Cloning and characterization of Xenopus Rsk2, the predominant p90 Rsk isozyme in oocytes and eggs

The 90-kDa ribosomal S6 kinases, the p90 Rsks, are a family of intracellular serine/threonine protein kinases distinguished by two distinct kinase domains. Rsks are activated downstream of the ERK1 (p44) and ERK2 (p42) mitogen-activated protein (MAP) kinases in diverse biological contexts, including progression through meiotic and mitotic M phases in Xenopus oocytes and cycling Xenopus egg extracts, and are critical for the M phase functions of Xenopus p42 MAPK. Here we report the cloning and biochemical characterization of Xenopus Rsk2. Xenopus Rsk1 and Rsk2 are specifically recognized by commercially available RSK1 and RSK2 antisera on immunoblots, but both Rsk1 and Rsk2 are immunoprecipitated by RSK1, RSK2, and RSK3 sera. Rsk2 is about 20-fold more abundant than the previously described Xenopus Rsk1 protein; their concentrations are approximately 120 and 5 nm, respectively. Rsk2, like Rsk1, forms a heteromeric complex with p42 MAP kinase. This interaction depends on sequences at the extreme C terminus of Rsk2 and can be disrupted by a synthetic peptide derived from the C-terminal 20 amino acids of Rsk2. Finally, we demonstrate that p42 MAP kinase can activate recombinant Rsk2 in vitro to a specific activity comparable to that found in Rsk2 that has been activated maximally in vivo. These findings underscore the importance of the Rsk2 isozyme in the M phase functions of p42 MAP kinase and provide tools for further examining Rsk2 function.