KLPs are expressed in a cell cycle-dependent manner. TKRP125, a member of the BimC subfamily and NCAK from tobacco and two KLPs from Arabidopsis (KCBP, KatB/C) show a high level of expression in M-phase of the cell cycle (Asada, 1996, Bowser and Reddy, 1997, Mitsui
et al., 1996, Nishihama et al., 2002). DcKRP120-1 and DcKRP120-2 are not detectable in non-dividing cells (Barroso et al., 2000). TKRP125 localizes to the cortical
MTs in the S phase, along MTs of pre-prophase band (PPB) and perinuclear
MTs in prophase, the equatorial plane of spindle MTs in metaphase and anaphase,
and the phragmoplast MTs in telophase and cytokinesis. A carrot homolog of
TKRP125 (DcKRP120-1) also localizes to the cortical MTs, the pre-prophase
band, spindle and the phragmoplast (Barroso et al., 2000). Antibodies to another
member of the BimC subfamily (DcKRP120-2) stained the spindle and phragmoplast, predominantly in the mid-line of the phragmoplast where plus ends of the
MTs overlap (Barroso et al., 2000). AtPAKRP1, a non-member of BimC, which resembles an ungrouped Xklp2 from Xenopus, also localizes to the mid-line of the
phragmoplast. KatA, a C-terminal motor, localizes near the mid-zone at metaphase
and anaphase and the phragmoplast in Arabidopsis and tobacco BY-2 suspension
453
454
18.4 Cellular Roles of Motors
cells (Liu et al., 1996). Anti-KCBP antibodies stain the pre-prophase band, the spindle and the phragmoplast (Bowser and Reddy, 1997). The KCBP antibody did not
stain the cell plate region of the phragmoplast but MT bundles on either side of the
cell plate are strongly labeled (Fig. 18.6). Localization of KCBP in Haemanthus also
showed localization to mitotic MTs arrays. In Haemanthus anti-KCBP staining is
seen almost exclusively at the spindle poles in late anaphase (Smirnova et al.,
1998). The assembly of the acentriolar spindle in plants may involve convergence
of MT minus-ends leading to the formation of spindle poles (Franklin and Cande,
1999, Smirnova and Bajer, 1992). The fact that KCBP is a minus-end-directed
motor (Song et al., 1997) and localizes to the spindle poles suggests that it may
be involved in acentriolar spindle formation in plants. Between early and late
anaphase, the localization of KCBP shifts toward the spindle pole, supporting a
role for KCBP in the formation of a converging bipolar spindle (Smirnova et al.,
1998, Smirnova and Bajer, 1992). Localization of minus-end (KatA and KCBP) and
plus-end (TKRP125) motors in the spindle suggests their involvement in counterbalancing forces generated by plus- and minus-end motors to stabilize the spindle.
Constitutive activation of KCBP during late prophase caused premature breakdown of the nuclear envelope and arrest of cells at pro-metaphase whereas activation of KCBP at late metaphase or anaphase did not effect the progression of anaphase but caused aberrant phragmoplasts formation and delayed cytokinesis (Vos
et al., 2000). This study suggests that KCBP is differentially active during the various phases of cell division. Its activity is downregulated in metaphase and telophase and upregulated in anaphase most likely by changes in cytosolic calcium
level (Vos et al., 2000). Co-localization of some of the KLPs with mitotic MT arrays
suggests that they may have a role in forming these arrays by bundling MTs. The
motor domain of KCBP has been shown to bundle MTs (Kao et al., 2000).
