MAP Kinase Signaling During M Phase Progression 245
Function of phosphorylated NtMAP65-1 during M phase.InBY-2cells,
overexpression of mutant NtMAP65-1a proteins that cannot be phosphory-
lated by NRK1 delays M phase progression and phragmoplast expansion,
whereas mutants that cannot be phosphorylated by CDKs do not affect either
process (Sasabe et al. 2006). Similar delays in expansion are often observed
in cells overexpressing dominant-negative mutants of NACK1, NPK1, and
NQK1 (Nishihama et al. 2001, 2002; Soyano et al. 2003). Further analysis
has revealed that the cortical MTs and phragmoplast MTs in cells overex-
pressing an NRK1-non-phosphorylatable mutant are resistant to the MT-
depolymerizing drug propyzamide, suggesting that the MTs are stabilized by
the non-phosphorylatable mutant of NtMAP65-1a.
It is well known that the MAP65/Ase1p/PRC1 protein family can bind and
bundle MTs. Recombinant NtMAP65-1a also has MT binding and bundling
activity in vitro. On the other hand, phosphorylation of NtMAP65-1a by
NRK1 reduces its bundling activity (Fig. 5C). This is not due to the phos-
phorylation of CDKs, which suggests that the stability of MT-containing
structures depends on the MT-bundling activity of MAP65. Recently, three
groups have reported that the MTs bundled by MAP65 proteins have in-
creased resistance to some chemicals or environmental stresses in vivo and
in vitro (Wicker-Planquart et al. 2004; Van Damme et al. 2004b; Mao et al.
2005). The phenotypes of various organisms with loss-of-function mutations
of MAP65/Ase1/PRC1 imply that these proteins maintain the distribution of
the spindle or phragmoplast MTs (Jiang et al. 1998; Smertenko et al. 2000;
2004; Mollinari et al. 2002; Schuyler et al. 2003; Verbrugghe and White 2004;
Vernì et al. 2004; Müller et al. 2004). The activity of MT-bundling seems
to be concerned with the stability of MTs. Thus, the simplest model to ex-
plain the molecular effect of phosphorylation of NtMAP65 by NRK1/NTF6 is
that, during cytokinesis, it suppresses the stability of the phragmoplast at the
midzone, resulting in increased MT turnover and, therefore, phragmoplast
expansion. Whether the phosphorylation of MAP65 by CDKs is important
during the progression of M phase in plants remains to be determined. In
animals, the phosphorylation of PRC1 by CDK before metaphase appears to
be important for progression from metaphase to anaphase and may suppress
MT bundling in the mitotic spindle (Mollinari et al. 2002). In addition, mam-
malian PRC1 interacts separately with many other proteins that are involved
in the formation of the midzone of the central spindle, and its interaction
appears to be involved in the phosphorylation of CDK (Ban et al. 2004;
Kurasawa et al. 2004; Zhu and Jiang 2005). These reports suggest that regu-
lation of the dynamics of MT-containing structures must be very complex.
Further investigations of MAP65-interacting proteins and the NACK-PQR
pathway-related factors are required if we are to fully understand the regula-
tion of mitosis.