G proteins and low-molecular-weight GTP-binding proteins in platelets

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.

Association of a 24-kDa GTP-binding protein, Gn24, with human platelet alpha-granule membranes

Human platelets were disrupted using nitrogen cavitation and fractionated on sucrose density gradients to permit isolation of alpha-granules, the major secretory granule of platelets. Membrane proteins prepared from intact alpha-granules by alkali extraction were separated by SDS-polyacrylamide gel electrophoresis, transferred to nitrocellulose and the blot probed for the presence of GTP-binding proteins using [alpha-32P]GTP. Two low molecular mass GTP-binding proteins with molecular mass of 27 and 24 kDa, respectively, were identified on the alpha-granule membrane. In contrast to the 27-kDa protein which was present in significant amounts in the plasma membrane-enriched fraction, the 24-kDa protein showed a preferential association with the alpha-granule membrane. On immunoblotting with specific antiserum, the 24-kDa GTP-binding protein was found to be distinct from rab3A. To the best of our knowledge, the present report represents the first identification of low molecular mass GTP-binding proteins associated with a platelet secretory granule.