Characterization of the γ chain platelet binding site on fibrinogen fragment D

Abstract

Glycoprotein (GP) IIb/IIIa on adenosine diphosphate (ADP)-activated human platelets interacts with specific sites on the fibrinogen molecule leading to aggregation. We characterized the platelet-binding site on the γ chains of fibrinogen using plasmic fragments DγA and Dγ′. Fragment DγA, which contains the carboxy terminal γA400-411 platelet-binding sequence (HHLGGAKQAGDV), was 70-fold more active than the synthetic γA400-411 peptide in inhibiting ADP-induced platelet aggregation. Fragment DγA inhibited fibrinogen binding and also bound directly to ADP-activated platelets. The Kd values determined for fibrinogen and fragment DγA binding were 0.55 μmol/L and 1.2 μmol/L, respectively. In contrast, fragment Dγ′, which differs from fragment DγA with respect to its γ chain sequence from position 408 to the COOH-terminus at position 427, did not inhibit platelet aggregation or fibrinogen binding, and did not bind directly to the platelet surface. Denaturation of fragment DγA with guanidine-HCl caused a loss of inhibitory activity in platelet aggregation assays. These data indicate that the native conformation of the γ chain platelet-binding site on fibrinogen is important for optimal binding to GPIIb/IIIa. © 1992 by The American Society of Hematology.