CA2-KINASE IN PERMEABILIZED SEA-URCHIN EMBRYOS( TRIGGERS PREMATURE INACTIVATION OF THE CDC2 PROTEIN)

Exit from mitosis requires inactivation of the cyclin B-p34(cdc2) prot ein kinase complex. Since increased cytosolic Ca2+ has been implicated as a potential trigger of mitotic progression, we directly tested the possibility that Ca2+ triggers the pathway responsible for inactivati ng the cdc2 kinase, using sea urchin embryos permeabilized at various stages of the cell cycle. In cells permeabilized during late interphas e and prophase, micromolar Ca2+ induced premature inactivation of the cdc2 kinase without affecting the absolute amount of p34(cdc2) protein . Inactivation was selective for the cdc2 kinase, as elevated Ca2+ had no effect on cAMP-dependent protein kinase activity. Premature cdc2 k inase inactivation did not require cyclin B destruction, but did coinc ide with the dissociation of cyclin B-q34(cdc2) complexes. In cells pe rmeabilized during prometaphase and metaphase, cdc2 kinase inactivatio n was Ca2+-independent, presumably because at these later times the in activating pathway had been enabled prior to permeabilization. This wo rk provides evidence that Ca2+ is the physiological trigger enabling c dc2 kinase inactivation during mitosis.