Cytoskeletal Motor Proteins in Plant Cell Division 183
subfamily so that they have been suggested to reside in a plant unique sub-
family (Dagenbach and Endow 2004; Lawrence et al. 2004). They interact
with the MAPKKK through a domain in their non-motor region (Nishihama
et al. 2002). These kinesins colocalize with aforementioned MAPKKK at the
cell division site (Nishihama et al. 2002; Yang et al. 2003). InA. thaliana,
theyhaveredundantfunctionsincytokinesisofallcelltypesasloss-of-
function mutations at either loci only lead to partial failure in cytokinesis
(Tanaka et al. 2004). However, NACK1/HIN functions more pronouncedly in
somatic cytokinesis during embryogenesis, while STD/TES/NACK2 seems to
be more critical for male meiotic cytokinesis (Hülskamp et al. 1997; Nishi-
hama et al. 2002; Spielman et al. 1997; Strompen et al. 2002). These kinesins
play a role in the localization of the MAPKKK at the division site, and con-
sequently they are required for the expansion of the cell plate (Nishihama
et al. 2002). A recent study shows that NtMAP65-1 is a substrate of this MAP
kinase cascade in tobacco (Sasabe et al. 2006). Native MAP65-1 acts as a mi-
crotubule stabilizing agent by promoting microtubule polymerization and
bundling, and is targeted to the spindle midzone during mitosis (Mao et al.
2005a,b; Smertenko et al. 2000). Upon phosphorylation by the MAP kinase
NRK1/NTF6, MAP65-1’s microtubule bundling activity is significantly down
regulated, and is more restricted to the middle region of the phragmoplast
where microtubule plus ends are located (Sasabe et al. 2006). Therefore, the
phosphorylation of MAP65-1 promotes the turnover of phragmoplast micro-
tubules.
Intriguingly, the depolymerization initiates at the minus ends, and pro-
gresses towards the plus end (Zhang et al. 1993). To our knowledge, none of
the plant kinesins has been shown with a microtubule depolymerase activ-
ity. In animal cells, Kinesin-13 acts as the depolymerase with microtubule
end-stimulated ATPase activity (Walczak 2003). In certain cells like those of
the frog Xenopus, a given Kinesin-13 can act at both the plus and the mi-
nus end of microtubules (Desai et al. 1999; Walczak et al. 1996; Wordeman
and Mitchison 1995). In Drosophila cells, however, one Kinesin-13 acts at the
plus end, while another one acts at the minus end to depolymerize micro-
tubules (Rogers et al. 2004). However, an Arabidopsis Kinesin-13 acts at the
Golgi stacks, and does not play a role in cytokinesis (Lu et al. 2005). Whether
other plant kinesins possess a depolymerase activity awaits further analysis.
It will also be interesting to learn whether plant cells use the same kinesin(s)
for depolymerizing microtubules in the spindle and the phragmoplast.
Concomitant with the depolymerization, new microtubules are polymer-
ized at the peripheral region of the phragmoplast. A recent observation
indicates that the Kinesin-14 ATK5 appears at the leading edge of the phrag-
moplast microtubule array (Ambrose et al. 2005). On the basis of its potential
role in promoting polymerization as shown by its Drosophila homolog NCD,
ATK5 may act to promote microtubule polymerization at the periphery of the
phragmoplast.