Dimerization of tissue factor supports solution-phase autoactivation of factor VII without influencing proteolytic activation of factor X

Tissue factor (TF) is a transmembrane receptor that initiates the thromboge nic cascade by assembly with the serine protease factor VII or VIIa (VII/VI Ia) resulting in formation of the bimolecular active complex TF.VIIa. Chemi cal cross-linking studies identified that a minor population of TF forms di mers on the surface of cells, possibly influencing TF.VIIa proteolytic func tion as a result of dimerization. We here investigate the effects of dimeri zation of the extracellular domain of TF on the proteolytic function of the TF.VIIa complex. The leucine zipper dimerization domain of the yeast trans criptional factor GCN4 (LZ) was genetically fused at the C-terminus of the extracellular domain of TF separated by a short linker (TF(L)LZ). TF(L)LZ h omodimerized with a K-d similar to that of the LZ peptide. Tryptophan fluor escence indicated that the two TF moieties were in close proximity and para llel orientation in TF(L)LZ. TF(L)LZ dimers bound two molecules of VIIa, an d VIIa binding did not influence the TF dimer equilibrium. Dimerization inf luenced neither amidolytic nor the factor X activation activities of the TF .VIIa complexes. Notably, dimer TF(L)LZ efficiently promoted the autoactiva tion of VII to VIIa in solution in contrast to monomeric TF(L)LZ or TF1-218 . Thus, TF dimerization on cells may serve to "prime" the initiation of the coagulation pathway by generating active TF.VIIa complexes for the subsequ ent activation of downstream macromolecular substrates.