Article
Fibrinogen structure in projection at 18 Å resolution: Electron density by co-ordinated cryo-electron microscopy and X-ray crystallography

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Abstract

Electron microscope images of frozen-hydrated crystals of a proteolytically modified fibrinogen show excellent preservation of the structure. An electron density map of the key centric projection of the crystal at 18 Å resolution has been obtained by combining the phases derived from cryo-electron microscopy with X-ray amplitudes. Simulation methods developed in earlier studies have been used to interpret the map. In contrast to the earlier images, the map allows us to visualize the coiled-coil region of the molecule and possible substructure in the β domains. The map also shows that there is a marked difference in density in the two regions corresponding to the molecular ends where the γ domains interact. A possible interpretation of this finding is provided by assuming substructure in the γ domains and the breaking of molecular symmetry where these domains interact. Some additional constraints useful for the determination of the three-dimensional structure were obtained from cryo-electron micrographs of a perpendicular view at 25 Å resolution. Implications of this working model for the molecular length and contacts in the filaments in both the crystal and fibrin are described. The data used here will be valuable as a starting point for obtaining the three-dimensional structure.

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Supported by grant AR17346 from NIH to C.C. Funds to purchase and maintain the computer system were obtained from a shared instrumentation grant 1-S10-RR04671-01 awarded to David J. DeRosier by the National Institutes of Health. Funds to purchase and maintain the Philips EM420T transmission electron microscope were obtained from a shared instrumentation grant 1-S10-RR02464-01 awarded to C.C. by the National Institutes of Health.

Present address: Texas A&M University, Department of Chemistry, College Station, TX 77843-3255, U.S.A.

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Present address: Albert Einstein College of Medicine, 1300 Morris Park Ave, Bronx, NY 10461, U.S.A.

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