Cytoskeletal and Vacuolar Dynamics During Plant Cell Division 131
ganization, and characterization of the corresponding gene revealed that
the deduced amino acid sequence of MOR1 had similarity toXenopus
MAP215 (XMAP215) and a human homolog of TOG1p (Charrasse et al.
1998; Tournebize et al. 2000). Biochemical identification of microtubule-
associated proteins from tobacco BY-2 cells revealed proteins with a molecu-
lar mass of about 200 kDa, which were found to be tobacco homologs of
MOR1/XMAP215 (Yasuhara et al. 2002; Hamada et al. 2004). Purified tobacco
MOR1 proteins accelerated the polymerization, lengths and numbers of MTs
(Hamada et al. 2004). Although a role for MOR1 in CMT organization was
initially implied by the mutant phenotype, anArabidopsismutant ofgem1-1
that affects cytokinesis and cell division patterns at pollen mitosis was found
to be allelic tomor1(Twell et al. 2002). Recently, Kawamura et al. (2006) re-
ported short and abnormally organized mitotic spindles and phragmoplasts
in themor1-1mutant, in which these MT structures persisted longer than in
WT plants. Therefore, MOR1 is now implicated in mitosis and cytokinesis as
well as in CMT organization.
26S proteasomes.During cell cycle progression, increased evidence indi-
cates the rapid and timely degradation of cell cycle regulatory proteins by
the ubiquitin-proteasome pathway in mammalian and yeast cells. In higher
plant cells, degradation of cyclins A and B by the 26S proteasome has also
been reported (Genschik et al. 1998; Criqui et al. 2000). The 26S proteasome
is a highly organized protein-degrading machinery that catalyzes the ATP-
dependent degradation of ubiquitinated proteins (Coux et al. 1996; Hershko
and Ciechanover 1998). In tobacco BY-2 cells, the 26S proteasome was local-
ized to nuclear envelopes and the MT structures of the PPBs, mitotic spindles
and phragmoplasts (Yanagawa et al. 2002). The proteasome inhibitor, MG-
132, exclusively caused cell-cycle arrest at the early stages of PPB formation
at metaphase and prior to entering the G1 phase, which appears to be closely
related to proteasome distribution in the cells. In BY-GT cells treated with MG-
132 at metaphase, after formation and collapse of the original phragmoplast,
phragmoplast-like structures appeared again (Oka et al. 2004). In such cases,
the CMTs never became reorganized and the phragmoplast-like structures had
the ability to form cell plates as did the original phragmoplast. Associated with
phragmoplast collapse was the disappearance of the kinesin-related protein,
TKRP125, which co-localized with phragmoplast microtubules, demonstrated
possible microtubule translocation activity, and was implicated in the forma-
tion and/or maintenance of the bipolar structure of phragmoplasts (Asada
et al. 1997). As the TKRP125 protein level remained constant following MG-
132 treatment and was localized to the extra phragmoplast (Oka et al. 2004),
it is possible that TKRP125 is one of the targets of the ubiquitin-proteasome
degradation pathway during M/G1 transition and that degradation of MT-
associated proteins is an indispensable process in cell cycle progression.
Protein kinases.MT dynamics during cell cycle progression have been
shown to be regulated by several protein kinases. Mutagenesis of the con-