A Cluster of Basic Amino Acid Residues in the γ370−381 Sequence of Fibrinogen Comprises a Binding Site for Platelet Integrin αIIbβ3 (Glycoprotein IIb/IIIa)

Adhesive interactions of platelet integrin αIIbβ3 with fibrinogen and fibrin are central events in hemostasis and thrombosis. However, the mechanisms by which αIIbβ3 binds these ligands remain incompletely understood. We have recently demonstrated that αIIbβ3 binds the γ365−383 sequence in the γC-domain of fibrin(ogen). This sequence contains neither the AGDV nor the RGD recognition motifs, known to bind αIIbβ3, suggesting the different specificity of the integrin. Here, using peptide arrays, mutant fibrinogens, and recombinant mutant γC-domains, we have examined the mechanism whereby αIIbβ3 binds γ365−383. The αIIbβ3-binding activity was localized within γ370−381, with two short sequences, γ370ATWKTR375 and γ376WYSMKK381, being able to independently bind the integrin. Furthermore, recognition of αIIbβ3 by γ370−381 depended on four basic residues, Lys373, Arg375, Lys380, and Lys381. Simultaneous replacement of these amino acids and deletion of the γ408AGDV411 sequence in the recombinant γC-domain resulted in the loss of αIIbβ3-mediated platelet adhesion. Confirming the critical roles of the identified residues, abnormal fibrinogen Kaiserslautern, in which γLys380 is replaced by Asn, demonstrated delayed clot retraction and impaired αIIbβ3 binding. Also, a mutant recombinant fibrinogen modeled after the naturally occurring variant Osaka V (γArg375 → Gly) showed delayed clot retraction and reduced binding to purified αIIbβ3. These results identify the γ370−381 sequence of fibrin(ogen) as the binding site for αIIbβ3 involved in platelet adhesion and clot retraction and define the new recognition specificity of this integrin.