236 M.Sasabe·Y.Machida
Fig. 2 The direct interaction of NPK1 and NACK1 at the equator of the phragmoplast
in telophase.ADomain organization in the NPK1 and NACK1 proteins. The coiled-coil
structures (green) were predicted by the COILS program (Lupas et al. 1991), and the
nuclear localization signal (red) and NACK1/NPK1-binding sites (bars) were determined
in our previous studies (Ishikawa et al. 2002).BColocalization of NPK1 and NACK1 at
the equator of the phragmoplast. BY-2 cells expressing GFP-NPK1 (green) were double-
stained with rabbit antibodies against NACK1 (red)and4′-6-diamidino-2-phenylindole
(DAPI) for nuclei (blue).CThe model for the activating mechanism of the NACK1/NPK1
complex followed by the activation of the MAPK cascade during plant cytokinesis. KD,
kinase domain; RD, regulatory domain; MD, motor domain; ST, stalk domain. See text
for details
is localized in the cytoplasm at metaphase (Nishihama et al. 2001). During
cytokinesis,whenthekinaseactivityofNPK1increases,NPK1shiftstothe
leading edge of the equatorial zone of the phragmoplast (Fig. 2B; Nishihama
et al. 2001). Because of this pattern of localization, this kinase was designated
nucleus- andphragmoplast-localized proteinkinase 1 (NPK1).
Function of NPK1. In BY-2 cell lines overexpressing a kinase-defective
mutant of NPK1 (NPK1:KW), a significant fraction of cells are binucleate
or multinucleate and unusually large (Nishihama et al. 2001). Staining with
calcofluor, which bindsβ-glucans in cell walls, or aniline blue, which binds