The objective of this research project is to determine the molecular structures and interactions of several large blood clotting proteins. A major goal is elucidation of the molecular packing in the fibrin clot. Important interactions in the clot may be revealed by image processing of electron micrographs of various crystal forms of fibrinogen, some of which appear to be closely related to fibrin. New, more highly ordered arrays of fibrin itself wil be studied by electron microscopy and X-ray fiber diffraction. The bonding in the clot will also be analyzed by crystallization of specific fragments of cross-linked fibrin obtained by plasmin cleavage. Detailed structural information about the fibrinogen molecule will be derived by coordination of electron microscopy/image processing of ordered arrays with X-ray crystallographic analysis. Likewise, heavy atom markers that bind to specific parts of the fibrinogen molecule may be used as derivatives for the X-ray studies as well as for localization of different functional regions by direct visualization of the complexes by electron microscopy of crystals. A similar approach will be used for research on the interaction of fibrinogen with other proteins, including plasmin and Streptococcal M protein. Structural analysis of the specific binding of these and other proteins will be carried out by electron microscopy/image processing of ordered aggregates of fibrinogen to which one of these proteins has been added. Pilot crystallizations, the first step in any structural study, will be initiated for a few other large blood clotting proteins that appear to be similar in size and some other biochemical properties to fibrinogen. In particular, the large co-factor protein, Factor V, will be examined in the electron microscope and attempts will be made to crystallize it. The long term goal of this project is to relate the structures of these proteins to their roles at the molecular level in the mechanism of coagulation. All of the clotting components play vital roles in hemostasis and thrombosis. The complex clotting system is unbalanced in many pathological conditions, including coronary heart disease and cancer. In order to develop more effective and specific methods to control these processes, it is necessary to understand the molecular structures and interactions of the proteins involved.