Interactions in platelets between G proteins and the agonists that stimulate phospholipase C and inhibit adenylyl cyclase.

摘要: Platelet responses to agonists are believed be mediated by at least two pertussis toxin-sensitive guanine nucleotide-binding (G) proteins: Gi which inhibits adenylyl cyclase and Gp, stimulates phospholipase C. The present studies compare the properties of Gp examine their interactions with receptors for various platelet agonists. In permeabilized platelets membranes, toxin [32P]ADP-ribosylated a protein(s) (alpha 41) migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis fractionally below rabbit bovine alpha i (Mr = 41,000). Prior exposure an agonist inhibited [32P]ADP-ribosylation 41 extent correlated pattern that agonist. Thrombin, elicited were both decreased radiolabeling greater than 90%. Epinephrine, was functionally coupled only Gi, 50%, as did vasopressin platelet-activating factor (PAF), Gp. U46619, thromboxane analog neither cAMP formation nor caused phosphoinositide hydrolysis, had no effect 32P-ADP-ribosylation. These results suggest either G regulates more one enzyme or subunits from protein comigrate within 41. Two-dimensional used test latter possibility. Upon isoelectric focusing, resolved into distinct subspecies. However, these appear minor variants rather since: 1) proteins produced same proteolytic fragments after digestion chymotrypsin Staphylococcus aureus V8 protease 2) preincubation agonists, including those interact in intact solely (PAF vasopressin) (epinephrine), extent. functional some found depend upon conditions assay. Although unable inhibit platelets, PAF inhibition isolated membranes. Collectively, observations single toxin-sensitive, 41-containing may involved regulation C additional constraints altered during membrane isolation help determine is