Molecular Analysis of the Cell Plate Forming Machinery 307
of protein coat on the surface of the tubular structures (Samuels et al. 1995),
while the hourglass-shaped structures have two collars at the neck regions
(Segui-Simarro et al. 2004). These protein coats and collars have been shown
to be DRP1 proteins using antibodies against DRP1a (Otegui et al. 2001).
DRP1 proteins have been proposed to act as a tubulase that mediates vesicle
tubulation and thus prevent ballooning of the fused structures(Verma, 2001;
Verma and Hong 2005). Animal dynamins are pinchases (see below), but can
also act as a tubulase under certain circumstances such as in the presence
of GTP-γ-S (Takei et al. 1995), overexpression of a clathrin antisense RNA
(Iversen et al. 2003) or in vitro polymerization on tubular liposomes (Takei
et al. 1998; Sweitzer and Hinshaw 1998; Stowell 1999). Thus, plant DRP1 fam-
ily is a dedicated group of proteins that are responsible for the formation of
tubular structures from fused vesicles at the forming cell plate. This novel
mechanism serves as a space-filling model from the architectural view and is
ideal for building a cell plate structure de novo.
2.3
DRP2 Pinchase is Required for Membrane Recycling at the Cell Plate
ArabidopsisDRP2 proteins are thebona fideplant dynamins which appear
to function as pinchases (Fig. 2; Verma and Hong 2005). The dynamin pin-
chase hydrolyzes GTP and provides mechanical force to pinch the vesicles off
the plasma membrane, an essential step in endocytosis and membrane re-
cycling (Sweitzer and Hinshaw 1998; McNiven 1998; Verma et al. 2005). Cell
plate formation involves the de novo construction of two “flat” layers of the
plasma membrane using phospholipids and membrane proteins that are de-
livered via vesicles. Coalescing of the tubular structure described above leaves
behind extensive membrane which needs to be recycled. It has been esti-
mated that 10-14-fold more membranes are required to build one unit of the
plasma membrane, suggesting that most membrane constituents that have
been delivered to the cell plate through the exocytic pathway need to be re-
cycled back through endocytosis and membrane recycling pathways. DRP2A
has been shown to be associated with the cell plate and may be responsible
for pinching-off of the vesicles from the forming cell plate to recycle the extra
membrane compartment (Hong et al. 2003b).
3
Homotypic and Heterotypic Membrane Fusion at the Cell Plate
Cell plate formation involves intensive activities of membrane fusion events
that can be grouped into two categories: homotypic and heterotypic mem-
brane fusions. Whereas the heterotypic fusion at the cell plate may employ
redistributed fusion machinery from the plasma membrane, homotypic fu-