An analysis of the role of guanine nucleotide binding proteins in antigen receptor/CD3 antigen coupling to phospholipase C.

In permeabilized human T lymphocytes, phospholipase C (PLC)-mediated metabolism of polyphosphatidylinositols can be stimulated by triggering the T cell antigen receptor/CD3 antigen complex (Ti/CD3) with the CD3 antibody UCHT1 or by activation of G proteins with the non-hydrolyzable guanine nucleotide analogue, guanosine 5'-O-(3-thiotrisphosphate) (GTP[S]). Ti/CD3 induction of inositol phosphate production demonstrated no dependence on exogenous guanine nucleotides. Furthermore, Ti/CD3 stimulation did not influence the kinetics or dose-response of GTP[S]-induced inositol phosphate production, suggesting that the Ti/CD3 complex does not regulate guanine nucleotide exchange on the G protein pool stimulated by GTP[S]. These data indicate that the Ti/CD3 complex is not G protein-linked to PLC in a manner analogous to the G protein linkage of receptors to adenylate cyclase. However, the inhibitory guanine nucleotide, GDP, antagonizes not only GTP[S]-induced polyphosphatidylinositol hydrolysis but also UCHT1-induced inositol phosphate production. These data infer that a G protein can modulate the coupling of the Ti/CD3 complex to PLC and that there may be some "cross-talk" between Ti/CD3 and G protein PLC coupling mechanisms.