The efficient activation of p90 ^rsk by MAP kinase requires their interaction through a docking site located at the C-terminal end of p90 ^rsk . The MAP kinase p42 ^mpk1 can associate with p90 ^rsk in G ₂ -arrested but not in mature Xenopusoocytes. In contrast, an N-terminally truncated p90 ^rsk mutant named D2 constitutively interacts with p42 ^mpk1 . In this report we show that expression of D2 inhibitsXenopus oocyte maturation. The inhibition requires the p42 ^mpk1 docking site. D2 expression uncouples the activation of p42 ^mpk1 and p34 ^cdc2 /cyclin B in response to progesterone but does not prevent signaling through p90 ^rsk . Instead, D2 interferes with a p42 ^mpk1 -triggered pathway, which regulates the phosphorylation and activation of Plx1, a potential activator of the Cdc25 phosphatase. This new pathway that links the activation of p42 ^mpk1 and Plx1 during oocyte maturation is independent of p34 ^cdc2 /cyclin B activity but requires protein synthesis. Using D2, we also provide evidence that the sustained activation of p42 ^mpk1 can trigger nuclear migration in oocytes. Our results indicate that D2 is a useful tool to study MAP kinase function(s) during oocyte maturation. Truncated substrates such as D2, which constitutively interact with MAP kinases, may also be helpful to study signal transduction by MAP kinases in other cellular processes.