A novel p34(cdc2)-binding and activating protein that is necessary and sufficient to trigger G(2)/M progression in Xenopus oocytes

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 a pe induced to enter M phase of meiosis by progesterone stimulation. This pr ocess is known as meiotic maturation and requires the translation of specif ic 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 hom ologies 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, ablatio n 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 activ ates 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 act ivating protein that regulates the G(2)/M transition during oocyte maturati on.