310 Z. Hong · D.P.S. Verma
Homotypic fusion is a complex and highly ordered process consisting of
three stages, priming, docking and membrane fusion. Fusion of mitotic vac-
uoles in yeast is a well studied case of homotypic fusion. Both types of v-
and t-SNARE proteins are present on the same vesicle and can form stable
cis-SNARE complexes. At the priming step, thecis-SNARE complex is dis-
assembled into activated t- and v-SNAREs. The activated t-SNARE on one
vesicle can interact with a v-SNARE on another vesicle, forming atrans-
SNARE complex. The formation oftrans-SNARE renders irreversible docking
and fusion of two vesicles. Nearly two dozen proteins are required for the ho-
motypic vesicle fusion in a cell-free assays in yeast (Wickner and Haas 2000;
Wickner 2002). Understanding the precise role of each of these proteins in
plants is essential in order to know how different phases of cell plate building
are orchestrated and completed in a time-bound manner.
3.3
Heterotypic Membrane Fusion
Heterotypic membrane fusion, i.e., the incorporation of two different types
of membranes such as vesicles and the plasma membrane, is mediated by
avesicle-associated SNARE (v-SNARE) and atarget membrane-associated
SNARE (t-SNARE) (Hawes et al. 1999; McNiven et al. 2000). The growing
cell plate is actually a tubular network (Staehelin and Hepler 1996; Verma
2001), and a large quantity of vesicles carrying proteins and cell wall mate-
rials fuse directly with this network, eventually leading to the formation of
the mature and “flat” plasma membrane, a step referred to as the fenestra-
tion of tubular network (Staehelin and Hepler 1996). The protein complexes
required for docking of exocytic vesicles to the plasma membrane are be-
lieved to be employed for the fenestration of tubular network during cell plate
formation.
3.4
Membrane Fusion Machinery
A growing number of SNAREs and other proteins involved in vesicle dock-
ing/fusion events localize to the cell plate and are critical for normal cy-
tokinesis. SNAREs comprise a large diverse group of proteins including syn-
taxins, membrins, VAMPS and others (Sanderfoot et al. 2000). In plants,
the cytokinesis-specific syntaxin Knolle (Q-SNARE) interacts with AtSNAP33
(SNAP25 homolog), AtNPSN11 (plant-specific SNARE), AtCDC48, and Keule
protein (Sec1 homolog) (Lukowitz et al. 1996; Lauber et al. 1997; Waizeneg-
ger et al. 2000; Assaad et al. 2001; Heese et al. 2001; Rancour et al. 2002; Zheng
et al. 2002; Jurgens 2004).
Yeast Cdc48 has been implicated in the ER-to-ER homotypic membrane
fusion (Latterich et al. 1995), and itsArabidopsishomolog (AtCdc48) colocal-