The Binding Site for Fibrin‐Derived Peptide γ 370–383 (P3) in the α IIb β‐Propeller of Integrin α IIb β 3

The interaction between a fibrin‐specific sequence γ370–383 (P3) in the γC‐domain of fibrinogen and integrin α IIb β 3 supports platelet adhesion and platelet‐mediated fibrin clot retraction. The mechanism whereby α IIb β 3 binds P3 is different from that involved in binding of the AGDV, a binding site in soluble fibrinogen responsible for α IIb β 3 ‐mediated platelet aggregation. We have found that P3 can bind to multiple binding sites within the α IIb β 3. Using peptide libraries spanning the entire sequence (residues 1–451) of the α IIb β‐propeller, 8 amino acid residues in several noncontiguous segments were identified as candidate contact sites for P3 binding. Synthetic peptides duplicating two of these segments, α IIb 94–108 and α IIb 229–237, inhibited platelet‐mediated fibrin clot retraction and α IIb β 3 ‐mediated cell adhesion, supporting the role of these regions in P3 recognition. Substitution of residues identified as critical for P3 binding in the α IIb β‐propeller of the receptors expressed on the surface of HEK293 cells resulted in a 40–60% loss of cell adhesion to an immobilized fibrinogen fragment, a mimic of fibrin. These residues are clustered within the W2:4‐1/1–2 and W4:4‐1/1–2 loops of the “W” repeats at the edge of α IIb β‐propeller distant from the interface with the I‐like domain of the β 3 subunit where the binding site for RGD was previously localized. Supported by SDG AHA 0835257N.