Role of interchain disulfide bonds on the assembly and secretion of human fibrinogen.

The cysteines involved in joining the 2 half-molecules of fibrinogen and also those located on either side of the alpha-helical coiled-coil region, were substituted, by site-directed mutagenesis, with serine. Fibrinogen assembly and secretion were determined in transiently transfected COS cells. Our studies indicate that in order to assemble the 2 half-molecules into a dimer, it is not sufficient to only have the disulfide linkages which keep the 2 half-molecules intact. The disulfide rings which flank the coil-coiled region also play important roles in dimer assembly. Intact interchain disulfide linkages at the NH2-terminal end of the coiled-coil region are essential for assembly of the 2 half-molecules. Disruption of these disulfide rings leads to the formation and secretion of half-molecules. Disruption of the interchain disulfide rings at the COOH-terminal end of the coiled-coil region allows dimer formation, but the 6-chain molecule which is assembled is not secreted. Disruption of both disulfide rings at either end of the coiled-coil region disallows assembly of half-molecules and of dimeric fibrinogen.