Alteration of receptor/G-protein interaction by putative endogenous protein kinase activity in Dictyostelium discoideum membranes.

Membranes of Dictyostelium discoideum cells were incubated under phosphorylation conditions and washed, and the effects on cAMP binding to chemotactic receptors in the absence and presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) were investigated. Most experiments were done with adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), which is a good substrate for many kinases, but the product, protein phosphorothioate, is not easily hydrolyzed by phosphatases. Pretreatment of membranes under phosphorylating conditions with MgATP gamma S alters the site heterogeneity of the cAMP-binding forms, without a significant effect on the total number of binding sites. A similar effect was induced by GTP gamma S under nonphosphorylation conditions. The effects of MgATP gamma S were rapid (t1/2 = 1 min), irreversible, and not induced by Mg2+ or ATP gamma S alone or by magnesium adenylyl imidodiphosphate and magnesium adenylyl (beta, gamma-methylene)diphosphate. MgATP induced a smaller inhibition than MgATP gamma S, which was potentiated by the addition of exogenous cAMP-dependent protein kinase. The effect of MgATP was rapidly reversible; reversibility was reduced by the phosphatase inhibitor NaF. These results suggest that the effects of MgATP gamma S are mediated by an endogenous protein kinase. The major 35S-thiophosphorylated band detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was a protein with Mr = 36,000. The phosphorylation of a protein with the molecular weight of the cAMP receptor (Mr = 40,000-45,000) was not observed.