A Constitutively Active Form of the Protein Kinase p90Rsk1 Is Sufficient to Trigger the G2/M Transition in Xenopus Oocytes

The protein kinase p90Rsk has previously been implicated as a key target of the MAPK pathway during M phase of meiosis II in Xenopus oocytes. To determine whether Rsk is a mediator of MAPK for stimulation of the G2/M transition early in meiosis I, we sought to generate a form of Rsk that would be constitutively active in resting, G2 phase oocytes. Initial studies revealed that an N-terminal truncation of 43 amino acids conferred enhanced specific activity on the enzyme in G2 phase, and stability was highest if the C terminus was not truncated. The full-length enzyme is known to be activated by phosphorylation at five sites. Two of these sites and flanking residues were replaced with either aspartic or glutamic acid, and Tyr699 was mutated to alanine. The resulting construct, termed fully activated (FA) Rsk, had constitutive activity in G2 phase, with a specific activity equivalent to that of wild type Rsk in M phase. In eight independent experiments ∼45% of oocytes expressing FA-Rsk underwent germinal vesicle breakdown (GVBD, the G2/M transition) in the absence of progesterone, and this effect could be observed even in the presence of the MAPK kinase inhibitor U0126. Moreover, the specific activity of FA-Rsk in vivo was unaffected by U0126. In oocytes that did not undergo GVBD with FA-Rsk expression, subsequent treatment with progesterone resulted in a very rapid rate of GVBD even in the presence of U0126 to inhibit the endogenous MAPK/Rsk pathway. These results indicate that Rsk is the mediator of MAPK effects for the G2/M transition in meiosis I and in a subpopulation of oocytes Rsk is sufficient to trigger the G2/M transition.