THE ROLE OF AMINO-TERMINAL DISULFIDE BONDS IN THE STRUCTURE AND ASSEMBLY OF HUMAN FIBRINOGEN

Human fibrinogen contains two half-molecules, each composed of an α, β, and γ chain linked by disulfide bonds. The two half-molecules (αβγ) are held together in the native protein by additional disulfide bonds located in the amino terminus of each chain. Site-directed mutagenesis, in which the amino-terminal Cys residues (α-Cys28 and 36, β-Cys65, and γ-Cys8 and 9) were converted to Ser, was carried out in order to study the role of the amino-terminal disulfide bonds in the structure and assembly of fibrinogen. An analysis of the fibrinogen synthesized in transfected baby hamster kidney (BHK) cells employing various combinations of these mutations revealed that α-Cys36 and β-Cys65 form disulfide bonds between two αβγ half-molecules, rather than within the same half-molecule; furthermore, these two disulfide bonds are sufficient to hold the two αβγ half-molecules together as intact fibrinogen. Disulfide bonds formed by γ-Cys8 and 9 were also sufficient to hold the two fibrinogen αβγ half-molecules together, while the disulfide bond between the two α-Cys28 residues failed to form in the absence of the disulfide bonds linking the α and β chains and the two γ chains. © 1993 Academic Press, Inc.