Further evidence that the tyrosine phosphorylation of glycogen synthase kinase-3 (GSK3) in mammalian cells is an autophosphorylation event

Phosphorylation of the endogenous GSK3alpha (glycogen synthase kinase-3alpha) at Tyr(279) and GSK3beta at Tyr(216) was suppressed in HEK293 or SH-SY5Y cells by incubation with pharmacological inhibitors of GSK3, but not by an Src-family inhibitor, 4-amino-5(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), or a general protein tyrosine kinase inhibitor (genistein). GSK3,beta transfected into HEK-293 cells or Escherichia coli became phosphorylated at Tyr(216), but catalytically inactive mutants did not. GSK3beta expressed in insect Sf21 cells or E. coli was extensively phosphorylated at Tyr(216), but the few molecules lacking phosphate at this position could autophosphorylate at Tyr(216) in vitro after incubation with MgATP. The rate of autophosphorylation was unaffected by dilution and was suppressed by the GSK3 inhibitor kenpaullone. Wild-type GSK3 was unable to catalyse the tyrosine phosphorylation of catalytically inactive GSK3beta lacking phosphate at Tyr(216). Our results indicate that the tyrosine phosphorylation of GSK3 is an intramolecular autophosphorylation event in the cells that we have studied and that this modification enhances the stability of the enzyme.