Phosphorylation of Cdc25C by pp90Rsk contributes to a G2 cell cycle arrest in Xenopus cycling egg extracts.

In unfertilized Xenopus eggs, the p42 mitogen activated protein kinase (p42MAPK) pathway is known to maintain cell cycle arrest at metaphase of meiosis II. However, constitutive activation of p42MAPK in post-meiotic, cycling Xenopus egg extracts can lead to either a G2 or M-phase arrest of the cell cycle, depending on the timing of p42MAPK activation. Here, we examined the molecular mechanism by which activation of the p42MAPK pathway during interphase leads to cell cycle arrest in G2. When either a recombinant wild type Cdc25C(WT) or a mutated form of Cdc25C, in which serine 287 was replaced by an alanine (S287A), was added to cycling egg extracts, S287A accelerated entry into M-phase. Furthermore, the addition of S287A overcame the G2 arrest caused by p42MAPK, driving the extract into M-phase. p90Rsk a kinase that is the target of p42MAPK, was phosphorylated and activated (pp90Rsk) in the G2-arrested egg extracts, and was able to phosphorylate WT but not S287A in vitro. 14-3-3 proteins were associated with endogenous Cdc25C in G2-arrested extracts. Cdc25C(WT) that had been phosphorylated by pp90(Rsk) bound 14-3-3zeta, whereas S287A could not. These data suggest that the link between the p42MAPK signaling pathway and Cdc25C involves the activation of pp90Rsk and its phosphorylation of Cdc25C at S287, causing the binding of 14-3-3 proteins. We propose that the binding of 14-3-3 proteins to pp90Rsk phosphorylated-Cdc25C results in a G2 arrest in a manner similar to the cell cycle delays induced by differentiation signals that occur later in embryonic development.