AbstractInteraction between von Willebrand factor (VWF) and platelet GP Ib-IX-V is required for hemostasis, in part because intracellular signals from VWF/GP Ib-IX-V activate the ligand-binding function of integrin αIIbβ3. Because they also induce tyrosine phosphorylation of the ADAP adapter, we investigated ADAP's role in GP Ib-IX-V signal transduction. Fibrinogen or ligand-mimetic POW-2 Fab binding to αIIbβ3 was stimulated by adhesion of ADAP+/+ murine platelets to dimeric VWF A1A2 but was significantly reduced in ADAP−/− platelets (P < .01). αIIbβ3 activation by ADP or a Par4 thrombin receptor agonist was also decreased in ADAP−/− platelets. ADAP stabilized the expression of another adapter, SKAP-HOM, via interaction with the latter's SH3 domain. However, no abnormalities in αIIbβ3 activation were observed in SKAP-HOM−/− platelets, which express normal ADAP levels, further implicating ADAP as a modulator of αIIbβ3 function. Under shear flow conditions over a combined surface of VWF A1A2 and fibronectin to test interactions involving GP Ib-IX-V and αIIbβ3, respectively, ADAP−/− platelets displayed reduced αIIbβ3-dependent stable adhesion. Furthermore, ADAP−/− mice demonstrated increased rebleeding from tail wounds. These studies establish ADAP as a component of inside-out signaling pathways that couple GP Ib-IX-V and other platelet agonist receptors to αIIbβ3 activation.