Vesicle Traffic at Cytokinesis 293
cell-plate-fusion machinery represent homologues of proteins used for vesicle
trafficking elsewhere.
3.1
KNOLLE, the First Piece of the Puzzle
The first component of the cell-plate-specific vesicle trafficking machinery to
be identified was the Qa-SNARE (syntaxin) KNOLLE (Lukowitz et al. 1996).
First identified in a screen for embryo/seedling lethal mutants,knollemutants
die as young seedlings, with many cell wall stubs, and incomplete cytokine-
sis (Lukowitz et al. 1996).KNOLLEis only expressed in dividing cells, and the
KNOLLE protein accumulates quickly on the cell plate, as well as the Golgi
and endosome-like structures (Lauber et al. 1997). Control is not only medi-
ated at the message level, since the KNOLLE protein quickly disappears at the
end of cytokinesis (Lauber et al. 1997), indicating that the protein must also
contain signals (unknown at this point) for cell-cycle-specific destruction. As
a Qa-SNARE, KNOLLE was well suited for a role in recognizing and driving
fusion of incoming vesicles to the cell plate (Lauber et al. 1997), just as related
syntaxins drive fusion of vesicles at the neuronal synapse (for example, Li and
Chin 2003). Thus, discovery of KNOLLE drove research towards discovering
other aspects of the SNARE-machinery that may be involved.
SNAREs are a well-conserved family of proteins that contain a diagnos-
tic coiled-coil motif (a SNARE-helix) that is the mechanism used to assemble
specific heteromeric SNARE complexes. On the basis of the sequences of the
SNARE helices from many eukaryotes, Bock et al. (2000) classified SNARE he-
lices into four groups: Qa, Qb, Qc, and R. Appropriately, all known SNARE
complexes are made up from four helices, with one from each of the four
classes. For example, the well-known synaptic SNARE complex is made from
Syntaxin 1 (Qa), SNAP25 (a peptide containing two SNARE helices, an N-
terminal Qb, and a C-terminal Qc), and synaptobrevin (R). As shown in
Fig. 2A, the presynaptic membrane (a specialized PM) serves as the loca-
tion where syntaxin and SNAP25 coil together creating a Qa + Qb + Qc
heterotrimer of SNARE helices (called a “target-(t)-SNARE complex”). This
complex contains a surface groove into which the R-helix of synaptobrevin
can specifically fit. Synaptobrevin is found on the synaptic vesicles, such that
when the vesicle docks, its R-helix begins to coil into the groove. The coil-
ing motion of the SNARE helices acts like a “molecular twist tie”, bringing
the vesicle membrane and the target membrane into close proximity (Hong
et al. 2005). The coiling also releases large amounts of free energy (Fasshauer
et al. 2002), enough to drive vesicle fusion in vitro (for example, McNew et al.
2000). Similar heterotetrameric complexes, each having their own unique set
of Qa + Qb + Qc + R-SNAREs, have been identified in vesicle trafficking
among endomembrane compartments, suggesting that this model may be the
basis for all vesicle trafficking in the cell (e.g. Mc New et al. 2000; Parlati et al.