ROLE OF FIBRINOGEN ALPHA-CHAIN AND GAMMA-CHAIN SITES IN PLATELET-AGGREGATION

Fibrinogen (Fbg) mediates platelet aggregation by its interaction with the platelet glycoprotein IIb-IIIa (integrin alpha(IIb)beta3). Peptides containing the amino acid sequence RGD derived from the alpha chain (residues alpha95-97 and residues alpha572-574) and the sequence HHLGGAKQAGDV derived from the carboxyl terminus of the gamma chain of Fbg (residues gamma400-411) inhibit these interactions. To determine the role of these sequences in intact Fbg, recombinant human Fbg (rFbg), mutant rFbgs with an RGD --> RGE substitution at either position alpha97 or alpha574, and a rFbg gamma'-containing variant that has a carboxyl-terminal interruption in the HHLGGAKQAGDV sequence have been expressed in transfected BHK cells. Purified rFbg and the two RGE mutant Fbgs were similar to plasma Fbg in platelet aggregation assays. In contrast, the gamma' variant Fbg was markedly defective in platelet aggregation. These data support the proposals that the carboxyl-terminal region of the gamma chain of Fbg is essential for optimal platelet aggregation and that the alpha-chain RGD sequences are neither necessary nor sufficient for platelet aggregation.