A new G-protein was detected in human platelets which was ADP-ribosylated in a pertussis-toxin-dependent manner, was located in the supernatant of saponized platelets and was of a slightly lower molecular mass (40 kDa) than platelet membrane Gi alpha. This soluble ADP-ribosylated protein was immunoprecipitated by an antiserum to Gi alpha, but not by one to Go alpha. Prior thrombin stimulation of platelets led to an inhibition of the ADP-ribosylation of this protein. This inhibition was evident even under conditions which abolished the thrombin-stimulated inhibition of membrane Gi alpha ADP-ribosylation. These results indicate that the platelet thrombin receptor is coupled to two structurally and functionally distinct Gi alpha proteins: a major Gi alpha protein present in platelet membranes, and a minor Gi alpha protein detectable in the platelet soluble fraction.
