Expression of human soluble tissue factor in yeast and enzymatic properties of its complex with factor VIIa.

The extracellular domain of human tissue factor (TF, amino acids 1-217) was expressed in Saccharomyces cerevisiae, using the inducible yeast acid phosphatase promoter and the yeast invertase signal sequence to direct its secretion into the culture broth. Two active soluble forms sTF alpha (high molecular weight form) and sTF beta (low molecular weight form) were purified, the yield being approximately 10 and 1 mg/liter of culture supernatant, respectively. sTF alpha had an apparent molecular mass of 150 kDa on SDS-polyacrylamide gel electrophoresis and contained more than 200 residues of mannose/mol of protein. sTF beta had an apparent molecular mass of 37 kDa and contained 22 residues of mannose/mol of protein. N-Glycosidase F treatments of both rTFs reduced the apparent molecular mass to 35 kDa. The amino-terminal sequences and amino acid compositions of sTF alpha and sTF beta were consistent with those deduced from the cDNA sequence, thereby indicating that the difference in molecular mass is caused by heterogeneity of oligosaccharide structures. Of these recombinant TFs, sTF beta enhanced factor VIIa-amidolytic activity 40-fold toward the chromogenic substrate and 147-fold toward the fluorogenic substrate, affecting mainly the kcat value. The enhancement was comparable with that of TF purified from human placenta. The TF-mediated enhancement of factor VIIa-amidolytic activity was inhibited by heparin-activated antithrombin III, forming a high molecular weight complex. As treatment of sTF beta with denaturants such as guanidine hydrochloride or urea led to a biphasic loss of the activity, the extracellular domain of TF probably consists of two discrete domains. This expression system provides a significant amount of the extracellular domain of TF so that studies of interactions with factor VII are feasible.     

Activation of blood coagulation factor VIIa with cleaved tissue factor extracellular domain and crystallization of the active complex

Exposure of blood to tissue factor leads to the formation of a high affinity tissue factor/factor VIIa complex which initiates blood coagulation. As a first step toward obtaining structural information of this enzyme system, a complex of active-site inhibited factor VIIa (F.VIIai) and soluble tissue factor (sTF) was prepared for crystallization. Crystals were obtained, but only after long incubation times. Analysis by SDS-PAGE and mass spectrometry indicated the presence of sTF fragments similar to those formed by proteolytic digestion with subtilisin (Konigsberg, W., Nemerson, Y., Fang, C., Lin, T.-C. Thromb. Haemost. 69:1171, 1993). To test the hypothesis that limited proteolysis of sTF facilitated the crystallization of the complex, sTF fragments were generated by subtilisin digestion and purified. Analysis by tandem mass spectrometry showed the presence of nonoverlapping N- and C-terminal sTF fragments encompassing more than 90% of the tissue factor extracellular domain. Enzymatic assays and binding studies demonstrated that an equimolar mixture of N- and C-terminal fragments bound to factor VIIa and fully restored cofactor activity. A complex of F.VIIai and sTF fragments was prepared for crystallization. Crystals were obtained using microseeding techniques. The best crystals had maximum dimensions of 0.12 × 0.12 × 0.6 mm and showed diffraction to a resolution of 3 Å. © 1995 Wiley-Liss, Inc.     

Site-directed fluorescence probing to dissect the calcium-dependent association between soluble tissue factor and factor VIIa domains

We have used the site-directed labeling approach to study the Ca(2+)-dependent docking of factor VIIa (FVIIa) to soluble tissue factor (sTF). Nine Ca(2+) binding sites are located in FVIIa and even though their contribution to the overall binding between TF and FVIIa has been thoroughly studied, their importance for local protein-protein interactions within the complex has not been determined. Specifically we have monitored the association of the gamma-carboxyglutamic acid (Gla), the first EGF-like (EGF1), and the protease domains (PD) of FVIIa to sTF. Our results revealed that complex formation between sTF and FVIIa during Ca(2+) titration is initiated upon Ca(2+) binding to EGF1, the domain containing the site of highest Ca(2+) affinity. Besides we showed that a Ca(2+)-loaded Gla domain is required for an optimal association of all domains of FVIIa to sTF. Ca(2+) binding to the PD seems to be of some importance for the docking of this domain to sTF.     

