MAP Kinase Signaling During M Phase Progression 237
Fig. 3Specific activation of NPK1, NQK1, and NRK1 at late M phase.AThe graph shows
a plot of the mitotic indices of BY-2 cells synchronized at M phase. The cell cycle was ar-
rested at prometaphase by propyzamide after release from an aphidicolin block.BThe
protein kinase activities of NPK1, NQK1, and NRK1 were determined by immunocom-
plex kinase assays using recombinant kinase-negative NQK1, kinase-negative NRK1, and
myelin basic proteins as substrates, respectively
callose, a polysaccharide synthesized at the initial stage of vesicle fusion, re-
veals the formation of incomplete cell plates in these multinucleated cells
(Nishihama et al. 2001). Such multinucleated cells could also be observed
in transgenic tobacco plants harboring NPK1:KW. The cotyledons of these
NPK1:KW plants develop with a rough surface, and the stomata of the cotyle-
dons consistently contained many binucleated guard cells and cells without
a nucleus (Nishihama et al. 2001). Thus, expression of NPK1:KW inhibits the
lateral expansion of the cell plate in both tobacco cells and plants.
Arabidopsishomologs of NPK1 (ANP1, ANP2, and ANP3) also appear to
be involved in cytokinesis. Loss of function of two of the three homologs of
NPK1 (ANP2 and ANP3) causes defects in cytokinesis, specifically, the for-
mation of multinucleated cells with incomplete cell walls (Krysan et al. 2002).
These results suggest that the MAPK cascade including the NPK1 MAPKKK
family is necessary for the progression of cytokinesis in plants.
2.2
Activator of NPK1 MAPKKK: NACK1, a KLP
As described above, deletion of the regulatory domain in the carboxy-
terminal half of NPK1 increases kinase activity, indicating that this domain