Collagen-induced platelet aggregation and release reaction: role of ras protein

The relationship between the platelet collagen receptor and ras protein was examined by immunoprecipitation of control and collagen stimulated platelets with both antibodies. Both ras protein and receptor are coprecipitated by both antibodies. The coprecipitated samples contained GTP and GDP which were separated by thin layer chromatography. The effect of guanine nucleotide binding protein (G-protein, 21ras) on platelet aggregation was examined by using a guanine triphosphate analogue (GTP-gamma-S). Results of streptolysin O permeabilized experiments show that the addition of the analogue to permeabilized platelet-rich plasma causes platelet aggregation and release of ATP in a dose-dependent fashion. The aggregation induced by GTP-gamma-S could not be obtained with GDP, GDP-beta-S, or 5'-GMP, suggesting that the effect of GTP-gamma-S is specific. The platelet aggregation induced by the analogue was inhibited in a dose-dependent manner by phenylmethanesulfonyl fluoride (PMSF) but not apyrase. GTP-gamma-S induced thromboxane B2 formation was also decreased by PMSF. Formation of thromboxane B2 was also blocked by the addition of PMSF and ethanol suggesting that GTP-gamma-S increases arachidonic acid release from permeabilized platelets. These results suggest that both GTP-gamma-S and GTP enhance phospholipase A2 activity which releases arachidonic acid in permeabilized platelets and subsequently causes platelets to aggregate.