Numerous forces are at play in the development, maintenance, and function of
the phragmoplast. Golgi-derived vesicles are transported to the forming cell plate
where the plus-ends of phragmoplast MTs interdigitate. It is likely that MT motors
are involved in forming the phragmoplast and powering the movement of the
vesicles along the MTs. Seven plant KLPs (Kat A, TKRP125, AtKCBP, AtPAKRP1,
AtPAKRP2, DcKRP120-2, HINKEL/AtNACK1) including plus- and minus-end
motors and some plant-specific KLPs are associated with the phragmoplast and
so are expected to function in cytokinesis (Asada, 1996, Barroso et al., 2000,
Bowser and Reddy, 1997, Lee and Liu, 2000, Lee et al., 2001, Liu et al., 1996, Nishihama et al., 2002, Smirnova et al., 1998, Strompen et al., 2002). The localization
patterns with various KLPs differed considerably. Immunolocalization studies as
well as phragmoplast MT gliding assay in the presence of TKRP antibodies indicates its involvement in the organization of MT arrays especially in phragmoplast
function (Asada et al., 1997). The polarity of MTs and microfilaments in the phragmoplast (Euteneur et al., 1982, Staehelin and Hepler, 1996) indicates that plus-end
motors are likely to be involved in the transport of vesicles to the cell plate. Of the
seven KLPs that localize to the phragmoplast, five are N-terminal motors
(TKRP125, PAKRP1, DcKRP120-2, PAKRP2, HIK/NCAK) and two are C-terminal
(KatA and KCBB). TKRP125 is a plus-end motor, and the other four N-terminal
18 Molecular Motors in Plant Cells
DAPI
MT
Triple localization of nucleus, MTs
and KCBP in a cytokinetic cell of tobacco. Nuclei
stained with DAPI (DAPI), MTs (MT) are localized with a monoclonal antibody to b-tubulin
and KCBP (KCBP) was localized with affinitypurified anti-KCBP antibody. Secondary antibodies conjugated to FITC and Cy3 were used to
detect MTs and KCBP, respectively. From Bowser
and Reddy, 1997.
Figure 18.6.
KCBP
KLPs are likely plus-end motors. One or more of these are likely to be involved in
vesicle transport to the cell plate. The punctate staining pattern of PAKRP2 and its
association with vesicles, suggest that it is involved in transporting vesicles to the
cell plate (Lee et al., 2001). It is unlikely that KCBP and KatA play a role in vesicle
transport to the cell plate as these are known to be minus-end motors and the MTs
are oriented with their plus ends facing the cell plate (Euteneur et al., 1982, Liu
et al., 1996, Song et al., 1997). The possible functions of minus-end motors are stabilization of the phragmoplast and/or recycling of Golgi vesicle membranes from
the expanding cell plate. Functional analysis of TKRP125 indicates its role in crosslinking and sliding of anti-parallel MTs in the phragmoplast (Asada et al., 1997,
Asada and Shibaoka, 1994). Immunofluorescence with AtPAKRP1 antibodies
showed staining of interzonal MTs at late anaphase and later on its localization
is restricted to the plus ends of interdigitating MTs in the phragmoplast (Lee
and Liu, 2000). Functional studies with this KLP indicates its involvement in maintaining the integrity of phragmoplast MTs (Lee and Liu, 2000). In order for the
growth of the phragmoplast to be achieved, a rapid turnover of phragmoplast
MTs must occur (Hepler and Hush, 1996, Hush et al., 1994). The expansion of
455
456
18.4 Cellular Roles of Motors
the phragmoplast from the center to the periphery involves disassembly of the MTs
inside and assembly of MTs at the outside. Blocking of disassembly in the center by
MT stabilizing drugs causes cessation of phragmoplast expansion and formation of
incomplete cell walls (Nishihama and Machida, 2001, Yasuhara et al., 1993). Phragmoplast-associated KLPs may play a role in the MT assembly and disassembly associated with its growth. The loss of MTs from the center of the phragmoplast as it
grows does not occur in hik mutants, suggesting that it controls directly or indirectly the dynamics of MTs in the phragmoplast (Strompen et al., 2002). HIK
does not appear to be involved in either vesicle transport or organization of MTs
in the phragmoplast. Studies with tobacco NCAK1, a homolog of HINKEL, showed
its localization to the equatorial zone of the phragmoplast but not in the inner regions of the phragmoplast where the cell plate matures (Nishihama et al., 2002). It
appears that NCAK through a protein kinase, NPK1, regulates depolymerization of
MTs in the center of the phragmoplast as it expands (Nishihama et al., 2001, 2002).
There is some evidence for the presence of a myosin in the phragmoplast (Parke
et al., 1996), suggesting that actin-based motors may also be involved in the transport of vesicles on the phragmoplast. During cell division the anti-class VIII myosin staining remains confined to the transverse cell walls and is strongest in the
newly formed cell wall (Reichelt et al., 1999). Immunogold electron microscopy
showed labeling of class VIII myosin associated with the plasma membrane and
plasmodesmata. These studies suggest that class VIII myosins may be involved
in new cell wall formation and transport in the plasmodesmata.
