Cytoskeletal Motor Proteins in Plant Cell Division 179
Plant Kinesin-5 was first isolated from the tobacco BY-2 cells as TKRP125
(Asada et al. 1997). Later two similar proteins were identified in carrot sus-
pension cells by a biochemical approach (Barroso et al. 2000). Structurally,
they are very similar to their counterparts in fungi and animals. Among 61
kinesins inA. thaliana, four belong to Kinesin-5 with the signature BIMC box
(Lee and Liu 2001, 2004). Immunolocalization data indicate that one or more
Kinesin-5 decorates midzone microtubules (Asada et al. 1997; Barroso et al.
2000), suggesting they may play a role in microtubule sliding there (Fig. 4B).
Unfortunately, functional characterization of Kinesin-5s inA. thalianaby ge-
netic means has been lagging behind, likely due to functional redundancy of
multiple homologs.
Intriguingly, the activity of Kinesin-5 is antagonized by Kar3p/NCD-like
kinesins in the Kinesin-14 subfamily in both fungi and animals (Hoyt et al.
1993; O’Connell et al. 1993; Sharp et al. 2000a). The current model is that
Kinesin-5 generates outward force along microtubules to allow anti-parallel
microtubules to slide against each other, while Kinesin-14 acts as a brake by
generating inward forces (Sharp et al. 2000c). Four members of the Kinesin-
14 subfamily inA. thaliana, K ATA / AT K 1 , K AT B , K ATC , a n d AT K 5 , s t r u c -
turally resemble Kar3p/NCD (Lee and Liu 2001, 2004). They decorate midzone
microtubules in mitotic cells by both immunofluorescence and fluorescent
protein tagging (Ambrose et al. 2005; Liu et al. 1996; Liu and Palevitz 1996;
Mitsui et al. 1994). Whether these four kinesins function antagonistically
against the four Kinesin-5s awaits further studies.
In summary, to our knowledge members of Kinesin-5 and Kinesin-14
may have similar functions in mitosis and meiosis as their animal and fun-
gal counterparts. Kinesins of other subfamilies which have been proven to
function in spindle operation in fungal and animal cells either do not have
homologs in plant cells, or their plant counterparts do not function in mitosis
or meiosis.
3
Kinesins in Plant Cytokinesis
Concomitant with the shortening of the kinetochore fibers during mitosis,
midzone microtubules develop into prominent tight bundles because of new
microtubule polymerization and coalescence (Zhang et al. 1993). The polarity
of these midzone microtubules is later sorted out with their plus ends facing
the future division site (Fig. 5). It had been speculated that phragmoplast mi-
crotubules interdigitated in the middle. However, a recent study indicates that
the anti-parallel microtubules in the phragmoplast do not overlap, and are
spaced by the proposed “cell plate assembly matrix” (Austin et al. 2005).
Phragmoplast microtubules serve as tracks along which Golgi-derived
vesicles are transported towards microtubule plus ends for assembling the cell