Dynamic character of the complex of human blood coagulation factor VIIa with the extracellular domain of human tissue factor: a normal mode analysis

As an attempt to investigate the dynamic interactions between plasma serine protease, coagulation factor VIIa (VIIa) and its cofactor, tissue factor (TF), we performed normal mode analysis (NMA) of the complex of VIIa with soluble TF (the extracellular part of TF; sTF). We compared fluctuations of Calpha atoms of VIIa or sTF derived from NMA in the VIIa-sTF complex with those of VIIa or sTF in an uncomplexed condition. The atomic fluctuations of the Calpha atoms of sTF complexed with VIIa did not significantly differ from those of sTF without VIIa. In contrast, the atomic fluctuations of VIIa complexed with sTF were much smaller than those of VIIa without sTF. These results suggest that domain motions of VIIa molecule alone are markedly dampened in the VIIa-sTF complex and that the sTF molecule is relatively more rigid than the VIIa molecule. This may indicate functions of TF as a cofactor.     

Tissue factor is not involved in the mitogenic activity of factor VIIa

The present study was undertaken to evaluate in vitro the importance of tissue factor in the mitogenic effect of factor VIIa for embryonic fibroblasts. For that purpose, embryonic fibroblasts were isolated from either wild-type or transgenic mice showing a single inactivation of the tissue factor gene or expressing a truncated form (lacking the cytosolic domain) of this protein. Factor VIIa stimulated in a dose-dependent manner the growth of the 3 types of fibroblasts, thus showing that TF is not involved in the mitogenic activity of factor VIIa. The mitogenic activity of factor VIIa disappeared in serum immunopurified in factor X and was almost totally inhibited by DX9065, a selective factor Xa inhibitor, showing that this effect of factor VIIa occurred via factor Xa generated during the incubation period. Hirudin did not show any significant effect on factor VIIa-induced fibroblast proliferation, thus showing that the effect observed for factor VIIa was selectively mediated by factor Xa and was not due to thrombin formation. Our results therefore represent the first evidence for the possible importance of factor Xa in the mitogenic effect of factor VIIa and show the negligible role of tissue factor in this process.     

Relations between factor VIIa binding and expression of factor VIIa/tissue factor catalytic activity on cell surfaces.

The kinetics of the binding of rVIIa to cell surface tissue factor (TF) and the resultant expression of VIIa/TF activity were studied. Binding of 125I-rVIIa (10 nM) to cell surface TF required 30-60 min for saturation, whereas VIIa/TF activity was fully expressed toward factor X (F X) on intact monolayers after only 1 min of incubation. At the time only 10-20% of the total VIIa TF complexes present at saturation had formed. Freeze-thawing the monolayers before assay increased VIIa/TF activity up to 30-fold, and the time course of its expression was similar to that of TF-specific binding of VIIa to the monolayers. Equilibrium binding revealed a single high affinity binding class of TF sites on intact monolayers for rVIIa with a Kd of 1.6 nM. Experiments with active-site inhibited rVIIa yielded evidence for two populations of VIIa. TF complexes on intact monolayers: (1) a minor population (less than 20%) that formed within 1 min of incubation and accounted for all VIIa/TF activity toward F X present on the intact monolayers, and (2) a major population that was inactive toward F X on intact monolayers but which was fully active after the monolayers were lysed. Tissue factor pathway inhibitor (TFPI).F Xa complexes inhibited the VIIa/TF activity of the first population, i.e. of the complexes active on intact monolayers, half maximally at a concentration of 0.2 nM TFPI. TFPI/Xa also bound to the second population of VIIa.TF complexes on intact monolayers and inhibited their expression of VIIa/TF activity following cell lysis with a half-maximal inhibitory concentration of 2.0 nM. The potential physiologic implications of these findings are discussed.     

Crystals of the complex between human growth hormone and the extracellular domain of its receptor.

Single crystals suitable for high-resolution diffraction studies have been grown of the human growth hormone (hGH) complexed to the extracellular domain of its cloned receptor from the human liver (hGHbp), using the technique of repeat seeding. The crystals are in space group P2(1)2(1)2, with a = 145.8 A, b = 68.6 A, c = 76.0 A, and diffract to at least 2.7 A resolution on a rotating anode X-ray source. Analysis of the composition of these crystals showed the stoichiometry of the complex to be hGH: (hGHbp)2. This finding, coupled with biochemical data on the complex in solution, indicates that the biologically significant dimerization of the growth hormone receptor is mediated through a single hormone molecule. Structure determination of the complex is currently being completed.     

