304 Z. Hong · D.P.S. Verma
Fig. 1 Subcellular localization of phragmoplastin, the first protein marker of the cell
plate.A–DA cytokinetic tobacco BY-2 cell was stained with DAPI for nuclear DNA (A),
anti-tubulin monoclonal antibody for phragmoplast microtubules (B)oranti-soybean
phragmoplastin polyclonal antibodies (C). The fluorescence signals were recorded sepa-
rately in black-and-white photographs. Artificial colors are assigned to DAPI staining (in
blue), microtubules (inred)andcellplate(ingreen). The three photographs are superim-
posed to indicate the relative subcellular localization of phragmoplastin (D;GuandVerma
1996). N, nuclear DNA. M, phragmoplast microtubules. CP, cell plate.EGreen fluorescent
protein (GFP)-tagged phragmoplastin expressed in a tobacco BY-2 cell (Gu and Verma
1997)
rich (PR) domain, which both are characteristic of dynamins (Fig. 2; see
below). It is present largely in the membrane fractions (presumably the Golgi
vesicles) and can also be detected in the cytosol (Gu and Verma 1995). It
contains two separableself-assembly (SA1 and SA2) domains that are re-
sponsible for the formation of phragmoplastin polymers (Zhang et al. 2000).
Intermolecular interaction between SA1 and SA2 leads to the formation of
staggered helical polymer structures that wrap around vesicles. Polymeriza-
tion of phragmoplastin is dependent on GTP binding (Zhang et al. 2000)
and may be regulated by other protein components (Verma 2001; Verma
and Hong 2005). Phragmoplastin has five closely related homologs (DRP1A-
E) inArabidopsis(see below; Hong et al. 2003a), which appear to act as
tubulase in creating dumbbell-shaped tubular structures at the cell plate
(Samuels et al. 1995; Verma 2001; Segui-Simarro et al. 2004; Verma and Hong
2005). Formation of such tubuler structures is essential to prevent balloon-
ing of the fused vesicles and to give rise to a plate, instead of large vacuole
(Verma 2001).