A cluster of basic amino acid residues in the γ370-381 sequence of fibrinogen comprises a binding site for platelet integrin α _IIbβ₃ (glycoprotein IIb/IIIa)

Adhesive interactions of platelet integrin α_IIbβ ₃ with fibrinogen and fibrin are central events in hemostasis and thrombosis. However, the mechanisms by which α_IIbβ ₃ binds these ligands remain incompletely understood. We have recently demonstrated that α_IIbβ₃ 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 α_IIbβ₃, suggesting the different specificity of the integrin. Here, using peptide arrays, mutant fibrinogens, and recombinant mutant γC-domains, we have examined the mechanism whereby α_IIbβ₃ binds γ365-383. The α_IIbβ₃-binding activity was localized within γ370-381, with two short sequences, γ³⁷⁰ ATWKTR ³⁷⁵ and γ³⁷⁶WYSMKK³⁸¹, being able to independently bind the integrin. Furthermore, recognition of α_IIbβ₃ by γ370-381 depended on four basic residues, Lys³⁷³, Arg³⁷⁵, Lys³⁸⁰, and Lys ³⁸¹. Simultaneous replacement of these amino acids and deletion of the γ⁴⁰⁸AGDV⁴¹¹ sequence in the recombinant γC-domain resulted in the loss of α_IIbβ₃- 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 α_IIbβ₃ 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 α_IIbβ₃. These results identify the γ370-381 sequence of fibrin(ogen) as the binding site for α_IIbβ₃ involved in platelet adhesion and clot retraction and define the new recognition specificity of this integrin.