THE BETA-CHAIN OF CHICKEN FIBRINOGEN CONTAINS AN ATYPICAL THROMBIN CLEAVAGE SITE

A cDNA corresponding to almost the entire coding region of the mRNA for the beta-chain of chicken fibrinogen was sequenced. At the protein level, significant homology to the beta-subunits of other vertebrate fibrinogens was found, with the highest degree of amino acid identity localized in the C-terminal region. In general, features conserved in the fibrinogens from other species also characterize the chicken sequence, including the cysteine motifs bordering an alpha-helical permissive region of fixed length and a single glycosylation site in the C-terminal region. However, the site of thrombin-catalyzed cleavage, which in other species consists of an Arg-Gly peptide bond, is instead an Arg-Ala bond in the chicken beta-chain. The Ala was confirmed directly from a sequencing analysis of the purified beta-chain of chicken fibrin. This finding may explain the observed slow clotting time of chicken fibrinogen relative to that of other species.