Immunoprecipitation of GPV from biotinylated H and R platelet proteins. H (lane 1) and R (lanes 2 to 9) platelets were surface labeled with biotin and treated with saline (lanes 1 to 5) or with 5 U/mL thrombin (lanes 6 to 9). Platelets were washed, lysed with Triton X-100, and the proteins immunoprecipitated with MoAb V.1 against human GPV (lane 1), MoAbs RPM.4 (lanes 2 and 6), RPM.9 (lanes 3 and 7), and RPM.11 (lanes 4 and 8) against R GPV, and MoAb RPM.10 against R GPIIb-IIIa (lanes 5 and 9). MoAb V.1 immunoprecipitated a 82-kD band from human resting platelets (lane 1). MoAbs RPM.4, RPM.9, and RPM.11 immunoprecipitated a broad band centered at 88 kD from R unstimulated platelets (lanes 2 to 4). This band disappeared from the surface of thrombin-stimulated platelets (lanes 6 to 8). These results confirm that R GPV is specifically cleaved by thrombin and that the epitopes for the R GPV MoAbs are located NH2-terminal from the cleavage site. Under these conditions, RPM.10 gave a weak signal at 100 kD (lane 5) corresponding to GPIIIa and no labeling of GPIIb.