The tissue factor region that interacts with factor Xa in the activation of factor VII

Tissue factor is the cell membrane-anchored cofactor for factor VIIa and tr iggers the coagulation reactions. The initial step is the conversion of fac tor VII to factor VIIa which, in vitro, is efficiently catalyzed by low con centrations of factor Xa. To identify the tissue factor region that interac ts with the activator factor Xa during this process, we evaluated a panel o f soluble tissue factor (1-219) mutants for their ability to support factor Xa-mediated activation of factor VII. The tissue factor residues identifie d as most important for this interaction (Tyr157, Lys159, Ser163, Gly164, L ys165, Lys166, and Tyr185) were identical to those found to be important fo r the interaction of substrate factor X with the tissue factor factor VIIa complex. The residues form a continuous surface-exposed patch with an area of about 500 Angstrom (2), which appears to be located outside the tissue f actor-factor VII contact zone. In agreement, the two monoclonal antibodies 5G6 and D3H44-F(ab')(2), whose epitopes overlap with this identified region , inhibited the rates of factor VII activation by 86% and 95%, respectively . These antibodies also strongly inhibited the conversion of I-125-labeled factor VII when cell membrane-expressed, full-length tissue factor (1-263) was employed. Together the results suggest the usage of a common surface re gion of tissue factor in its dual role-as a cofactor for factor Xa-mediated factor VII activation and as a cofactor for factor VIIa-mediated factor X activation. The finding that factor Xa and factor X may engage in similar, if not identical, molecular interactions with tissue factor further indicat es that factor Xa and factor X are similarly oriented toward their respecti ve interaction partners in the ternary catalytic complexes.