Inhibitors of factor VIIa affect the interface between the protease domain and tissue factor

Blood coagulation is triggered by the formation of a complex between factor VIIa (FVIIa) and its cofactor, tissue factor (TF). The gamma-carboxyglutamic acid-rich domain of FVIIa docks with the C-terminal domain of TF, the EGF1 domain of FVIIa contacts both domains of TF, and the EGF2 domain and protease domain (PD) form a continuous surface that sits on the N-terminal domain of TF. Our aim was to investigate the conformational changes that occur in the sTF.PD binding region when different types of inhibitors, i.e., one active-site inhibitor (FFR-chloromethyl ketone (FFR)), two different peptide exosite inhibitors (E-76 and A-183), and the natural inhibitor tissue factor pathway inhibitor (TFPI), were allowed to bind to FVIIa. For this purpose, we constructed two sTF mutants (Q37C and E91C). By the aid of site-directed labeling technique, a fluorescent label was attached to the free cysteine. The sTF.PD interface was affected in position 37 by the binding of FFR, TFPI, and E-76, i.e., a more compact structure was sensed by the probe, while for position 91 located in the same region no change in the surrounding structure was observed. Thus, the active site inhibitors FFR and TFPI, and the exosite inhibitor E-76 have similar effects on the probe in position 37 of sTF, despite their differences in size and inhibition mechanism. The allosteric changes at the active site caused by binding of the exosite inhibitor E-76 in turn induce similar conformational changes in the sTF.PD interface as does the binding of the active site inhibitors. A-183, on the other hand, did not affect position 37 in sTF, indicating that the A-183 inhibition mechanism is different from that of E-76.     

Disulfide locked variants of factor VIIa with a restricted beta-strand conformation have enhanced enzymatic activity

Proteolytic processing of zymogen Factor VII to Factor VIIa (FVIIa) is necessary but not sufficient for maximal proteolytic activity, which requires an additional allosteric influence induced upon binding to its cofactor tissue factor (TF). A key conformational change affecting the zymogenicity of FVIIa involves a unique three-residue shift in the position of beta-strand B2 in their zymogen and protease forms. By selectively introducing new disulfide bonds, we locked the conformation of these strands into an active TF*FVIIa-like state. FVIIa mutants designated 136:160, 137:159, 138:160, and 139:157, reflecting the position of the new disulfide bond (chymotypsinogen numbering), were expressed and purified by TF affinity chromatography. Mass spectrometric analysis of tryptic peptides from the FVIIa mutants confirmed the new disulfide bond formation. Kinetic analysis of amidolytic activity revealed that all FVIIa variants alone had increased specific activity compared to wild type, the largest being for variants 136:160 and 138:160 with substrate S-2765, having 670- and 330-fold increases, respectively. Notably, FVIIa disulfide-locked variants no longer required TF as a cofactor for maximal activity in amidolytic assays. In the presence of soluble TF, activity was enhanced 20- and 12-fold for variants 136:160 and 138:160, respectively, compared to wild type. With relipidated TF, mutants 136:160 and 137:159 also had an approximate threefold increase in their V(max)/K(m) values for FX activation but no significant improvement in TF-dependent clotting assays. Thus, while large rate enhancements were obtained for amidolytic substrates binding at the active site, macro-molecular substrates that bind to FVIIa exosites entail more complex catalytic requirements.     

Substitution of valine for leucine 305 in factor VIIa increases the intrinsic enzymatic activity

Factor VII requires the cleavage of an internal peptide bond and the association with tissue factor (TF) to attain its fully active factor VIIa (FVIIa) conformation. The former event alone leaves FVIIa in a zymogen-like state of relatively low specific activity. We have designed a number of FVIIa mutants with the aim of mimicking the effect of TF, that is, creating molecules with increased intrinsic (TF-independent) enzymatic activity. Based on a possible structural difference between free and TF-bound FVIIa (Pike, A. C. W., Brzozowski, A. M., Roberts, S. M., Olsen, O. H., and Persson, E. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 8925--8930), we focused on the helical region encompassing residues 307-312 and residues in its spatial vicinity. For instance, FVIIa contains Phe-374 and Leu-305, whereas a Phe/Tyr residue in the position corresponding to 374 in homologous coagulation serine proteases is accompanied by Val in the position corresponding to 305. This conceivably results in a unique orientation of this helix in FVIIa. Substitution of Val for Leu-305 in FVIIa resulted in a 3--4-fold increase in the intrinsic amidolytic and proteolytic activity as compared with wild-type FVIIa, whereas the activity in complex with soluble TF remained the same. In accordance with this, L305V-FVIIa exhibited an increased rate of inhibition as compared with wild-type FVIIa, both by d-Phe-Phe-Arg-chloromethyl ketone and antithrombin III in the presence of heparin. The increased FVIIa activity upon replacement of Leu-305 by Val may be mediated by a movement of the 307--312 helix into an orientation resembling that found in factors IXa and Xa and thrombin. The corresponding shortening of the side chain of residue 374 (Phe --> Pro) had a smaller effect (about 1.5-fold increase) on the intrinsic activity of FVIIa. Attempts to increase FVIIa activity by introducing single or multiple mutations at positions 306, 309, and 312 to stabilize the 307-312 helix failed.     

