152 J.C. Ambrose · R. Cyr
(Walczak et al. 1998; Vos et al. 2000; Chen et al. 2002; Ambrose et al. 2005;
Ambrose and Cyr 2007). In animals and fungi, these inward-directed sliding
forces generated by Kinesin-14 counterbalance the outward pushing forces
provided by Kinesin-5 (BimC) family members, as evidenced by the finding
that loss of Kinesin-14 partially rescues the spindle-collapse phenotype found
with Kinesin-5 disruption (Hoyt et al. 1993; O’Connell et al. 1993; Mountain
et al. 1999; Sharp et al. 1999, 2000). Although the balance of inward forces gen-
erated by Kinesin-14 with outward forces generated by Kinesin-5 has not yet
been identified in plant spindles, it probably exists based on the similarity of
Kinesin-14 phenotypes (Chen et al. 2002; Marcus et al. 2003; Ambrose et al.
2005; Ambrose and Cyr 2007) and the known localization of Kinesin-5 family
members to the mitotic spindle in tobacco BY-2 cells and carrot cells (Asada
et al. 1997; Barroso et al. 2000).
It has been suggested that the abundance of Kinesin-14 family members
in plants accounts for the absence of the other known eukaryotic minus end-
directed motor, dynein (Reddy and Day 2001; Ambrose et al. 2005). While
dynein-containing fungi and animal genomes generally contain only one or
a few dyneins, the varied association of components of the dynein regulatory
complex, dynactin, probably accounts for the regulation and modulation of
the diverse cellular activities of cytoplasmic dynein. In higher plants these
multiple roles may be taken on by different Kinesin-14 family members, in-
stead of one or a few minus end motors with complex regulatory subunits.
Indeed, Kinesin-14 and cytoplasmic dynein share similar functions within
the spindle apparatus in acentrosomalXenopusegg extracts, where the de-
pletion of both cytoplasmic dynein and the Kinesin-14 XCTK2 leads to an
exacerbation of the splayed-pole phenotype observed with inhibition of ei-
ther one alone (Walczak et al. 1998).
The Kinesin-14 ATK5 is a +TIP that localizes to the mitotic spindle
midzone from early prometaphase until telophase, where it generates in-
ward forces and organizes spindle MTs (Ambrose et al. 2005; Ambrose and
Cyr 2007). Spindles fromatk5-1null mutants are elongated and bent dur-
ingprometaphase,andlaterbecomeabnormallybroadatthemidzoneand
poles, consistent with a loss of MT organizing activity. Additionally, although
prophase spindle poles appear normal, the broadening of poles inatk5-
1 spindles appears during prometaphase, suggesting that ATK5 is a major
pole determinant during prometaphase (Ambrose and Cyr 2007). The ob-
servation that poles become splayed during prometaphase, when the PPB
has already disappeared, but remain tight during prophase, when the PPB
is present, illustrates a redundancy between the PPB and Kinesin-14 in pole
formation. Similarly, theatk1-1phenotype, which affects pole formation, is
more pronounced in meiotic cells, which lack PPBs (Chen et al. 2002). In
the next section, we discuss how different mechanisms and pathways in-
teract and supplement one another in the formation of the plant mitotic
spindle.