PROGRESS IN DETERMINING THE STRUCTURE OF FRAGMENT D FROM HUMAN FIBRINOGEN. G. Spraggon, S. J. Everse, R. F. Doolittle, Center for Molecular Genetics, Univ. Calif. San Diego, La Jolla, CA, USA 92093-0634

Fragment D is a complex, large molecular weight (86,000 Da) fragment derived from vertebrate fibrinogen by limited proteolysis with plasmin or trypsin. Its three constituent polypeptide chains have long been supposed to form a coiled-coil at their aminoterminal ends and to be cemented by a ring-shaped triple set of disulfide bonds. The carboxyl-terminal portions of two of the chains make up two homologous globular domains. Recently we reported the crystallization of fragment D from human fibrinogen and some preliminary characterization (Everse et al. Prot. Sci., 4:1013-16,1995). The space group is P2₁, unit cell dimensions a = 107.7, b = 48.0, c = 167.6, beta = 105.7 In the interim, we have identified several isomorphous derivatives and have been able to calculate preliminary low resolution phases. The molecular envelope determined by solvent flattening of an initial fourier map has revealed an silhouette in which the boundaries of the distal domains and coiled coil are reasonably delineated. It also reveals, in contrast to our initial finding, that there is only one molecule per asymmetric unit. The solvent flattened map itself contains several features which can be attributed to secondary structure, including portions of the coiled-coil. Efforts to improve the phasing are under way.

Supported by N.I.H. Grant HL-26873. SJE was supported by a Fellowship from the American Heart Association, California Affiliate.