c-Mos proteolysis is independent of the Ca2+ rise induced by 6-DMAP in Xenopus oocytes

In Xenopus oocytes, metaphase II arrest is due to a cytostatic factor (CSF) that involves c-Mos, maintaining a high MPF (cdk1/cyclin B) activity in th e cell. At fertilization, a rise in intracellular calcium triggers the prot eolysis of both cyclin B and c-Mos. The kinase inhibitor 6-dimethylaminopur ine (6-DMAP) is also able to release matured Xenopus oocytes from metaphase II block. This is characterized by c-Mos proteolysis without degradation o f cyclin B, We hypothesized that 6-DMAP induced an increase in intracellula r calcium. Using the calcium-sensitive fluorescent dye Fura-2, we observed a systematic increase in intracellular calcium following 6-DMAP application . In matured oocytes previously microinjected with the calcium chelator BAP TA no calcium changes occurred after 6-DMAP addition; however, c-Mos was st ill proteolysed. In oocytes at the GVBD stage, c-Mos proteolysis occurred i n response to 6-DMAP but not to calcium ionophore treatment. We suggest tha t c-Mos proteolysis is rather controlled by a phosphorylation-dependent pro cess. (C) 2001 Academic Press.