Structural requirements for the interaction between tissue factor and factor VII: characterization of chymotrypsin-derived tissue factor polypeptides.

Tissue Factor (TF) is the cellular receptor for coagulation Factor VII/VIIa (FVII/VIIa). TF binds to FVIIa and promotes the rapid activation of the zymogen substrates Factors IX and X (FIX and FX) to the respective serine proteinases. In order to probe structure-function relationships in TF, we have subjected the truncated membrane-bound variant, TF 1-243, to proteolytic digestion in SDS-containing gels. Three major polypeptide fragments were generated by proteolysis of TF 1-243 with chymotrypsin, producing cleavages C-terminal to residues 34, 76 and 103. All three polypeptides, TF 35-243, 77-243 and 104-243, bound biotinylated human FVII in a highly specific ligand blot assay. High-performance electrophoretic chromatography was used to isolated chymotrypsin-derived fragments of TF. These purified fragments bound FVII in ligand blots, and two of the three polypeptides exhibited much reduced, but significant, procoagulant activity in a chromogenic assay for the generation of Factor Xa in the presence of FVIIa and Ca2+. The smallest chymotrypsin-derived TF polypeptide, TF 104-243, showed reduced binding of FVII in ligand blot analyses, inhibited the activity of the full-length molecule, but had no procoagulant activity. These data suggest that a part of the binding site for FVII is contained within the TF sequence 104-243. The sequence TF 1-34 either contains a part of the FVII-binding domain or its removal leads to dysfunctional folding, disrupting binding sites elsewhere in the molecule.