148 J.C. Ambrose · R. Cyr
3
Microtubule-Associated Proteins in Plant Mitotic Spindles
Identification of the molecular players involved in plant mitosis has tradition-
ally lagged behind that of animals and fungi; however, with the sequencing of
theArabidopsisand rice genomes, the advent of GFP technology, and an ex-
plosion in proteomic technologies, advances are being made at the forefront
of the mitosis field. Since a number of excellent reviews are available on the
molecular characteristics of animal mitosis (Gadde and Heald 2004; Kline-
Smith and Walczak 2004), this section will emphasize the available data from
plants.
Control of MTs dynamics and organization within the plant mitotic spindle
is accomplished in part with a diverse ensemble of structural and regula-
tory microtubule-associated proteins (MAPs) (Wick 2000; Hussey et al. 2002).
MAPs can be divided roughly into two main groups: structural MAPs and
motor MAPs.
3.1
Structural MAPs
Structural MAPs can be further subdivided into three functional classes:
(1) MAPs that affect MT dynamics (e.g. stabilize or destabilize MTs);
(2) MAPs that crosslink or bundle MTs; and (3) MT-severing MAPs. It should
be noted, however, that these groupings are not mutually exclusive.
MAP-65.To date, nine MAP-65 proteins have been identified or predicted
inArabidopsis, three in tobacco BY-2 suspension cells, and three in carrot
suspension cells (Hussey et al. 2002). These proteins share homology to hu-
man PRC1 and yeast Ase1p, both of which contribute to the formation and
integrity of antiparallel midzone MTs via crosslinking and stabilization (Mol-
linari et al. 2002; Schuyler et al. 2003; Zhu and Jiang 2005). In plants, several
members of the MAP-65 family have also been localized at the midzone,
suggesting they share a similar function (Muller et al. 2004; Chang et al.
2005; Mao et al. 2005). Several other members localize to the cortical ar-
rays, where they have been implicated in MT bundling (Van Damme et al.
2004b; Mao et al. 2005); indeed, purified carrot MAP-65 bundles MTs in
vitro, forming 20 – 25 nm crossbridges between MTs (Chan et al. 1999). In the
case of AtMAP65-4, localization to the spindle is observed, but midzone en-
richment is not seen (Van Damme et al. 2004b). The presence or absence
of CDK phosphorylation sites between different MAP-65 family members
may contribute to their differentially modulated localization dynamics. In-
terestingly, mutation of the MAP consensus Cdk site to an unregulated form
results in the premature accumulation of AtGFP-MAP65-1 to the midzone in
early prometaphase, whereas the wild-type version doesn’t appear there until
anaphase (Mao et al. 2005).