Mitosis-specific phosphorylation of caldesmon: possible molecular mechanism of cell rounding during mitosis.

One of the profound changes in cellular morphology during mitosis is a massive alteration in the organization of microfilament cytoskeleton. It has been recently discovered that nonmuscle caldesmon, an actin and calmodulin binding microfilament-associated protein of relative molecular mass Mr = 83,000, is dissociated from microfilaments during mitosis, apparently as a consequence of mitosis-specific phosphorylation. cdc2 kinase, which is a catalytic subunit of MPF (maturation or mitosis promoting factor), is found to be responsible for the mitosis-specific phosphorylation of caldesmon. Because caldesmon is implicated in the regulation of actin myosin interactions and/or microfilament organization, these results suggest that cdc2 kinase directly affects microfilament re-organization during mitosis.