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Characterization of a 67-kDa S6 protein kinase from rat liver.

Fractionation of rat liver cytosol on DEAE-cellulose resolved two S6 kinases eluting at 25 mM KCl (peak I) and 100 mM KCl (peak II). The apparent molecular weights of the peak I and peak II kinases are 26,300 and 67,000, respectively. The peak II kinase was further purified and characterized. Incubation of the kinase with [gamma-32P] ATP and Mg2+ resulted in the incorporation of 32P predominantly into a 67-kDa band. Optimal activity of the kinase was observed in the presence of 5 mM Mg2+ and in the pH range of 8.0-8.5. The Km for ATP and 40S subunit were 7.3 microM and 1.5 microM, respectively. The Mg(2+)-stimulated kinase activity was inhibited by various divalent metals, NaF, and polyamines. The properties of the peak II S6 kinase are very similar or identical to the previously described mitogen-activated S6 protein kinase and may represent the nonactivated form of this enzyme.