Molecular Analysis of the Cell Plate Forming Machinery 313
ing similarity with annexin and protein kinase C (Andrawis et al. 1993). This
annexin-like protein has GTPase activity which is inhibited by Ca2+and stim-
ulated by Mg2+(Shin and Brown 1999). Rop1 may act as a spatial regulator of
CalS as Rho has been shown to be a regulatory subunit ofβ-1,3-glucan syn-
thase in yeast (Qadota et al. 1996). Phragmoplastin may activate this enzyme
by squeezing the vesicles into tubules (Gu and Verma 1996, 1997; Zhang et al.
2000) at the forming cell plate (Verma 2001) or direct interaction via UGT1 as
has been demonstrated (Hong et al. 2001b).
4.2
Microtubule-Associated Proteins
Theγ-tubulin has been localized towards the minus ends of the phragmo-
plast microtubules (Joshi and Palevitz 1996). AIR9, a 187 kDa microtubule-
associated protein, decorates the preprophase band (PPB) and appears in the
junction between the outwardly growing cell plate and the mother cell wall
(Buschmann et al. 2006). Kinesin-related proteins with either the plus-end,
or the minus-end directed microtubule motors have been found to be as-
sociated with the phragmoplast. TKRP125, a tobacco kinesin-related protein
and a plus-end directed motor, requires another protein of 120 kD to display
microtubule-translocating activity (Asada and Shibaoka 1994; Asada et al.
1997). KatA, anArabidopsiskinesin-like protein and a minus-end-directed
motor, is localized in the midzone of an anaphase cell and is associated
with phragmoplast during cytokinesis (Liu et al. 1996). AnotherArabidopsis
kinesin-like protein, KCBP, binds to calmodulin and is confined to the ma-
ture phragmoplast (Bowser and Reddy 1997). AtPAKRP2, which localizes to
Brefeldin A-sensitive puncta during early plate development, is a likely can-
didate for the motor that drives vesicle movement along the phragmoplast
microtubules (Lee et al. 2001; Smith 2002), and the kinesin-related protein,
product of the HINKEL/NACK1 genes, functions in microtubule dynamics
during plate expansion in collaboration with a mitogen-activated protein
kinase-signaling pathway (Nishihama et al. 2002; Strompen et al. 2002). The
dynamics of these various motor proteins may facilitate not only the mobi-
lization of tubulin cytoskeleton but also vesicle traffic.
4.3
Cytokinesis-Related Signaling Proteins
Vesicle trafficking and fusion events during cell plate formation may also be
regulated by MAP kinases through phosphorylation of rab and dynamin-like
GTPases as shown in animals (Earnest et al. 1996, Cormont et al. 1994). A to-
bacco MAP kinase p43Ntf6is activated in late anaphase and early telophase
and is localized to the cell plate in anaphase cells (Calderini et al. 1998). The
enzymatic activity of MMK3, an alfalfa MAP kinase associated with cell plate