Cytoskeletal Motor Proteins in Plant Cell Division 185
vesicular network, and ultimately to the cell plate (Bednarek and Falbel 2002;
Jürgens 2005; Otegui and Staehelin 2000; Verma 2001).
To date, very little has been known about kinesins in vesicle transport
in the phragmoplast. Many questions are still standing. How many kinesins
are involved in vesicle transport? Are all vesicles transported by identical ki-
nesins? What is the mechanism of vesicle–kinesin interaction? How are such
kinesins activated for vesicle transport? Are vesicles transported processively
along phragmoplast microtubules? Whether, and if yes how, these kinesins are
recycled during cytokinesis?
An immunocytochemical screen for kinesins in the phragmoplast has re-
vealed a plant-specific kinesin AtPAKRP2 which is localized on vesicle-like
structures in the phragmoplast (Lee et al. 2001). This N-terminal motor
kinesin is very divergent from all other studied kinesins among eukary-
otes,thusitisconsideredasanorphankinesin.AtPAKRP2isassociated
with Golgi-derived vesicles, and the association apparently is required for
AtPAKRP2 to be localized along phragmoplast microtubules. This evidence
suggests that AtPAKRP2 is most likely activated by its cargo (vesicle) binding.
Further functional analysis is necessary for addressing how AtPAKRP2 works
in the phragmoplast.
The membrane system in the phragmoplast is highly dynamic, and vesicu-
lation is probably continuously taking place (Verma and Hong 2005). Besides
relatively long distance transport along phragmoplast microtubules, there
might be short distance microtubule-based transports within the phragmo-
plast. However, our knowledge in this aspect is vague. Obviously, more ki-
nesins are yet to be characterized for their roles in vesicle trafficking in the
phragmoplast.
3.4
Kinesins for Spatial Regulation of Cytokinesis?
One of the most intriguing questions regarding plant cell division is the fore-
cast of the cell division plane by the PPB, whose assembly and disassembly
take place before the breakdown of the nuclear envelope. Apparently, the
phragmoplast is able to “read” the information left by the PPB at the corti-
cal division site. A recent report on a predicted minus end-directed kinesin
KCA1 suggests that this kinesin may be involved in the spatial regulation of
cell division inA. thaliana(Vanstraelen et al. 2006). KCA1 and its homolog
KCA2 contain typical motor and neck sequences as the Kinesin-14 mem-
ber KATA/ATK1, but in addition they have an extended coiled-coil domain
followed by a domain with two CDK-phosphorylation sites and a nuclear lo-
calization sequence (Vanstraelen et al. 2004). Using a heterologous expression
system of tobacco cells, a GFP-KCA1 fusion protein shows a localization pat-
tern at the cell cortex and the cell plate, but not at the cortical site which the
PPB occupies (Vanstraelen et al. 2006). While such a localization pattern is