Processing of the carboxyl 15-amino acid extension in the α-chain of fibrinogen

The cDNA for the α-chain of human fibrinogen codes for 15 amino acids at the carboxyl terminus that are not found in the native protein circulating in plasma. In the present studies, experiments were designed to test whether the carboxyl extension was required for fibrinogen assembly or was simply the result of limited proteolysis and maturation of the protein during or following secretion. Baby hamster kidney cells were transfected with cDNAs coding for the α-, β-, and γ-chains, and the recombinant fibrinogen secreted into the cell culture medium was identified either with an antibody against the mature molecule or with an antibody directed toward the carboxyl extension of the α-chain. The secreted protein contained primarily two species of α-chains, including one with the carboxyl extension and one that was the same as that in plasma fibrinogen. A mutant fibrinogen, in which Arg-611 at the putative cleavage site in the α-chain was converted to Gly, was also readily assembled, and, in this case, the protein was secreted with the α-chain carboxyl extension. Similarly, a mutant fibrinogen that completely lacked the carboxyl extension of the α-chain was assembled and secreted from the transfected baby hamster kidney cells. These two mutants with modified α-chains were readily clotted by thrombin and crosslinked by factor XIIIa. These results indicate that the α-chain carboxyl extension in fibrinogen is not required for assembly or secretion of the molecule. Accordingly, it was concluded that the cleavage of the α-chain removing the carboxyl-terminal 15 amino acids is a normal and specific processing event occurring during the maturation of the protein.