The fibrinogen RIBS-I epitope (γ373–385) appears proximate to the γ408–411 adhesive domain but is not involved in interaction between receptor-bound or surface-adsorbed fibrinogen and platelet GPIIbIIIa

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The carboxyl terminus of the fibrinogen (Fg) γ chain (γ400–411) is necessary and sufficient to support platelet aggregation and adhesion. However, a monoclonal antibody (mAb) to the Fg RIBS-I epitope (γ373–385), the anti-Fg-RIBS-I, which binds only to platelet-bound or surface-adsorbed Fg but not soluble Fg, inhibits platelet aggregation. In this study, we showed that this same antibody also inhibits the adhesion of platelets to Fg-coated polystyrene beads. We then investigated the mechanisms by which the anti-Fg-RIBS-I antibody inhibits platelet aggregation and adhesion. The Fg RIBS-I epitope does not interact with platelet GPIIbIIIa, since recombinant Fg missing the last four amino acids, the Ala-Gly-Asp-Val, on the carboxyl terminus of its γ chains supports neither platelet aggregation nor adhesion to surfaces, nor GPIIbIIIa binding, while it binds anti-Fg-RIBS-I normally. Purified, soluble GPIIbIIIa (265 kDa) inhibits the binding of both the anti-Fg-RIBS-I and 4A5 (a mAb specific to γ408–411 of Fg); however, peptide G13 (1.5 kDa), corresponding to the Fg γ chain binding domain on GPIIba (GPIIb300–312), only inhibits the binding of 4A5, and does not affect the binding of the anti-Fg-RIBS-I to Fg. The anti-Fg-RIBS-I reduces the on-rate of the 4A5 binding to Fg with no measurable changes in the dissociation of the Fg-bound 4A5. These data indicate that the inhibition of platelet aggregation and adhesion by the anti-Fg-RIBS-I antibody is due to the steric hindrance of the Fg γ400–411 to platelet GPIIbIIIa. Thus the Fg RIBS-I epitope (γ373–385) does not appear to be involved in direct interaction with platelet GPIIbIIIa, leaving the γ408–411 of Fg as the sole domain mediating platelet aggregation and adhesion.