18.4.2
Cell Polarity and Morphogenesis
Both actin and MT cytoskeleton have been implicated in cell polarity and morphogenesis (Mathur and Chua, 2000, Mathur et al., 1999, Reddy and Day, 2000, Szymanski et al., 1999, Vidali and Hepler, 2001). The first indication that a KLP
might play a role in morphogenesis came from studies with the zwichel mutant.
In wild-type Arabidopsis, trichomes are unicellular with a stalk and three branches
(Oppenheimer, 1998). In zwichel (zwi) mutants trichomes are abnormal with a
short stalk and one or two branches depending on the severity of the allele
(Fig. 18.7). Cloning of ZWICHEL has indicated that it is identical to KCBP, a calmodulin-binding KLP (Oppenheimer et al., 1997), suggesting the requirement of
this motor for expansion of the stalk and branching. Using paclitaxel, a known
MT stabilizer, zwichel mutants were induced to form similar growth points indicative of branch formation in normal trichomes (Mathur and Chua, 2000). Transient
stabilization of MTs could compensate for KCBP/ZWI activity suggesting that it
may be involved in stabilization of MTs. The two MT-binding sites of KCBP (Narasimhulu and Reddy, 1998) could be involved in stabilization or possibly a KCBP/
ZWI-interacting protein(s) could be responsible for the stabilization. Several alleles
of zwi have been characterized. All zwi mutants grew normally with no apparent
defects in cell division except that they contain abnormal trichomes (Krishnakumar
and Oppenheimer, 1999, Oppenheimer et al., 1997). The lack of a phenotype in
18 Molecular Motors in Plant Cells
Scanning electron micrographs
of Arabidopsis trichomes of wild-type and KLP
(zwichel) mutants. (a) Wild-type; (b) zwi; (c) zwi
w2; (d) zwi9311-11. In zwi w2 most trichomes
have two branches but the length of the second
branch is varied. In the zwi9311-11 mutant
Figure 18.7.
about 40 % of trichomes are unbranched and
the rest have two branches. Seeds of zwichel
mutants were kindly provided by Dr David
Oppenheimer and Dr Martin Hülskamp. Scale
bar ˆ 100 mm. From Reddy and Day, 2000.
other tissues suggests that either KCBP is non-essential or another motor with
overlapping functions may substitute for ZWI function in other tissues. In Arabidopsis there are 61 KLPs including several C-terminal motors. Three extragenic
suppressors (suppressor of zwichel-3; suz1, suz2 and suz3) that rescued the trichome
branch number defect in a zwichel mutant have been isolated (Krishnakumar and
Oppenheimer, 1999). All three suppressors were found to be allele specific, indicating direct interactions between these proteins. The suz1 zwi-3 double mutants are
male sterile due to a defect in pollen germination and pollen tube growth, suggesting a role for these genes in pollen germination and tube growth (Krishnakumar
and Oppenheimer, 1999). At least two proteins, a plant-specific protein kinase and
a protein required for trichome branching (ANGUISTIFOLIA), have been shown
to interact with KCBP/ZWI (Day et al., 2000, Folkers et al., 2002).
18.4.3
Cytoplasmic Streaming
Cytoplasmic streaming in plant cells is important in intracellular transport. The
force that drives the cytoplasmic streaming appears to be dependent on actin
and actin-based motors (Shimmen and Yokota, 1994, Staiger and Schilwa, 1987,
Williamson, 1976). Insight into the function of plant myosins in cytoplasmic
streaming has been gained primarily by studies carried out with algae and pollen
tubes. Characean cells exhibit a very rapid cytoplasmic streaming (Z 70 mm s 1).
