Surface labeling of αIIbβ3 and P-selectin. (A) Fibrinogen binding to platelets activated with AYPGKF: platelets from WT and SG^−/− mice were treated with the PAR4 agonist peptide AYPGKF at the concentrations indicated, followed by incubation with FITC-labeled fibrinogen as described in “Flow cytometry.” Binding was quantitated by flow cytometry. Each assay represents 50 000 platelets. Results shown are representative of 2 independent experiments with very similar results. (B) Exposure of P-selectin. Platelets were treated with 1 U/mL thrombin and stained with anti–P-selectin antibody. Each point represents platelets from a single mouse.