MAP KINASE DOES NOT INACTIVATE, BUT RATHER PREVENTS THE CYCLIN DEGRADATION PATHWAY FROM BEING TURNED ON IN XENOPUS EGG EXTRACTS

Unfertilized frog eggs arrest at the second meiotic metaphase, due to cytostatic activity of the c-mos protooncogene (CSF). MAP kinase has b een proposed to mediate CSF activity in suppressing cyclin degradation , Using an in vitro assay to generate CSF activity, and recombinant CL 100 phosphatase to inactivate MAP kinase, we confirm that the c-mos p roto-oncogene blocks cyclin degradation through MAP kinase activation, We further show that for MAP kinase to suppress cyclin degradation, i t must be activated before cyclin B-cdc2 kinase has effectively promot ed cyclin degradation. Thus MAP kinase does not inactivate, but rather prevents the cyclin degradation pathway from being turned on, Using a constitutively active mutant of Ca2+/calmodulin dependent protein kin ase II, which mediates the effects of Ca2+ at fertilization, we furthe r show that the kinase can activate cyclin degradation in the presence of both MPF and the c-mos protooncogene without inactivating MAP kina se.