Role of ADAP in fibrinogen binding to platelets. (A-B) Effect of monovalent POW-2 Fab on binding of multivalent fibrinogen. ADAP^+/+ and ADAP^−/− platelets were stimulated with AYPGKF in the presence or absence of POW-2 Fab and binding of FITC-fibrinogen was quantified by flow cytometry. Results are presented as specific FITC-fibrinogen binding, as defined in Materials and methods. Note the complete inhibition of FITC-fibrinogen binding by POW-2 Fab at relatively low (≤100 μM) (A) but not high (≥160 µM) (B) concentrations of AYPGKF. Also note the reduced fibrinogen binding to ADAP^−/− platelets compared with ADAP^+/+ platelets, with or without POW-2 Fab. Results represent the mean ± SEM of at least 3 experiments. Statistical analysis results are presented in supplemental Table 1. (C-D) Irreversible fibrinogen binding. ADAP^+/+ and ADAP^−/− platelets were incubated with ADP (C) or AYPGKF (D) together with FITC-fibrinogen for 5 or 45 minutes before the addition of EDTA or buffer as a control. Platelets were then incubated for an additional 10 minutes, and irreversible FITC-fibrinogen binding was quantified. Results represent the mean ± SEM of 6 experiments (*P < .05, **P < .01).