SEVERELY IMPAIRED POLYMERIZATION OF RECOMBINANT FIBRINOGEN GAMMA-364 ASP-]HIS, THE SUBSTITUTION DISCOVERED IN A HETEROZYGOUS INDIVIDUAL

During blood coagulation, soluble fibrinogen is converted to fibrin mo nomers that polymerize to form an insoluble clot, Polymerization has b een described as a two-step process: the formation of double-stranded protofibrils and the subsequent lateral aggregation of protofibrils in to fibers, Previous studies have shown that gamma chain residues Tyr-3 63 and Asp-364 have a significant role in polymerization, most likely in protofibril formation, To better define the role of these residues, me synthesized three fibrinogens with single substitutions at these t wo positions: Tyr-363 --> Ala, Asp-364 --> Ala, and Asp-364 --> His, W e found that the release of fibrinopeptides A and B was the same for t hese variants and normal recombinant fibrinogen, showing that all vari ants had normal fibrin formation, In contrast, we found that polymeriz ation was significantly delayed for both Ala variants and was almost n onexistent for the His variant, Clottability for the Ala variants was only slightly reduced, and fibrin gels were formed. Surprisingly, clot tability of the His variant was substantially reduced, and fibrin gels were not formed, Our data suggest that both protofibril formation and lateral aggregation were altered by these substitutions, indicating t hat the C-terminal domain of the gamma chain has a role in both polyme rization steps.