Factor XI (fXI) is the zymogen of a homodimeric plasma protease, factor XIa (fXIa) that contributes to hemostasis by converting factor IX (fIX) to the protease fIXa. While fXI deficiency causes a relatively mild bleeding disorder, there is mounting evidence that this protein contributes substantively to pathologic processes such as thrombosis and disseminated intravascular coagulation. In addition to its dimeric structure, fXIa has structural features that are distinct from those of vitamin K-dependent proteases. In this proposal, we address questions regarding mechanisms of fXI activation, and the interactions of fXIa with fIX and platelets, during hemostasis. Classically, fXI is converted to fXIa by factor XIIa (fXIIa). However, fXII deficiency does not cause abnormal bleeding. -thrombin activates fXI in solution, but the premise that this occurs in plasma has been challenged recently. Because it is a homodimer, fXIa has two active sites per molecule. Recently we showed that conversion of fXI to fXIa proceeds through a species with one active site (1/2-fXIa) that is functionally similar to fXIa. In Aim 1, we address the hypotheses that fXI is activated by -thrombin in plasma, and that other products of prothrombin activation, such as meizothrombin and -thrombin, activate fXI. We will examine the kinetics of activation of each fXI subunit, comparing activation by fXIIa, -thrombin, -thrombin, meizothrombin, and -thrombin with mutations in anion binding exosites; and will study the importance of the fXI dimeric structure to activation in purified and plasma systems. FIX activation involves sequential proteolytic cleavage, first after Arg145 producing the intermediate fIX, and then after Arg180 generating fIXa. We have shown that an exosite on the fXIa heavy (non-catalytic) chain is required for fIX conversion to fIXa. New evidence indicates a second site, probably on the catalytic domain but distinct from the active site, is also required. We hypothesize that this second site is required for conversion of fIX to fIX, while the heavy chain exosite makes contributions to both cleavages. In Aim 2 we will study the role of exosite interactions in fIX activation by fXIa, delineate the importance of exosites on the fXI heavy chain and catalytic domains to cleavage of the two fIX activation sites, and determine if cleavage at Arg145 is a prerequisite for cleavage at Arg180. FXI binds to platelet glycoprotein 1b (GP1b). While it was proposed that this interaction facilitates fXI activation, this now is uncertain. Preliminary work indicates fXI also binds to a receptor for apolipoprotein E (ApoER2) on platelets. ApoER2 can form a complex with GP1b on platelets, and the two may be components of a fXI binding site. We hypothesize that the fXI-ApoER2 interaction involves a binding site on fXI distinct from the one required for binding to GP1b. In Aim 3 we will assess the interaction of fXI and the active species fXIa and 1/2-fXIa with platelets, study the importance of platelets to fXI activation and activity in a flow-based system, and identify the binding site on fXI involved in interactions with platelet ApoER2. PUBLIC HEALTH RELEVANCE: Plasma factor XIa makes modest contributions to normal blood coagulation (hemostasis) at a site of vessel injury, but appears to make a disproportionately greater contribution to coagulation in pathologic processes such as vascular thrombosis. This protease, therefore, is a legitimate target for novel anti- thrombotic therapies that should carry a lower risk of life-threatening bleeding, when compared to currently available anticoagulants.