In vivo regulation of MPF in Xenopus oocytes.

Entry into M phase in the eukaryotic cell cycle is controlled by the oscillating activity of MPF. The active component of MPF is now known to be the p34cdc2 protein kinase originally found in yeast. The p34cdc2 protein kinase displays a characteristic M-phase-specific histone H1 kinase activity when it interacts with cyclins, which are proteins that oscillate through the cell cycle and are thought to regulate p34cdc2 activity. Cyclins can induce M phase when introduced into fully grown Xenopus oocytes and cyclin may play a role in normal oocyte maturation. Small Xenopus oocytes do not mature in response to the hormonal triggers which act on stage 6 oocytes. We introduced cyclin into stage 4 (small) Xenopus oocytes and showed that it activates MPF in these cells, probably by interacting with endogenous p34cdc2 kinase. We made labelled extracts from cyclin-mRNA-injected stage 4 oocytes and used them to show differential stability of clam cyclins A and B at oocyte maturation. The relative stability of the two forms of cyclin related directly to their ability to stabilize crude MPF preparations from injected stage 6 oocytes.