A protein kinase from Xenopus eggs specific for ribosomal protein S6.

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Abstract
A protein kinase specific for ribosomal protein S6 was purified from eggs of X. laevis. As visualized on a Ag-stained polyacrylamide gel, the major protein in the preparation migrated with a MW of 90,000. Incubation of the enzyme preparation with [.gamma.-32P]ATP led to phosphorylation of this protein on Ser resudies. Upon glycerol gradient centrifugation, the S6 kinase activity and the MW 90,000 protein both sedimented with a MW of 50,000-55,000. Two-dimensional gel electrophoresis demonstrated that up to 4-5 phosphate groups per S6 molecule could be incorporated with this enzyme in vitro, and 2-dimensional peptide mapping demonstrated that the phosphopeptides from S6 labeled in vitro with the enzyme comigrated with those from highly phosphorylated S6 labeled in vivo in response to progesterone treatment. The purified S6 protein kinase did not phosphorylate at a significant rate ribosomal protein S10, histone H1, histone H4, mixed histones, casein, or phosvitin, indicating a high degree of substrate specificity. Activation of a single S6 protein kinase may be sufficient to account for increased S6 phosphorylation after a growth stimulus.