Deletion of the membrane anchoring region of tissue factor abolishes autoactivation of factor VII but not cofactor function. Analysis of a mutant with a selective deficiency in activity

The activation of human blood coagulation factor VII can occur by the feedback activity of either factor VIIa (autoactivation) or factor Xa. Both of these reactions are known to be enhanced by the presence of tissue factor, an integral membrane protein and the cofactor for factor VIIa. We examine here the activation of ¹²⁵I-factor VII by both factor VIIa and factor Xa employing a mutant soluble form of tissue factor which has had its transmembrane and cytoplasmic domains deleted (sTF_1-219). This mutant soluble tissue factor retains cofactor activity toward factor VIIa in a single-stage clotting assay but shows a strong dependence on initial plasma levels of factor VIIa (from 1 to 10,000 ng/ml) when compared to wild-type tissue factor. We show that this dependence is due to a deficiency of sTF(1- 219) in ability to both promote autoactivation and enhance the factor Xa- catalyzed activation of ¹²⁵I-factor VII. sTF_1-219 does not, however, inhibit the tissue factor-independent activation of ¹²⁵I-factor VII by factor Xa. The results strongly suggest that the phospholipid anchoring region of tissue factor is essential for autoactivation and beneficial for factor Xa-catalyzed activation of ¹²⁵I-factor VII. In addition, when taken together with the dependence of clotting times on initial factor VIIa levels observed with sTF_1-219, these results indicate that factor VII autoactivation may be of greater importance in the initiation of blood coagulation via tissue factor than has been previously realized.