Plant Cytokinesis – Insights Gained from Electron Tomography Studies 269
In in vitro studies, dynamin has been shown to form spiral polymers
around lipid bilayer tubules, with GTP-dynamin producing tight spirals and
GDP-dynamin expanded ones (Stowell et al. 1999; Zhang et al. 2000). Based
on these observations, it has been proposed that GTP hydrolysis causes the
dynamin spirals to expand like a spring, going from a tight to an expanded
spiral configuration (Fig. 6). The energy released during this conformational
change appears to be used by cells to shape or fission membranes (Stowell
et al. 1999; Hong et al. 2001b). For example, the dynamin spirals that are
assembled around the necks of budding clathrin-coated vesicles have been
postulated to break the neck of the vesicle buds and thereby create free vesi-
cles (Stowell et al. 1999). In the cellularizing endosperm, the dynamin spirals
within the CPAM wrap around and pinch but do not sever the wide cell plate
tubules, a process that appears to be used to prevent the tubules from prema-
turely expanding into sheet-like structures (Otegui et al. 2001). In contrast,
during somatic type cytokinesis, dynamin spirals are employed to convert
the hourglass-shaped fused vesicles into dehydrated and mechanically sta-
ble, dumbbell-shaped vesicles (Fig. 6), which serve as the building blocks of
somatic-type cell plates (Seguí-Simarro et al. 2004). The rationale for this
type of vesicle transformation is discussed below. When the cell plate-forming
vesicles fuse outside the CPAM, they appear to give rise to larger round vesi-
cles and not to dumbbell-shaped vesicle intermediates (Seguí-Simarro et al.
2004). Only when vesicle fusion occurs inside the CPAM are dumbbell-shaped
vesicles produced. This suggests that the CPAM serves as a scaffold for con-
centrating the dumbbell vesicle-forming DRPs in the vicinity of the cell plate,
much like the accumulation of the proteins that complete the formation of the
vesicle-tethering exocyst complexes.
The maturation of both syncytial and somatic-type cell plates also in-
volves the budding of large numbers of clathrin-coated vesicles (Otegui et al.
2001; Otegui and Staehelin 2004; Seguí-Simarro et al. 2004; Seguí-Simarro
and Staehelin 2006a), and this budding process too seems to be mediated by
DRP rings and spirals. In fact, similar rings have been observed constricting
theclathrin-coatedvesiclebudsthatremovemembranefromthematuring
cell plates (Seguí-Simarro et al. 2004; Seguí-Simarro and Staehelin 2006a).
The molecular nature of these latter rings is unknown, but DRP2A, another
dynamin-related protein that has also been localized to cell plates is a likely
candidate (Konopka et al. 2006). The presence of a pleckstrin homology do-
main and of a proline-rich motif (Hong et al. 2003) relates DRP2A to animal
dynamins involved in membrane trafficking processes mediated by clathrin-
coated vesicles.