Evidence that a GTP binding protein regulates phosphorylation of the CD3 antigen in human T lymphocytes

The role of guanine nucleotide binding regulatory proteins (G proteins) in the regulation of phosphorylation of the γ subunit of the CD3 antigen has been examined. CD3 γ chain phosphorylation in isolated T cell microsomes was stimulated by the G protein activator guanosine 5′-0 thiotriphosphate (GTPγS), but cyclic adenosine monophosphate and guanosine 5′-diphosphate were ineffective at inducing γ chain phosphorylation. The effect of GTPγS was rapid and transient; a half maximal effect was observed with 50μM of the nucleotide. γ polypeptide phosphorylated in vitro in GTPγS stimulated microsomes incorporated phosphate on Serines 123 and 126. These data are consistent with the involvement of a G protein in the signalling mechanisms that regulate the phosphorylation of the CD3 γ chain.