An insulin-stimulated ribosomal protein S6 kinase from rabbit liver

In this report we describe an activated form of S6 protein kinase in rabbits treated acutely with insulin. The major insulin-stimulated activity in rabbit liver is increased 2- to 5-fold compared to material from untreated animals based on DEAE-cellulose profiles. The activity observed in DEAE-cellulose fractions can be separated into a major and minor peak, each having very similar chromatographic behavior. Chromatography on DEAE-cellulose, S-Sepharose, heptyl-Sepharose, heparin-agarose, and Mono Q results in greater than 20,000-fold purification of the insulin-stimulated enzyme with a 12% recovery. The stimulated activity has chromatographic properties similar to an S6 protein kinase studied previously in 3T3-L1 cells (Cobb, M.H. (1986) J. Biol. Chem. 261, 12994-12999) and other systems. The enzyme purified from insulin-treated animals contains a major band that migrates in sodium dodecyl sulfate-polyacrylamide gels with M(r) ~ 70,000; this band also appears in the control preparation. Treatment of the insulin-stimulated S6 kinase with the catalytic subunit of phosphatase 2a reduces its activity by 97%. The activity of the inactivated S6 kinase is stimulated nearly 5-fold by a 15-min preincubation with partially purified insulin-stimulated microtubule-associated protein-2 kinase.