314 Z. Hong · D.P.S. Verma
during cytokinesis, was found to be transient in mitosis and correlated with
the timing of phragmoplast formation (Bogre et al. 1999). In contrast to the
distribution of phragmoplastin, MMK3 is not redistributed from the center to
the periphery as the cell plate expands (Bogre et al. 1999).
Centrin, a protein involved in nucleation of centromeres in animal cells
and binds Ca2+, has been detected at the forming cell plate (Stoppin-Mellet
et al. 1999). This protein appears to be a component of the Ca2+-sensitive con-
tractile nanofilaments in the neck region of plasmodesmata as shown by EM
immunolocalization (Blackman et al. 1999). Using antibodies, a vinculin-like
protein has also been localized at the cell plate but its plant homolog has not
yet been identified (Endlé et al. 1998).
4.4
Cytokinesis-Related Small GTP-Binding Proteins
Rho, Rab and Ran, three families of small GTP-binding proteins that function
in membrane trafficking and mRNA transport, have also been shown to par-
ticipate in cell-plate biogenesis. Rho-related GTPases localize to the cell plate
(Molendijk et al. 2001; Brembu et al. 2006).ArabidopsisSCD1 encodes a pro-
tein that may interact with Rab proteins and regulate intracellular protein
transport (Falbel et al. 2003). Thescd1(stomatal cytokinesis-defective 1)mu-
tant displays severe cytokinesis defects in leaf guard mother cells, and affects
polar cell expansion in trichomes and root hairs.ArabidopsisRan proteins
and their GTPase-activating proteins (RanGAP) are localized to the forming
cell plate (our unpublished data; Jeong et al. 2005).
In addition, certain mRNAs species may become localized near the cell
plate as has been shown for the actin mRNA inFucus(Bouget et al. 1996).
In this regard, a phragmoplastin-interacting protein (PhrIP1) has been iden-
tified which has properties in common with RNA-binding proteins and is
shown to bind Ran RNA (our unpublished data). A LIM domain protein
(WLIM1) characterized by the presence of one or several double zinc finger
motifs has been isolated from sunflower (Mundel et al. 2000). This protein
accumulates in the phragmoplast in early telophase and may be involved in
anchoring the actin cytoskeleton to site of cell plate and plasma membrane
adhesion. Antibodies against MTOC (microtubule organizing center) and
phosphoprotein MPM-2 have been shown to be associated with the phragmo-
plast (Smirnova et al. 1995).
5
Future Perspectives
The presence of DRPs in plants with diverse functions in membrane tubu-
lation and pinching, as well as fission of organelles, suggests that the basic