Description of a protein kinase derived from insulin-treated 3T3-L1 cells that catalyzes the phosphorylation of ribosomal protein S6 and casein

Particulate preparations from insulin-treated 3T3-L1 cells retain the enhanced ability to incorporate ³²P from [γ-³²P]ATP into ribosomal protein S6. A cyclic AMP-independent protein kinase that phosphorylates S6 and casein and that may be involved in the increase in S6 phosphorylation produced by insulin has been isolated based upon the observation that there is 1.5-3.0-fold higher activity in particulate preparations derived from insulin-treated cells than there is in comparable preparations from control cells. The enzyme activity was purified 2071-fold by KCl extraction, phosphocellulose chromatography, and gel filtration. The S6 phosphorylating activity was also characterized by its behavior on casein-Sepharose and DEAE-cellulose chromatography and its sedimentation in glycerol gradients. None of these procedures resolved the S6 and casein kinase activities. Some of the properties of this kinase, including a molecular weight of about 35,000, inhibition of F^- or phosphate, chromatography on DEAE-cellulose and phosphocellulose, and insensitivity to inhibition by GTP, are similar to those of a previous described enzyme, casein kinase I.