Vesicle Traffic at Cytokinesis 299
lead to localization of SYP21 to the cell plate (Müller et al. 2003), indicating
that the signal for cell plate localization is a part of the proteins themselves,
similar to signals that target proteins to other organelles (see Sanderfoot and
Raikhel, 2002). Nonetheless, this putative signal remains unknown.
Aside from the cell plate SNAREs, the only known membrane protein to
be targeted as cargo to the cell plate is callose synthase. Because the cell plate
lumen fills with the carbohydrate-polymer callose during the tubulovesicular
stage (Samuels et al. 1995), such a protein had been assumed to be involved
for a long time. Still, it was not until Hong et al. (2001) identified a large fam-
ily of callose synthase-like enzymes in Arabidopsis that the identity of this
enzyme was known. Through their work, they showed that the Arabidopsis
CalS1 isoform is found on the cell plate and interacts with DRP1a (i.e. phrag-
moplastin) and an associated UDP-Glucose transferase (Hong et al. 2001).
Being such a large protein, it would be difficult to assign significance to small
regions of homology between CalS1 and KNOLLE are (i.e. potential “cell plate
signals”), and no such signals have been tested. Until more known cargo pro-
teins are identified, such searches for “cell plate signals” are unlikely to be
fruitful.
Other membrane proteins perhaps do not require “cell plate” signals, and
cansimplyarriveatthecellplatebydiffusionintheplaneofthemembrane
from the regular PM or through endo-/exocytosis (Dhonukshe et al. 2006)
from endosomal organelles. Even these proteins are few and poorly known.
Until some mechanism for isolating an enriched source of cell plates for pro-
teomic investigation, cell plate proteins will likely come one at a time through
genetic or candidate gene approaches.
5
Summary
The cell plate of the land plants is the most striking example of how vesicle
trafficking and the addition of new membrane is required for eukaryotic cy-
tokinesis. As discussed above, much recent work has begun to identify the
sources of the membrane that makes up the cell plate, though some questions
remain as to how much is derived from secretory or endocytic processes.
Considerable effort has also identified many of the molecular machines that
are involved in targeting and fusing these membranes at the cell plate, but
many components remain elusive, and many more are still completely un-
known. Finally, we still know little about what proteins are actually being
transported to the cell plate. It may turn out that the cell plate does not have
many unique cargo proteins, but more likely, we just have not looked hard
enough. Nonetheless, much progress has been made in our understanding of
cell plate assembly, and this progress may already be helping to illuminate the
study of cytokinesis in other organisms that have less obvious roles for vesi-