The hydrolysis of polyphosphoinositides (PI) by phospholipase C during platelet activation produces two key intracellular messengers, inositol triphosphate and diacylglycerol. This process is thought to be regulated by a guanine nucleotide binding protein referred to as Gp. Although the evidence that Gp exists is compelling, to date it has not been isolated. Uncertainty about its identity has been compounded by variations between tissues in the susceptibility of Gp to pertussis toxin and by reconstitution studies which show that pertussis toxin-inhibited PI hydrolysis can be restored by purified Gi, the pertussis toxin-sensitive G protein which inhibits adenylate cyclase. Therefore, it remains unclear whether Gp represents a new G protein or a second role for Gj. When platelets permeabilized with saponin were incubated with pertussis toxin and 32P-NAD, a single 42 kDa protein was 32P-ADP-ribosylated which co-migrated with the purified a subunit of Gi. Preincubating the platelets with an agonist inhibited labeling of this protein by dissociating the G protein into subunits. The extent of inhibition correlated with the number of toxin-sensitive functions caused by the agonist. Labeling was abolished by thrombin, which inhibited cAMP formation and caused toxin-inhibitable PI hydrolysis. Labeling was partially inhibited by vasopressin and platelet activating factor, which caused toxin-inhibitable PI hydrolysis, but had no effect on cAMP formation and by epinephrine, which inhibited cAMP formation, but did not cause PI hydrolysis. Labeling was unaffected by the TxA2 analog U46619, which neither caused toxin-sensitive PI hydrolysis nor inhibited cAMP formation. These observations suggest that the 42 kDa band may contain a subunits from both Gp and Gi and, in fact, 2D electrophoresis resolved the 42 kDa protein band into two proteins with distinct pi. However, those agonists linked functionally only to Gp or only to Gi decreased the labeling of both proteins. Therefore, our data suggest (1) that Gj and Gp are the same protein and (2) that whether a aiven platelet agonist affects adenylate cyclase or phospholipase C or both depends upon factors extrinsic to the G protein.
Novel bimodal effects of the G-protein tissue transglutaminase on adrenoreceptor signalling