To examine the role of fibrinogen gamma-chain residue 387Ile in the assembly and secretion of this multichain protein, we synthesized a series of variants with substitution at gamma387 by Arg, Leu, Met, Ala, or Asp. Only the variant gamma387Asp 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 gamma-chain for 4 variants. The degree of lateral aggregation of protofibrils into fibrin fibers was slightly reduced for gamma387Arg and Ala, and moderately reduced for gamma387Leu and Met. Although the FXIIIa-catalyzed cross-linking for all of the variants was slower than that for gamma387Ile, that of gamma387Arg was much more markedly impaired than that of the others. In summary, our studies demonstrated that the specific residue at gamma387 or the conformation of gamma388-411 residues, but not the length of the gammaC 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 gamma-chain. Interestingly, our results clearly indicate that the conformations critical for these 2 functions are different from each other.