Vesicle Traffic at Cytokinesis 297
vide some weak cytokinetic function (though SEC1a, one of the KEULE-like
SM proteins, did not interact with KNOLLE as KEULE did; Assaad et al. 2001).
Similarly, KNOLLE is one of nine PM-type Qa-SNAREs (Sanderfoot et al.
2000), though it appears to be the only one expressed in a cell-cycle specific
manner (Volker et al. 2001). Again, some minimal level of cytokinetic func-
tion may be supplied by the other PM Qa-SNAREs that allows survival to the
seedling stage. Still, the combination of these two proteins is unquestionably
important, sinceknolle keuledouble mutants are “synthetic lethal”—they ar-
rest as a multinucleate zygote that fails to complete even the first division
(Waizenegger et al. 2000).
3.4
Other Regulators of Vesicle Fusion
Beyond the SNAREs and SM-proteins, other regulators have been suggested
to be involved in vesicle tethering. Tomographic examination of cell plates has
suggested that vesicle-docking complexes, of a shape similar to the Sec6/8-
complex of mammalian cells, are involved in collecting the vesicles neces-
sary for forming the cell plate (Seguí-Simarro et al. 2004). Actual involvement
of these and other exocyst components in vesicle docking and fusion re-
lated to cell plate formation has not been shown, but one would expect that
something like the exocyst complex (perhaps made from cell-plate-specific
isoforms) would be involved. Vesicle docking and fusion also requires the ac-
tion of a Rab-GTPase, and the identity of this (or these) cell-plate-specific
Rab remains unknown. A very large group of Rab11-like GTPases (RABA in
Vernoud et al. 2003) is found in land plants, and is likely to be involved in
post-Golgi trafficking similar to the mammalian homologues. Could one of
these 26 proteins be involved in regulating cell plate formation? On the other
hand, there is also a group of Rab8-like GTPases (RABE in Vernoud et al.
2003) that may play an important role in polarized secretion based upon simi-
larity to their yeast and mammalian homologues. Unfortunately, each of these
(in fact all the Rabs in Arabidopsis) are present in very large gene families,
and the potential for redundancy prevents a simple investigation of the role
of these Rabs in cell plate formation (Vernoud et al. 2003). Considering the
large gene families typical in most land plants, this potential for redundancy
prevents simple characterization of many potential cell-plate-specific factors.
Undoubtedly, many other levels of regulation in vesicle docking and fusion
will be revealed by future research.
3.5
A Role for Dynamins in Turning Tubes into Sheets
Once the vesicles fuse, work remains before a macroscopic organelle like
the cell plate can be formed. The morphology of the small cell plate frag-