174 Y.-R.J.Lee·B.Liu
the C-terminus which is preceded with a minus end specific neck sequence
(Henningsen and Schliwa 1997). Besides its ATP-dependent microtubule-
binding site in the motor domain, Kinesin-14 also has an ATP-independent
microtubule-binding site located at the N-terminus (Karabay and Walker
1999; Narasimhulu and Reddy 1998).A. thalianahas four kinesins struc-
turally resembling Kinesin-14 from fungi and animals like Kar3p and NCD
(Reddy and Day 2001b). Two of them, KATA/ATK1 and ATK5 demonstrate
minus end-directed motility, and localize preferentially towards the plus ends
of spindle microtubules (Ambrose et al. 2005; Liu et al. 1996; Liu and Palevitz
1996; Marcus et al. 2002). These kinesins are able to translocate microtubules
towards the spindle poles and to form focused poles, which has been demon-
strated in the presence of excess ATP (Liu et al. 1996). Genetic evidence
supports the notion that KATA/ATK1 plays a role in spindle microtubule or-
ganization during mitosis and meiosis (Chen et al. 2002; Marcus et al. 2003).
A mutation at this locus leads to reduced microtubule accumulation at spin-
dle poles (Marcus et al. 2003). The meiotic spindle is dramatically affected by
the loss of this kinesin as spindle poles become splayed with no obvious poles,
which leads to frequent failure in meiosis (Chen et al. 2002). Plants carrying
loss-of-functionatk5mutations exhibit broad spindles in mitotic cells (Am-
brose et al. 2005). Similar roles of animal Kinesin-14 in spindle pole focusing
has been confirmed recently (Goshima et al. 2005). Despite the abnormal
spindle morphology caused byatk1/5mutations, mitosis proceeds normally.
Thus, other cellular factors, e.g. other members of the Kinesin-14 subfamily,
may have redundant roles as these two kinesins.
2.2
Kinesins in Nuclear Envelope Breakdown
Studies in animal cells indicate that cytoplasmic dynein functions in facili-
tating the breakdown of the nuclear envelope by promoting nuclear envelope
invaginations along astral microtubules (Salina et al. 2002). Because plant
cells lack cytoplasmic dynein, such a role by dynein may have been taken over
by one or more minus end-directed kinesins. Two lines of evidence would
support this postulation. First, microtubules are associated with the nuclear
envelope at prophase (Fig. 2). Second, kinesin(s) are present on the prophase
nuclear envelope as revealed by pan kinesin antibodies raised against peptide
conserved among all kinesins (Sawin et al. 1992) (Fig. 2).
A unique member of the plant Kinesin-14 subfamily, the calmodulin-
binding kinesin KCBP/ZWI may play a role in nuclear envelope breakdown.
KCBP/ZWI contains a C-ter minal Ca++/calmodulin-binding site and a talin-
like domain towards the N-terminus (Reddy et al. 1996; Reddy and Reddy
1999). Otherwise, it is a minus end directed kinesin similar to KATA/ATK1
with an N-terminal ATP-independent microtubule-binding site (Narasimhulu
et al. 1997; Song et al. 1997). Interestingly, its motor activity is inhibited upon