Also in pollen tubes, vesicles flow rapidly from the shank to the tip of the pollen
tube where they fuse to support extensive tip growth of these cells (Emons et al.,
1991, Heslop-Harrison and Heslop-Harrison, 1990). In addition to the bi-directional movement of vesicles, the generative cell and the nucleus move unidirectionally toward the tip (Mascarenhas, 1990, 1993). In Chara, an increase in Ca2‡ concentration causes cytoplasmic streaming to stop (Hayama et al., 1979). A myosin
isolated from the alga Chara corallina was shown to be responsible for cytoplasmic
457
458
18.4 Cellular Roles of Motors
streaming (Yamamoto et al., 1994, 1995, 1999). The myosin was cloned and found
to be a class XI myosin related to the Arabidopsis MYA myosins (Kashiyama et al.,
2000). Using immunofluorescence, myosin was localized to vesicles, organelles
and generative cells and vegetative nuclei in grass pollen tubes (Heslop-Harrison
and Heslop-Harrison, 1989). A myosin isolated from lily pollen has been shown
to be responsible for cytoplasmic streaming in pollen tubes and two myosins identified in tobacco cell cultures are also thought to participate in cytoplasmic streaming (Yokota and Shimmen, 1994, Yokota et al., 1999b). Myosin has been localized to
vesicles, chloroplasts and other organelles, indicating their involvement in transport of these organelles (Heslop-Harrison and Heslop-Harrison, 1989, Miller
et al., 1995). The stop-and-go movements of Golgi stacks in plants also appear to
be dependent on myosin (Nebenfuhr et al., 1999). However, specific myosin(s) responsible for the movement of these organelles have not been identified. Biochemical properties and the localization data of two lily pollen myosins (175 and
170 kDa) suggest that these myosins are at least partially responsible for cytoplasmic streaming, organelle transport and calcium sensitivity of cytoplasmic streaming (Yokota et al., 1999a,1999b, Yokota and Shimmen, 1999). It has been shown
that the directional movement of mitochondria is dependent on the F-actin and
myosin system (Van Gestel et al., 2002). Unlike in animals, the movement of peroxisomes in plants is actin based and actin polymerization appears to drive the
movement of these organelles (Mathur et al., 2002).
18.4.4
Microtubule Dynamics and Organization
Microtubules are highly dynamic structures with a half-life of an individual MT of
about 10 min. Drugs that disrupt the dynamics of MTs have been shown to severely
hamper the MT-dependent process (Reddy, 2001b). Treatment of dividing cells with
colchicine leads to disappearance of the mitotic spindle, suggesting that MTs in the
spindle undergo continuous polymerization and deploymerization. Taxol, in contrast to colchicine, binds to MTs and stabilizes them. Addition of taxol to dividing
cells leads to arrest of dividing cells in mitosis. The dynamics of the MT cytoskeleton during cell division in plant cells have been studied extensively (Baskin and
Cande, 1990, Hepler and Hush, 1996). The rapid turnover of plant MTs suggests
that the transition between MT arrays and change in orientations may involve depolymerization/re-polymerization of MTs and/or movement of MTs. Dynamic instability of MTs is important for their normal function. For example, stabilization
of MTs in elongating root hairs causes loss of growth directionality and promotes
branching (Bibikova et al., 1999). The MTs of the phragmoplast have a t of 60 s
(Hush et al., 1994). However, the mechanisms that regulate the formation and dynamic instability of MT arrays in plants are not understood. Several aspects of MT
dynamics such as cross-linking, zippering, bundling, sliding, and stability of MTs
is regulated by motor proteins (Goldstein and Philip, 1999). KCBP like Ncd in Drosophila can cross-link and zipper MTs to focus MTs to form spindle poles (Chandra
et al., 1993, Kao et al., 2000, Matthies et al., 1996, Smirnova et al., 1998). Two dis-
18 Molecular Motors in Plant Cells
tinct MT binding sites and minus-motor activity enable these proteins to perform
such a function. In addition C-terminal motors have been shown to bundle MTs
(Chandra et al., 1993, Kao et al., 2000). Some motors are implicated in polymerization and depolymerization of cytoskeletal elements. There is at least one example
where a KLP has no motor activity but controls MT dynamics (Desai et al., 1999).
Plant KLPs such as NCAK1 appear to be involved in MT dynamics (Nishihama
et al., 2002, Strompen et al., 2002). Hence, the paradigm that motors bind cargo
and move along cytoskeletal tracks may not explain the functions of all the motors.
