Direct measurement of a second fibrinogen alpha chain in lamprey blood plasma

Crystal structure of a recombinant alphaEC domain from human fibrinogen-420

The crystal structure of a recombinant alphaEC domain from human fibrinogen-420 has been determined at a resolution of 2.1 A. The protein, which corresponds to the carboxyl domain of the alphaE chain, was expressed in and purified from Pichia pastoris cells. Felicitously, during crystallization an amino-terminal segment was removed, apparently by a contaminating protease, allowing the 201-residue remaining parent body to crystallize. An x-ray structure was determined by molecular replacement. The electron density was clearly defined, partly as a result of averaging made possible by there being eight molecules in the asymmetric unit related by noncrystallographic symmetry (P1 space group). Virtually all of an asparagine-linked sugar cluster is present. Comparison with structures of the beta- and gamma-chain carboxyl domains of human fibrinogen revealed that the binding cleft is essentially neutral and should not bind Gly-Pro-Arg or Gly-His-Arg peptides of the sort bound by those other domains. Nonetheless, the cleft is clearly evident, and the possibility of binding a carbohydrate ligand like sialic acid has been considered.

The minor form alpha' chain from lamprey fibrinogen is rapidly crosslinked during clotting

Lampreys have two genes for the alpha chains of fibrinogen, the second of which encodes a minor form with a carboxyl-terminal domain homologous to the carboxyl-terminal domains of beta and gamma chains. Initially, we referred to the alternative chain as alpha-II; we now use the designation alpha' in order to facilitate reference to crosslinked dimers. Antisera raised to synthetic peptides based on the cDNA sequence confirmed that the alpha' chain was present in fibrinogen prepared directly from plasma. The same antibodies were used to determine the size and properties of the carboxyl-terminal domain after its release by mild tryptic digestion, a fragment of apparent molecular weight 35,000-40,000 being produced. Unlike fragment D generated in the same digestions, the alpha' fragment did not bind to Gly-Pro-Arg or Gly-Val-Arg peptide affinity columns. During clotting under conditions where factor XIII is active, the alpha' chains became crosslinked very much more rapidly than ordinary alpha chains, the principal product being an apparent dimer, but smaller amounts of higher multimers being detectable. The crosslinking was inhibited by various amines, as well as by peptides that prevent polymerization.