Cytoskeletal Motor Proteins in Plant Cell Division 171
a microtubule-binding site (Vale 2003). The ATPase activity residing in this
motor domain is activated upon nucleotide (ATP)-dependent binding to mi-
crotubules. The directionality of kinesins, or whether a particular kinesin
moves towards the plus or minus end of microtubules, is determined by
a short peptide known as the neck sequence (Endow 1999). Among kinesins,
amino acid sequences of non-motor domains are not conserved. Most ki-
nesins have a tripartite structure of the motor domain, coiled-coil domain
for dimerization, and a cargo-binding domain. On the basis of phylogenetic
analysis of the motor domain, a unified nomenclature has classified kinesins
into Kinesin-1 to Kinesin-14 subfamilies (Lawrence et al. 2004; Miki et al.
2005). However, members of the same subfamily may have unrelated func-
tions in different organisms. Kinesins which do not fit in these 14 subfamilies
are often treated as orphan kinesins.
TheArabidopsis thalianagenome contains 61 genes encoding kinesins
(Reddy and Day 2001b). The riceOryza sativaalso has more than 50 ki-
nesin genes (Richardson et al. 2006). Unfortunately, most of these genes do
not have their cDNA sequences determined. The number of plant kinesins
is overwhelming especially considering mammals like mouse and human
have significantly fewer kinesins (Miki et al. 2005; Vale 2003). As kinesins
from other kingdoms, both Arabidopsis and rice kinesins include those with
plus end-directed motor activity, and those with minus end-directed activity.
Compared to kinesins from animals, plant kinesins include a large number
of minus end-directed ones. For example, inA. thaliana21 kinesins are pre-
dicted to be minus end-directed motors (Reddy and Day 2001b). Most of
plant kinesins contain non-motor sequences not found in kinesins of other
kingdoms, such as the Armadillo repeat sequence and calponin-homology
domain (Reddy and Day 2001b; Richardson et al. 2006). Functions of only
a few kinesins have been experimentally determined (Lee and Liu 2004).
Among the 61 kinesins inA. thaliana,itisintriguinghowmanyarein-
volved in cell division. Unfortunately, the comparison of non-motor domains
of these kinesins to animal and fungal ones known to play a role in cell divi-
sion usually is not informative. In a recent study, three criteria have been used
to postulate that a given kinesin may play a role in mitosis: (1) the presence of
a mitosis specificcis-acting element; (2) the presence of one or more putative
cyclin-dependent kinase (CDK)-phosphorylation sites; and (3) the presence
of a mitotic destruction box (Vanstraelen et al. 2006). On the basis of these
criteria, 23 kinesins are designated as “mitotic kinesins”, and their expression
are indeed elevated during cell division. Some of the later mentioned kinesins
are included in these 23 mitotic kinesins.
This article summarizes functions of kinesins in plant cell division, which
have either been experimentally determined or hypothesized according to
particular microtubule-based motility at certain stages during cell division.
While some kinesins may only function at a certain stage of cell division,
others may be active at multiple stages.