18.4.5
Intercellular Transport
Plant cells are connected through plasmodesmata to form a continuous symplastic
network. Recent studies show that macromolecules such as RNA and protein move
from cell to cell through plasmodesmata (Lazarowitz and Beachy, 1999, Lucas et al.,
1995, McLean et al., 1995, 1997). The cytoskeleton and molecular motors have been
implicated in this regulated transport of macromolecules through plasmodesmata
(Blackman and Overall, 1998, McLean et al., 1995, White et al., 1994). The movement protein of viruses which enables the movement of viruses from cell to cell colocalizes to MTs, suggesting a role for cytoskeleton and possibly molecular motors
in the transport of protein from cell to cell through plasmodesmata (Heinlein et al.,
1995, Lazarowitz and Beachy, 1999, Mas and Beachy, 1999, 2000, McLean et al.,
1995). These studies hint at an interesting possibility that motors may play a
role in transporting macromolecules from cell to cell and influence cell-to-cell communication. Immunolocalization studies have also detected myosin associated with
plasmodesmata. A recent study using an antibody to a cloned class VIII Arabidopsis myosin ATM1 (At ATM) localized this myosin to the plasmodesmata and the
plasma membrane regions involved in the assembly of new cell walls (Reichelt
et al., 1999). Earlier work suggested that actin was involved in regulation of plasmodesmal transport (Ding et al., 1996). Other studies using antibodies to animal
myosins in root tissues of Allium cepa, Zea mays and Hordeum vulagare have also
indicated the presence of myosin in the plasmodesmata (Radford and White,
1998).
18.4.6
Other functions
It appears that certain plant motors may perform unexpected novel functions. For
example, two plant KLPs (NCAK1 and NCAK2) have been shown to interact with
NPK1, a MAP kinase kinase kinase, and stimulate its activity (Nishihama et al.,
2002). Motors in plants may also be involved in regulating cell polarity, development and differentiation by transporting the proteins or mRNA to localized regions within the cell (Arn and MacDonald, 1998). There is substantial indirect evidence indicating that asymmetric localization of mRNA (e. g. ASH1 mRNA in
yeast), protein (e. g. calmodulin in Drosophila photoreceptors, a MAPKKK in
459
460
18.5 Regulation of Motors
tobacco BY2 cells) and RNA protein complexes in some eukaryotic cells is dependent on cytoskeletal motors (Bertrand et al., 1998, Carson et al., 1997, 1998, Long
et al., 1997, Munchow et al., 1999, Nishihama et al., 2002, Oleynikov and Singer,
1998, Perhonen et al., 1998, Porter et al., 1993, 1995; Takizawa et al., 1997).
18.5
Regulation of Motors
Although a large number of KLPs have been characterized from diverse organisms
(www.blocks.fhcrc.org/kinesin/), the mechanisms that regulate the activity/function of MT-dependent motors is limited both in plants and animals (Reddy,
2001b, Reilein et al., 2001).
18.5.1
Calcium/Calmodulin
Calcium is a key messenger in transducing many hormonal and environmental
signals in plants (Reddy, 2001a). Calmodulin, a ubiquitous calcium-binding protein
in eukaryotes, is highly conserved in eukaryotes and regulates many diverse cellular functions by modulating the activity of the proteins that interact with it. Upon
binding calcium, calmodulin becomes activated via a conformational change and is
able to regulate the activity of its target proteins (Reddy, 2001a). Cytoplasmic
streaming of characean cells is regulated by the cytosolic calcium concentration
(Nagai, 1993, Shimmen and Yokota, 1994, Williamson and Ashley, 1982). Elevated
levels of calcium inhibit cytoplasmic streaming and the movement of organelles
(Hayama et al., 1979, Kikuyama and Tazawa, 1982, Kohno and Shimmen, 1987,
1988a, 1988b, Takagi and Nagai, 1986, Tominaga et al., 1983, Williamson and Ashley, 1982). Movement of pollen tube organelles along actin cables is inhibited by
calcium, suggesting that the activity of actin-based motors is sensitive to calcium
levels. The mechanisms by which calcium regulates the activity of these motors
is just beginning to emerge. All myosins in Arabidopsis possess two or more putative calmodulin-binding motifs, suggesting that myosins in plants are likely to be
regulated by calmodulin in response to changes in intracellular calcium. Two myosin heavy chains (170 and 175 kDa) in plants associate with calmodulin, indicating
that calmodulin serves as a light chain for these myosins. Yokota et al. (1999a,b)
using highly purified plant myosins have provided the first evidence that calcium
inhibits the myosin motor activity as well as myosin-activated ATPase activity in
plants. The inhibition of myosin activity in the presence of calcium appears to
be due to dissociation of calmodulin from the heavy chain since calcium inhibition
of motor activity can be restored by exogenous addition of calmodulin.
