Activation of protein kinase C alters p34(cdc2) phosphorylation state and kinase activity in early sea urchin embryos by abolishing intracellular Ca2+ transients.

The p34(cdc2) protein kinase, a universal regulator of mitosis, is controlled positively and negatively by phosphorylation, and by association with B-type mitotic cyclins. In addition, activation and inactivation of p34(cdc2) are induced by Ca(2+) and prevented by Ca(2+) chelators in permeabilized cells and cell-free systems. This suggests that intracellular Ca(2+) transients may play an important physiological role in the control of p34(cdc2) kinase activity. We have found that activators of protein kinase C can be used to block cell cycle-related alterations in intracellular Ca(2+) concentration (Ca(2+)) in early sea urchin embryos without altering the normal resting level of Ca(2+). We have used this finding to investigate whether Ca(2+) transients control p34(cdc2) kinase activity in living cells via a mechanism that involves cyclin B or the phosphorylation state of p34(cdc2). In the present study we show that the elimination of Ca(2+) transients during interphase blocks p34(cdc2) activation and entry into mitosis, while the elimination of mitotic Ca(2+) transients prevents p34(cdc2) inactivation and exit from mitosis. Moreover, we find that Ca(2+) transients are not required for the synthesis of cyclin B, its binding to p34(cdc2) or its destruction during anaphase. However, in the absence of interphase Ca(2+) transients p34(cdc2) does not undergo the tyrosine dephosphorylation that is required for activation, and in the absence of mitotic Ca(2+) transients p34(cdc2) does not undergo threonine dephosphorylation that is normally associated with inactivation. These results provide evidence that intracellular Ca(2+) transients trigger the dephosphorylation of p34(cdc2) at key regulatory sites, thereby controlling the timing of mitosis entry and exit.