294 A. Sanderfoot
2002). Whether in vivo SNAREs are the specificity determinant, the fusion
machinery, or both is a topic of great debate, though their importance in some
aspect of the process is not really questioned. Regardless, many different lev-
els of regulation of the SNARE recognition and assembly process have been
identified which would seem to assist in controlling the action of the SNARE
proteins. For example, members of the SM (Sec1p/Munc18)-family of proteins
seem to regulate the assembly state of the Qa-SNAREs (Toonen and Verhage
2003), members of the Rab-GTPase family seem to regulate vesicle migration
and choice of target membrane (Stenmark and Olkkonen 2001), and a long
list of “docking factors” seem to assist in the specificity of vesicle trafficking
(Whyte and Munro 2002).
3.2
Almost All the Other SNAREs Soon Follow
Qa-SNAREs need three other SNAREs to make a complex, so the search for
other partners for KNOLLE proceeded. Heese et al. (2001) showed that the
Qb+Qc SNARE SNAP33 was one SNARE partner with KNOLLE (Fig. 2B).
However, while SNAP33 is found on the cell plate in dividing cells, it does
not have a cell cycle-specific expression pattern, and is found on the PM in
similar abundance to the cell plate in dividing cells, indicating that it is not
cell plate specific (Heese et al. 2001). Moreover, in non-dividing cells, SNAP33
has been found to interact with other PM Qa-SNAREs as part of both regu-
lar (Kargul et al. 2001) and a form of defense-related polarized secretion
(Collins et al. 2003; Assaad et al. 2004). In addition,snp33mutants do not
have a seedling lethal phenotype; instead, they die at the plant equivalent
of “teenagers” from the spread of necrotic lesions across the surface of the
plant (Heese et al. 2001). These lesions do contain cell wall stubs, and signs
of cytokinetic defects (Heese et al. 2001), but it does not directly phenocopy
knollemutants. Again, redundancy may be the cause, since SNAP33 is one of
a three-member family of SNAREs, each of which also interacts with KNOLLE
to a similar extent as SNAP33 in yeast two-hybrid experiments (Heese et al.
2001). Nonetheless, a genetic interaction ofsnp33withknolleandkeulemu-
tants (Heese et al. 2001) showed that KNOLLE and SNAP33 likely represents
aQa+Qb+Qc-SNAREcomplexinvolvedincellplateformation.
Interestingly, another SNARE was also shown to interact with KNOLLE
and be found on the cell plate. Zheng et al. (2003) identified a Qb-SNARE
called NPSN11 which, like SNAP33, was found localized to the cell plate in
dividing cells and interacted with KNOLLE by immunoprecipitation, yet was
not expressed in a cell specific manner, and was found on other organelles
in non-dividing cells (Zheng et al. 2003). Again, NPSN11 was one of three
NPSN-like Qb-SNAREs in Arabidopsis (Sanderfoot et al. 2000). In this case
the redundancy seems more significant since there was no observed pheno-
type innpsn11mutants (Zheng et al. 2003); in fact, single mutants in each of