Symmetrical disulfide bonds are not necessary for assembly and secretion of human fibrinogen

Human fibrinogen is a dimer, and each half-molecule is composed of three different polypeptides (A alpha, B beta, and gamma). The two half-molecules are joined together at the amino-terminal ends by three symmetrical disulfide bonds between adjacent A alpha chains, at position 28, and between adjacent gamma chains at gamma 8 and 9. To determine the role of these disulfide bonds in fibrinogen assembly and secretion, site-directed mutagenesis was used to change cysteines of A alpha 28 and gamma 8 and 9 to serine, the mutant chains were coexpressed with normal fibrinogen chains in COS cells, and assembly and secretion of fibrinogen were measured. Elimination of the symmetrical disulfide bonds did not affect assembly of the chains and dimeric fibrinogen was secreted. Analysis by plasmin digestion indicated that the secreted mutant fibrinogens have a similar structure to normal fibrinogen. Our results indicate that other domains of fibrinogen participate in dimer formation and that the three symmetrical disulfide bonds are not crucial for assembly and secretion of fibrinogen.