DEGRADATION OF THE PROTOONCOGENE PRODUCT P39MOS IS NOT NECESSARY FOR CYCLIN PROTEOLYSIS AND EXIT FROM MEIOTIC METAPHASE - REQUIREMENT FOR A CA2+ CALMODULIN DEPENDENT EVENT

Exit from M phase, which requires cyclin degradation, is prevented from occurring in unfertilized eggs of vertebrates arrested at second meiotic metaphase due to a cytostatic factor recently identified as p39mos, the product of the proto-oncogene c-mos. Calpain can destroy both p39mos and cyclin in vitro in extracts prepared from metaphase-arrested Xenopus eggs, but only when free Ca2+ concentration is raised to the millimolar range. When free Ca2+ concentration is raised for only 30 s to the micromolar range, as occurs in physiological conditions after fertilization, cyclin degradation is induced, but p39mos is not degraded. Cyclin proteolysis at micromolar free Ca2+ is not inhibited by calpastatin, and therefore does not involve calpain. A cyclin mutant modified in the destruction box is found to be resistant at micromolar, but not millimolar free Ca2+, suggesting that the ubiquitin pathway mediates cyclin degradation at micromolar Ca2+ concentration whereas calpain is involved at the millimolar level. A synthetic peptide which binds Ca2+ -calmodulin with high affinity suppresses cyclin degradation at micromolar but not millimolar free Ca2+, and this only when it is present in the extract during the first 30 s after raising free Ca2+ concentration. The inhibition of the cyclin degradation pathway by the Ca2+ -calmodulin binding peptide can be overcome by adding calmodulin. These results strongly suggest that a Ca2+ -calmodulin process is required as an early event following fertilization to release the cyclin degradation pathway from inhibition in metaphase-arrested eggs. In contrast, p39mos degradation is not required.