Multisite phosphorylation of the α subunit of transducin by the insulin receptor kinase and protein kinase C

The GDP-bound α subunit of transducin, but not the guanosine 5'-[γ-thio]triphosphate-bound one, undergoes phosphorylation on tyrosine residues by the insulin receptor kinase and on serine residues by protein kinase C. Holotransducin is poorly phosphorylated by the insulin receptor kinase and is not phosphorylated by protein kinase C. Neither holotransducin nor any of its subunits were phosphorylated by the cAMP-dependent protein kinase. That a given subunit of transducin undergoes multisite phosphorylation depending on the type of nucleotide bound to it or the nature of the kinase suggests that hormone-dependent phosphorylation could provide a versatile mode for regulation of guanine nucleotide-binding protein (G protein) function. In particular, the findings that certain G proteins serve as substrates for both the insulin receptor kinase and protein kinase C implicate G proteins in playing a key role in mediating the action of insulin and ligands that act to activate protein kinase C.