Cytoskeletal Motor Proteins in Plant Cell Division 181
sorting of microtubule polarity and maintenance of the mirrored array. Ki-
nesinscouldbeinvolvedinalloftheseactivities.
Although tubulins can self-polymerize into microtubules, locally empha-
sized polymerization in the spindle midzone would certainly be assisted by
proteins that promote polymerization. Cross-linking microtubule-associated
proteins in the MAP65 family are key factors for the development of midzone
microtubules (Muller et al. 2004; Van Damme et al. 2004). Recent results show
Kar3p/NCD-like ATK5, a member of the Kinesin-14 subfamily, decorates the
plus end of midzone microtubules (Ambrose et al. 2005). The NCD kinesin
has been shown to function in stabilizing microtubules, and promoting the
assembly of a stable bipolar spindle in the absence of structurally defined
MTOCs like the centrosome (Matthies et al. 1996). So ATK5 may function in
midzone microtubule polymerization bystabilizing these microtubules. Such
a stabilizing activity may be similar to that of the microtubule plus end track-
ing protein EB1 (Bisgrove et al. 2004; Chan et al. 2005; Dixit et al. 2006; Van
Damme et al. 2004).
Midzone microtubules exhibit a phenomenon of lateral coalescence to lead
the formation of the phragmoplast microtubule array (Zhang et al. 1993).
Again, the Kinesin-14 member KATA/ATK1 plays a role in the lateral coa-
lescence activity (Chen et al. 2002). In mutants lacking KATA/ATK1, mid-
zone microtubules remain as discrete thin bundles, and often fail to develop
into the phragmoplast microtubule array during meiosis. But such a pheno-
type is not obvious in somatic mitosis in the same mutant (Marcus et al.
2003). This may likely be because of functional redundancy of the other
three Kar3p/NCD-like Kinesin-14s inA. thaliana. As a matter of fact, two of
Kinesin-14s, KATB and KATC exhibit a cell cycle-dependent expression pat-
tern with a peak during the M phase, which suggests potential roles in cell
division (Mitsui et al. 1996).
One of the most critical steps in establishing the phragmoplast micro-
tubule array is to sort out the polarity of midzone microtubules so that their
plus ends would be facing each other at the division site. Earlier efforts
have determined that microtubule motor(s) are involved in microtubule-
microtubule translocation activity (Asada et al. 1991). TKRP125A of the
BIMC/Kinesin-5 subfamily is a prominent phragmoplast kinesin (Asada
et al. 1997; Asada and Shibaoka 1994). When TKRP125’s activity was in-
hibited using antibodies in vitro, microtubule translocation activity was
blocked (Asada et al. 1997). In tobacco suspension cells, TKRP125 decorates
phragmoplast microtubules, which further supports its role in microtubule-
microtubule sliding. Intriguingly, immunolocalization of a similar kinesin in
carrot suspension cells indicates that a particular Kinesin-5 specifically deco-
rates overlapping microtubules in the phragmoplast, which supports a role in
sorting out the polarity of the microtubules (Barroso et al. 2000). As a bipo-
lar motor, Kinesin-5 would be a perfect player for establishing an anti-parallel
microtubule array with plus ends in the middle (Fig. 4B).