A novel p34^cdc2-binding and activating protein that is necessary and sufficient to trigger G₂/M progression in Xenopus oocytes

Abstract

The activation of maturation-promoting factor (MPF) is required for G₂/M progression in eukaryotic cells.Xenopus oocytes are arrested in G₂ 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 G₂/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 inoocytes), 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 G₂/M transition during oocyte maturation.

Keywords

• Cell cycle
• meiotic maturation
• MPF
• M phase
• oocyte
• Cdc2