KCBP and recently discovered kinesin C are the only KLPs among the kinesin
superfamily that bind calmodulin (Reddy et al., 1996b, Rogers et al., 1999). The
binding of activated calmodulin to KCBP inhibits its interaction with MTs via
the calmodulin-binding domain resulting in the inhibition of MT-dependent
18 Molecular Motors in Plant Cells
ATPase activity and motility (Deavours et al., 1998, Narasimhulu et al., 1997,
Narasimhulu and Reddy, 1998, Song et al., 1997). These studies strongly suggest
that the binding of calmodulin to KCBP affects MT binding regions resulting in
inhibition of KCBP binding to MTs and dissociation of the KCBP/MT complex.
Structural studies with KCBP in free and calmodulin-bound forms are needed to
verify these speculations. The calmodulin effect on KCBP suggests that activated
calmodulin can act as a molecular switch to downregulate the activity of KCBP.
Based on in vitro studies with KCBP it is reasonable to speculate that spatial and
temporal changes in free cytosolic calcium levels in response to signals are likely
to regulate KCBP activity in the cell.
18.5.2
Protein Phosphorylation
The members of the BimC family have a conserved ‘BimC Box’ in which a threonine residue is phosphorylated by cdc2-kinase during cell division. Targeting of the
KLP to the spindle is regulated by phosphorylation (Blangy et al., 1995, Sawin and
Mitchison, 1995). TKRP125, all three members of Arabidopsis TKRP-like proteins
(AtKRPa, b and c) and DcKRP120-1 and DcKRP120-2 that belong to BimC family
have the conserved BimC box, suggesting that phosphorylation of these KLPs may
be involved in targeting them to the right cellular location. Recently it was found
that the tail region of KCBP also interacts with a protein kinase (KIPK, KCBP interacting protein kinase) in the yeast two-hybrid system (Day et al., 2000). The association of KIPK with KCBP suggests regulation of KCBP or KCBP-associated proteins by phosphorylation and/or KCBP is involved in targeting KIPK to its proper
cellular location. The tobacco NPK1, a mitogen-activated protein kinase kinase
kinase, NPK1, interacts with NACKs. This interaction results in activation and
autophosphorylation of NPK1 and phosphorylation of NACK1 (Nishihama et al.,
2002). Although the significance of NACK1 phosphorylation is not known,
NPK1 phosphorylation targets it to the equatorial zone of the phragmoplasts
(Nishihama et al., 2002).
18.6
Concluding Remarks
The complete sequencing of several phylogenetically diverse model organisms ranging from a unicellular eukaryote to highly complex multicellular animals and a
plant has allowed comparative analysis of molecular motors in eukaryotes. Plants
contain a large number of motors. About 0.4 % of the predicted genes in Arabidopsis encode molecular motors. The comparative analysis provides a framework for
future functional studies with plant KLPs. The number of myosins in Arabidopsis
is comparable to other multicellular organisms of similar genome size. However,
plant myosins are unlike myosins from any other organism except algae. Nonplant myosins contain a variety of other known domains that are lacking in plants.
461
462
References
Furthermore, Arabidopsis has a surprisingly large number of KLPs among the
completed eukaryotic genomes. The role of molecular motors in many plant cellular processes is beginning to be unraveled. Although the functions of most of the
plant motors remain to be determined, phylogenetic analysis and known domains
in these proteins are providing clues to their function which can be tested empirically. Many Arabidopsis KLPs do not fall into any known subfamilies of KLPs and
several Arabidopsis KLPs are not present in other organisms and are likely to
represent new plant-specific subfamilies. Analysis of expression of each motor
and cellular localization, using reporter fusions coupled with studies using the
loss-of-function and gain-of-function mutants, should help to elucidate the
functions of these motors. Identification of the proteins that are associated with
motors is also important to our understanding of their roles in plants. Knockout mutants for almost all Arabidopsis motors are already available from SIGnAL
(http://signal.salk.edu/) or TMRI (http://www.tmri.org). Because of the large
number of motors and likely functional redundancy or overlap with other motors
it will be necessary to create not only single mutants but also double or triple
mutants. Analysis of the function and regulation of plant motors will be an exciting
area of research in plant cell biology and is bound to produce many surprises.
Acknowledgments
I thank members of my laboratory and all my collaborators for their contributions.
I wish to thank Irene Day for preparing the figures and critically reading the manuscript. Research on motors in my laboratory is supported by grants from the
National Science Foundation (MCB-9630782 and MCB-0079938).
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