306 Z. Hong · D.P.S. Verma
ticipate in a variety of membrane fusion and fission processes. Cell plate
formation involves at least two subfamilies of DRPs: tubulase (DRP1) and pin-
chase(DRP2).Eachofthesehaveisoformswhichmaydifferentiallyinteract
with other proteins in a cell-type-specific manner.
2.1
DRPs, A Superfamily of Dynamin-Related Proteins
Arabidopsiscontains 16 genes that may potentially encode proteins with
the dynamin-signature (L-P-[PK]-G-[STN]-[GN]-[LIVM]-V-T-R) (Prosite
PDOC00362). These proteins have been grouped into six functional sub-
families (DRP1-6) (Hong et al. 2003a). The DRP1 subfamily contains five
phragmoplastin-like members (DRP1A-E), and are localized at the forming
cell plate (Lauber et al. 1997; Kang et al. 2001, 2003a,b; Hong et al. 2003b).
They may also play specific roles in the plasma membrane (Kang et al.
2003a,b) and cytoskeleton (Hong et al. 2003b). The DRP2 subfamily con-
tains thebona fideplant dynamins that have a characteristic PH domain in
themiddleofthemoleculeandaPRmotif(RXPXXP)neartheC-terminus
(Fig. 2). DRP2 proteins may be involved in clathrin-coated vesicle trafficking
between Golgi, plasma membrane including cell plate, and in vacuole mem-
brane biogenesis (Hong et al. 2003b; Jin et al. 2001; Lam et al. 2002). The
DRP3 subfamily members do not contain PH or PR motifs, and have been
implicated in the division of mitochondria and peroxisomes (Arimura and
Tsutsumi 2002; Nishida et al. 2003; Mano et al. 2004; Kuravi et al. 2006). The
DRP4 subfamily is related to the animal Mx proteins that have antiviral ac-
tivity (Haller and Kochs 2002).ArabidopsisDRP5 subfamily and its ortholog
fromCyanidioschyzon merolae(CmDnm2) are localized to the chloroplast
division-ring and required for chloroplast division (Gao et al. 2003; Miyag-
ishima et al. 2003, 2006). The function of DRP6 remains to be determined.
2.2
DRP1 Tubulase is Required for the Formation of Tubular Structures at the Cell Plate
The growing edges of the cell plate are filled with the “hourglass”- or
“dumbbell-shaped” tubular structures (Samuels et al. 1995; Otegui et al. 2001;
Segui-Simarro et al. 2004; Otegui and Staehelin 2004). These tubular struc-
tures are membrane-based transient and fragile structures that are very sensi-
tive to chemical fixation treatments. They can be preserved in tissue samples
that are cryofixed (Segui-Simarro et al. 2004; Samuels et al. 1995; Otegui et al.
2001; Otegui and Staehelin 2004). These tubular structures serve as basic
building blocks for the formation of cell plate. The volume of one tubular
structure is approximately twice the size of a cell plate vesicle, suggesting
that the tubular structures are generated from fusion of two cell plate vesicles
(Otegui and Staehelin 2004). Dumbbell-shaped tubular structures have a layer