The role of GTP-binding proteins in platelet activation.

Platelet activation begins with the binding of an agonist to the cell surface and culminates in the events of platelet aggregation, secretion and clot formation. Recent studies have identified two large families of GTP-binding proteins in platelets that are thought to participate in the events of platelet activation. The first of these are the G proteins, heterotrimeric proteins which are best known for their ability to mediate the interaction between agonist receptors and intracellular enzymes such as adenylyl cyclase, phospholipase C and phospholipase A2. To date, at least six G proteins have been identified in platelets: Gs, Gz, three variants of Gi and either Gq or G11 (or both). An additional, pertussis toxin-resistant G protein, Gq, may also be present. The second group of GTP-binding proteins present in platelets is substantially smaller than the heterotrimeric G proteins, ranging in size from 21 to 28 kDa. At least 15 such low molecular weight GTP-binding proteins have been identified in platelets, many of which are homologous to the products of the ras proto-oncogenes. In cells other than platelets, low molecular weight GTP-binding proteins have been implicated in protein transport, cell activation events and malignant transformation. Their role in platelets is unknown.