G_β interacts with endogenous GRK2 in platelets ex vivo but G_qα does not. (Ai) GST-GRK2 was immunoprecipitated from human or mouse platelets, and samples were probed for G_qα or G_iα2. Glutathione S-transferase (GST) reprobe indicated the equal loading. n = 2. (Aii) Human platelet lysate from resting or 1 U/mL thrombin-stimulated platelets in the presence or absence of GDP+AlF4^– were subjected to immunoprecipitation of GRK2 and probed for G_q. n = 3. (B) GRK2 from resting and thrombin-stimulated (1 U/mL) human platelet lysate was immunoprecipitated with the specific mouse anti-GRK2 antibody and probed for G_β. GRK2 was reprobed with an anti-GRK2 antibody. n = 6. A marker line was excised as indicated by the vertical line.