Degradation of the proto‐oncogene product p39mos is not necessary for cyclin proteolysis and exit from meiotic metaphase: requirement for a Ca(2+)‐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, byt 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 Ca(2+)‐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 Ca(2+)‐calmodulin binding peptide can be overcome by adding calmodulin. These results strongly suggest that a Ca(2+)‐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.