Molecular Analysis of the Cell Plate Forming Machinery 311
izes at the cell plate with Knolle and another plate-associated SNARE, SYP31,
and may play a similar role in cell plate formation (Feiler et al. 1995; Ran-
cour et al. 2002). CDC48 is one of the AAA proteins (ATPases associated with
various cellular activities) that regulate vesicle fusion events by disassem-
bling the SNARE complexes to allow for subsequent rounds of fusion (May
et al. 2001).ArabidopsisPatellin 1 (PATL1), a Sec-14 related protein impli-
cated in membrane trafficking in yeast, is recruited from the cytoplasm to
the expanding cell plate and may play a role in membrane-trafficking events
associated with cell-plate maturation (Peterman et al. 2004). RanGAP, a nu-
clear membrane-associated protein is also observed at the early stages of cell
plate (Jeong et al. 2005), but its precise role in the cell plate formation process
is not yet clear.
4
Other Cell Plate-Associated Proteins
Several hundred proteins may participate in the processes of cytokinesis in
plants. Some of them have been identified by genetic studies (Assaad 2001,
Sollner et al. 2002). In addition to the two major categories of proteins (i.e.,
dynamin-related proteins and proteins involved in membrane fusion) men-
tioned above, several other groups of proteins have been implicated in cell
plate formation. They include proteins associated with the callose synthase
complex, microtubule-associated proteins, small GTP-binding proteins, and
cytokinesis signaling proteins. Since the building of cell plate also involves
formation of plasmodesmata channels, many other proteins involved in the
formation of these structures remain to be identified.
4.1
Cell Plate-Specific Callose Synthase Complex
Unlike many other cell wall polysaccharides that are synthesized in the Golgi
and delivered to the cell wall, callose and cellulose are synthesized by plasma
membrane enzymes and deposited directly to the cell plate. Callose is the first
polysaccharide synthesized at the growing cell plate (Northcote et al. 1989;
Samuels et al. 1995; Bowser and Reddy 1997). Synthesis of callose begins as
the tubular structure is formed and continues until the plate has touched the
parental cell wall. Callose appears to apply a spreading force and helps sta-
bilize the structure of the membranous tubular network. InArabidopsis,this
callose is synthesized by a cell plate-specific callose synthase (CalS1) that is
a member of the large callose synthase family (Hong et al. 2001a). A T-DNA
knockout mutant ofcals1did not display any observable cytokinesis phe-
notype and contains callose at the cell plate, suggesting that more than one
callose synthase is involved in cell plate formation. Callose is a multimeric en-