Analysis of fibrinogen variants at γ387Ile shows that the side chain of γ387 and the tertiary structure of the γC-terminal tail are important not only for assembly and secretion of fibrinogen but also for lateral aggregation of protofibrils and XIIIa-catalyzed γ-γ dimer formation

To examine the role of fibrinogen γ-chain residue 387Ile in the assembly and secretion of this multichain protein, we synthesized a series of variants with substitution at γ387 by Arg, Leu, Met, Ala, or Asp. Only the variant γ387Asp showed impaired synthesis in the cells and very low secretion into the medium. In addition, we performed thrombin-catalyzed fibrin polymerization and factor (F) XIIIa-catalyzed cross-linking of the γ-chain for 4 variants. The degree of lateral aggregation of protofibrils into fibrin fibers was slightly reduced for γ387Arg and Ala, and moderately reduced for γ387Leu and Met. Although the FXIIIa-catalyzed cross-linking for all of the variants was slower than that for γ387Ile, that of γ387Arg was much more markedly impaired than that of the others. In summary, our studies demonstrated that the specific residue at γ387 or the conformation of γ388-411 residues, but not the length of the γC tail, is critical for fibrinogen assembly and subsequent secretion. Moreover, this residue or the conformation is also important for not only the lateral aggregation of fibrin polymers but also the FXIIIa-catalyzed cross-linking of the γ-chain. Interestingly, our results clearly indicate that the conformations critical for these 2 functions are different from each other.