The activation of maturation-promoting factor (MPF) is required for G 2 /M progression in eukaryotic cells. Xenopus oocytes are arrested in G 2 and are induced to enter M phase of meiosis by progesterone stimulation. This process is known as meiotic maturation and requires the translation of specific maternal mRNAs stored in the oocytes. We have used an expression cloning strategy to functionally identify proteins involved in G 2 /M progression in Xenopus oocytes. Here we report the cloning of two novel cDNAs that when expressed in oocytes induce meiotic maturation efficiently. The two cDNAs encode proteins of 33 kD that are 88% identical and have no significant homologies to other sequences in databases. These proteins, which we refer to as p33 ringo (rapid inducer of G 2 /M progression in oocytes), induce very rapid MPF activation in cycloheximide-treated oocytes. Conversely, ablation of endogenous p33 ringo mRNAs using antisense oligonucleotides inhibits progesterone-induced maturation, suggesting that synthesis of p33 ringo is required for this process. We also show that p33 ringo binds to and activates the kinase activity of p34 cdc2 but does not associate with p34 cdc2 /cyclin B complexes. Our results identify a novel p34 cdc2 binding and activating protein that regulates the G 2 /M transition during oocyte maturation.

Ribonuclease inhibitor (RI) is a cytoplasmic protein (50 kDa) that inhibits a variety of pancreatic type RNases. The porcine inhibitor contains 30 cysteine residues, all of which occur in the reduced state. It is well known that in vitro modification of the thiol groups inactivates the protein and greatly increases its susceptibility to proteolysis. Here we show that oxidation of thiol groups in RI can also occur within the cell. Induction of an oxidative insult in cultured LLC-PK 1 cells, either with a general oxidant, H 2 O 2 , or with a thiolspecific oxidant, diamide, led to the loss of RI activity. By using specific antibodies it was demonstrated that the decrease correlated with a decline in the amount of RI protein in the cells. Furthermore, analysis of RI mRNA levels and half-life of the protein excluded inhibition of the synthesis of RI as the cause of its depletion. The results indicate that oxidation of thiol groups in RI is sufficient to cause its rapid inactivation and disappearance from the cell. Most likely this results from intracellular degradation of the protein.