Engineering of substrate selectivity for tissue factor.factor VIIa complex signaling through protease-activated receptor 2

The complex of factor VIIa (FVIIa) with tissue factor (TF) triggers coagulation by recognizing its macromolecular substrate factors IX (FIX) and X (FX) predominantly through extended exosite interactions. In addition, TF mediates unique cell-signaling properties in cancer, angiogenesis, and inflammation that involve proteolytic cleavage of protease-activated receptor 2 (PAR2). PAR2 is cleaved by FVIIa in the binary TF.FVIIa complex and by FXa in the ternary TF.FVIIa.FXa complex, but physiological roles of these signaling complexes are incompletely understood. In a screen of FVIIa protease domain mutants, three variants (Q40A, Q143N, and T151S) activated macromolecular coagulation substrates and supported signaling of the ternary TF.FVIIa-Xa complex normally but were severely impaired in binary TF.FVIIa.PAR2 signaling. The residues identified were located in the model-predicted S2' pocket of FVIIa, and complementary PAR2 P2' Leu-38 replacements demonstrated that the P2' side chain was indeed crucial for PAR2 cleavage by TF.FVIIa. In addition, PAR2 was activated more efficiently by FVIIa T99Y, consistent with further contributions from the S2 subsite. The P2 residue preference of FVIIa and FXa predicted additional PAR2 mutants that were efficiently activated by TF.FVIIa but resistant to cleavage by the alternative PAR2 activator FXa. Thus, contrary to the paradigm of exosite-assisted cleavage of PAR1 by thrombin, the cofactor-associated protease FVIIa recognizes PAR2 predominantly by catalytic cleft interactions. Furthermore, the delineated molecular details of this substrate interaction enabled protein engineering of protease-selective PAR2 receptors that will aid further studies to dissect the roles of TF signaling complexes in vivo.