Center for Molecular Genetics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0634, USA.
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.