The AGDV residues on the gamma chain carboxyl terminus of platelet-bound fibrinogen are needed for platelet aggregation

It has been clear that only the carboxyl terminus of fibrinogen (Fg) gamma chain is required for the initial binding of Fg from solution to its GPIIbIIIa (glycoprotein IIb and IIIa) receptor on activated platelets, whereas the two RGD sites on the A ct chain do not play any role. In this study, we examined the role of these three putative adhesive domains on Fg already bound to its receptors in mediating platelet aggregation. Activated platelets were first incubated with Fg to let the Fg bind, then with monoclonal antibodies (mAb) to block the putative adhesive domains, and the platelet suspension was then sheared or stirred to induce aggregation. The mAb 4A5, which recognizes the last four amino acid residues (AGDV) in a dodecapeptide (H12) on the carboxyl terminus of the Fg gamma chain, markedly inhibited platelet aggregation. Z69/8, a mAb whose epitope is also on the dodecapeptide but does not recognize the AGDV residues, did not have any inhibitory effect on aggregation. The anti-RGDS and anti-RGDF mAbs did not affect both macro-and micro-aggregation at all, whether tested singly or together. These results demonstrate that, similar to the situation for the initial binding of soluble Fg, only the gamma chain carboxyl terminus with the AGDV residues are needed for platelet-bound Fg to support aggregation, while the RGD sites on the A alpha chain do not seem to be required. (C) 1997 Elsevier Science B.V.