Mitotic Spindle Assembly and Function 151
chromosome movement via direct binding to distinct regions of chromo-
somes. Members of the Kinesin-7 (CENP-E) and Kinesin-13 (KinI/MCAK)
families associate with chromosomal kinetochores, where they mediate sta-
ble attachment of MTs to kinetochores and congression to the metaphase
plate (kinesin-7), or through MT depolymerase activity, modulate spindle MT
dynamics (Kinesin-13) to facilitate spindle bipolarity and length (Yen et al.
1991; Schaar et al. 1997; Goshima and Vale 2003; Sharp et al. 2005; Kapoor
et al. 2006). Similar to Kinesin-13, members of the Kinesin-8 (Kip3) family
also appear to act as kinetochore-localized MT depolymerases because their
depletion results in elongated spindles and failed chromosome congression
(West et al. 2001; Savoian et al. 2004; Goshima and Vale 2005).
Members of kinesin-4 (chromokinesin/KIF4) and kinesin-10 (Nod/Kid)
families bind directly to chromosomal arms, where they facilitate motility
along non-kinetochore MTs toward the metaphase plate, thus producing so-
called “polar ejection forces” (Funabiki and Murray 2000; Yajima et al. 2003).
In contrast to the above kinesin families, which directly influence chromo-
some behavior, members of the Kinesin-5 (BimC), Kinesin-6 (MKLP1), and
Kinesin-14 (C-terminal) families facilitate spindle organization and function
via crosslinking MTs and sliding them relative to one another, thereby indi-
rectly influencing chromosome motion and behavior.
Plant kinesins involved in mitotic spindle function.Of the 61 predicted
kinesins in theArabidopsisgenome, 18 have so far been reported on, and of
those, seven have been implicated directly in mitosis and genes from 23 have
been shown to be up-regulated during mitosis (Vanstraelen et al. 2006). Al-
though flowering plants lack four of the 14 kinesin families designated by
Lawrence et al. (2004), they do contain representatives of each of the eight ki-
nesin families that have so far been implicated in mitosis in animals or fungi.
Furthermore, plants contain several novel plant-specific families not included
in the 14-family nomenclature (Richardson et al. 2006), some of which may be
involved in mitosis.
Several kinesins have been localized to the PPB and/or spindle (Liu et al.
1996; Smirnova et al. 1998; Kong and Hanley-Bowdoin 2002; Vanstraelen et al.
2004; Ambrose et al. 2005), however, functional data has only been reported
for ATK1, ATK5, and KCBP (Vos et al. 2000; Marcus et al. 2003; Ambrose et al.
2005; Ambrose and Cyr 2007); therefore, the bulk of this section will focus
on this functional data within the broader context of common mechanisms in
eukaryotic mitosis.
Over a third of the 61 predicted kinesins in theArabidopsisgenome be-
long to the Kinesin-14 family, which comprises the sole group of minus
end-directed kinesins (Reddy 2001). Members of this family typically con-
tain a C-terminally located motor domain (although central- or N-terminal
locations are also present in some plant Kinesin-14s) and function in the
gathering of microtubule minus ends into poles and also in providing inward
forces between overlapping MT plus ends to facilitate spindle compaction