Involvement of the β3 E749ATSTFTN756 region in stabilizing integrin αIIbβ3-ligand interaction

Summary. Platelet integrin α_IIbβ₃ must be activated via intracellular mechanisms before it binds soluble ligands, and it is thought to be activated at its extracellular site by surface‐bound ligands. Integrin activation is associated with rearrangement of the cytoskeleton and phosphorylation of proteins that become localized in focal contacts. In these processes, the cytoplasmic tail of the β‐subunit plays a central role. We introduced peptides homologous to the E⁷⁴⁹ATSTFTN⁷⁵⁶ domain (E–N peptide) and the T⁷⁵⁵NITYRGT⁷⁶² domain (T–T peptide) of β₃ in streptolysin O‐permeabilized platelets and analyzed the initial interaction with soluble fibronectin, fibrinogen and PAC‐1 after stimulation with thrombin. E–N peptide left the initial binding of fibronectin intact but interfered with stable receptor occupancy. E–N peptide also inhibited fibrinogen binding, thereby reducing the formation of large aggregates. Strikingly, E–N peptide did not disturb the binding of PAC‐1, which is known to reflect activation of the integrin. E–N peptide also inhibited tyrosine phosphorylation of focal adhesion kinase, a response known to be dependent on α_IIbβ₃. T–T peptide did not affect these processes. In a model for outside‐in integrin activation, E–N peptide disrupted the binding of CHO cells expressing α_IIbβ₃ to surface‐bound ligand. Again, T–T peptide had no effect. We conclude that the E⁷⁴⁹ATSTFTN⁷⁵⁶ region of the β₃‐tail stabilizes the binding of soluble and surface‐bound ligand to integrin α_IIbβ₃ via a mechanism that involves the phosphorylation of FAK.