Almost every step in blood coagulation requires assembly of multiple proteins on membrane surfaces, yet protein-membrane interactions in blood clotting remain poorly understood at the molecular level. We are using new, high-resolution technologies including magic-angle spinning solid-state NMR (SSNMR) to probe the mechanisms by which blood clotting proteins interact with phospholipid surfaces, and how changes in membrane composition regulate clotting reactions. The primary focus of our studies is the membrane-bound complex of tissue factor and factor VIIa, the two-subunit enzyme responsible for triggering blood clotting in health and disease. Aim 1 will delineate, at atomic resolution, the changes in structure and dynamics of PS- rich membrane domains in the presence of Ca2+, and will examine the roles of phosphatidylserine, phosphatidylcholine and phosphatidylethanolamine in configuring the membrane to support high affinity binding of clotting factors. Aim 2 will investigate the role of phospholipid-phospholipid interactions in enhancing blood clotting reactions on membrane surfaces. Aim 3 will delineate the lipid environments/conformations induced when clotting proteins bind to bilayers. Aim 4 will solve the structure of tissue factor on the membrane surface and identify conformational changes in tissue factor when it interacts with ligands. Together these studies will provide valuable new insights into the role of the membrane surface in blood clotting, at atomic-scale resolution. PUBLIC HEALTH RELEVANCE: Disorders of the blood clotting system represent the leading cause of disability and death in the United States, but we still have a very incomplete understanding of blood clotting reactions at the molecular level. These studies will shed new light on the regulation of the blood clotting system, with a particular focus on achieving a detailed understanding of how and why blood clotting reactions occur on membrane surfaces.