Purification and characterization of ribosomal protein S6 kinase I from Xenopus eggs

Ribosomal protein S6 kinase I has been purified from unfertilized Xenopus eggs to near homogeneity as a Mr = 90,000 protein. S6 kinase I is phosphorylated when activated in vivo and can be phosphorylated by mitogen-activated protein kinase in vitro. The purified enzyme is inactivated upon treatment with protein phosphatase 2A. Immunological data and analysis of substrate specificity demonstrate that S6 kinase I is related to, but distinct from, the previously characterized S6 kinase II. Both enzymes are members of the ribosomal protein S6 kinase (rsk) gene family.