172 Y.-R.J.Lee·B.Liu
1.2
Myosin Superfamily Proteins in Plants
Motility along microfilaments is driven by myosins. The myosin motor do-
main of approximately 700 amino acids bears a nucleotide-binding motif and
an actin-binding site. Myosins contain a special motif known as the IQ repeat
for binding to calmodulin as one of their light chains. Myosins are classi-
fied into 24 subfamilies (Berg et al. 2001; Foth et al. 2006; Hodge and Cope
2000). To date, only Myosin VI has been determined as a pointed (minus)
end-directed motor, and others are either determined as or predicted to be
barbed (plus) end-directed motors (Berg et al. 2001). Plant cells are known
for rapid cytoplasmic streaming which is powered by myosins (Shimmen and
Yokota 2004). Compared to the complex figure of myosins in animals, plant
myosins are grouped into only two subfamilies, Myosin VIII and Myosin XI
(Reddy and Day 2001a). Plant myosins are more related to Myosin V than
to others, implying that they probably play a role in membrane trafficking
as animal and fungal Myosin Vs do. Because plant myosins have predicted
molecular masses of larger than 150 -kDa, a few of them have their cDNA se-
quences determined. Moreover, cellular activities of most plant myosins have
not been studied experimentally.
2
Kinesins in Plant Mitosis and Meiosis
The spindle apparatus in plant cells looks very similar to that of an animal
cell, except for having incompletely focused spindle poles (Fig. 1A). There-
fore, the plant spindle microtubule array only consists of kinetochore mi-
crotubules linking the kinetochore to the spindle pole, and inter-polar non-
kinetochore microtubules probably required for maintaining the overall
structureofthespindle.Kinesinswouldberequiredforthefollowingpro-
cesses: (1) organization of microtubules to form the two spindle poles dur-
ing spindle assembly; (2) nuclear envelope breakdown; (3) attachment of
sister chromatids to microtubules; (4) congression of chromosomes at the
metaphase plate; (5) disassembly of kinetochore fibers and movement of sis-
ter chromatid/homologous chromosomes towards the poles; and (6) spindle
elongation.
2.1
Kinesins for Spindle Pole Organization
Unlike in animal cells where microtubules often originated from the cen-
trosome prior to mitosis, plant somatic cells committed to mitosis have mi-
crotubules polymerized across the nuclear envelope, concomitantly with the