Positive Feedback between MAP Kinase and Mos duringXenopusOocyte Maturation

Mos is a serine–threonine protein kinase and a key regulator of meiosis. One function ofXenopusMos is to activate mitogen-activated protein kinase (MAPK) through direct phosphorylation and activation of MAPK kinase (MAPKK). All three members of this signal cascade can individually induce hormone-independent reentry of oocytes into meiosis I. However, their inducing efficiency is reduced in the absence of protein synthesis. Here we show thatde novoMos synthesis is required for induction of meiosis I by active MAPKK or Mos–MAPK coinjection. In addition, MAPK efficiently phosphorylates Mos at Ser-3in vitro.These results suggest that a positive feedback loop exists between MAPK and Mos during oocyte maturation.De novosynthesis of Mos, and other proteins, is required for progression from meiosis I to the metaphase arrest at meiosis II; therefore, one function of MAPK during normalXenopusoocyte maturation might be to stimulate the synthesis or accumulation of Mos that is required for the completion of meiosis.