Inactivation of an S6 Kinase by Protein Phosphatases
Stimulation of quiescent cells with agents such as serum,1,2 various growth factors and hormones, 3–5 oncogene products, and tumor promoters 2,5 leads to the activation of a kinase that phosphorylates ribosomal protein S6. Several enzymes, including cAMP-dependent protein kinase,6 protease- activated kinase II,7 and protein kinase C8 phosphorylate S6 in vitro, but it appears that the mitogen-stimulated kinase is distinct from these. Earlier we described a novel S6 kinase from Swiss mouse 3T3 cells that was stimulated up to 25-fold by serum, epidermal growth factor (EGF), or sodium orthovanadate.l, 3 EGF caused a rapid activation of the enzyme; maximal activity was expressed within 5–15 min and then there was a slow return to basal levels after 2–3 hours. Addition of orthovanadate at this time, but not EGF, led to a reactivation of the kinase. In order to recover the enzyme in an active state, phosphatase inhibitors such as p-nitrophenyl phosphate (pNPP) or ß-glycerophosphate (ß-GP) had to be present in the extraction buffer, otherwise a time-dependent loss of kinase activity occured. These results suggested that the S6 kinase might be regulated by phosphorylation-dephosphorylation. In this study we have examined this possibility by separating the cellular phosphatases from the S6 kinase and testing them for the ability to inactivate the enzyme. Inactivation by purified protein phosphatases is also discussed.
KeywordsProtein Phosphatase Phosphatase Inhibitor Fast Flow Rous Sarcoma Virus Peak Tube
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