Spectroscopic probing of the influence of calcium and the gla domain on the interaction between the first EGF domain in factor VIIa and tissue factor.

The binding of factor VIIa (FVIIa) to tissue factor (TF) initiates blood coagulation. The binary complex is dependent on Ca2+ binding to several sites in FVIIa and is maintained by multiple contacts distributed throughout the various domains. Although the contributions from various residues and domains, including the Ca2+ coordination, to the global binding energy have been characterized, their importance for specific local interactions is virtually unknown. To address this aspect, we have attached four spectroscopic probes to an engineered Cys residue replacing Phe140 in soluble TF (sTF). This allows the monitoring of local changes in hydrophobicity and rigidity upon complex formation at the interface between the first epidermal growth factor-like (EGF1) domain of FVIIa and sTF. The fluorescent labels used sense a more hydrophobic environment and the spin labels are dramatically immobilized when FVIIa binds sTF. The results obtained with a 4-carboxyglutamic acid (Gla)-domainless derivative of FVIIa indicate that the Gla domain has no or minimal influence on the interaction between EGF1 and sTF. However, there is a difference in local Ca2+ dependence between Gla-domainless and full-length FVIIa.     

Transition state analysis of the complex between coagulation factor VIIa and tissue factor: suggesting a sequential domain-binding pathway

Injury of a blood vessel exposes membrane-bound tissue factor (TF) to blood, which allows binding of coagulation factor VIIa (FVIIa). This initiation of the coagulation cascade is dictated by a specific multi-domain interaction between FVIIa and TF. To examine the energies involved in the transition state of the FVIIa:TF complex, various residues in the extracellular part of TF (sTF) that are known to interact with FVIIa were replaced with a smaller cysteine residue. Determination of Phi values in each of the positions using surface plasmon resonance measurements enabled us to characterize the transition state complex between the resulting sTF variants and FVIIa. We found that the interactions in the transition state seemed to be most pronounced between the protease domain of FVIIa and sTF while detailed specific interactions between the Gla-domain and sTF were missing. Thus, the transition state energy data indicate a sequential binding event between these two macromolecules.     

Ternary complex between placental lactogen and the extracellular domain of the prolactin receptor

The structure of the ternary complex between ovine placental lactogen (oPL) and the extracellular domain (ECD) of the rat prolactin receptor (rPRLR) reveals that two rPRLR ECDs bind to opposite sides of oPL with pseudo two-fold symmetry. The two oPL receptor binding sites differ significantly in their topography and electrostatic character. These binding interfaces also involve different hydrogen bonding and hydrophobic packing patterns compared to the structurally related human growth hormone (hGH)-receptor complexes. Additionally, the receptor-receptor interactions are different from those of the hGH-receptor complex. The conformational adaptability of prolactin and growth hormone receptors is evidenced by the changes in local conformations of the receptor binding loops and more global changes induced by shifts in the angular relationships between the N- and C-terminal domains, which allow the receptor to bind to the two topographically distinct sites of oPL.     

Chimeric interferon-gamma receptors demonstrate that an accessory factor required for activity interacts with the extracellular domain.

We determined the species specificity and function of structural domains of the interferon-gamma receptor (IFN-gamma R) by construction of human/murine chimeric IFN-gamma R cDNA clones and their expression in various cells. We demonstrate that we can reconstitute a biologically active IFN-gamma R in eukaryotic cells with chimeric receptors as long as the extracellular domain and an accessory factor are from the same species. These results indicate that the extracellular domain of the receptor interacts directly or indirectly with the species-specific accessory factor.     

Interaction of the extracellular domain of the epidermal growth factor receptor with gangliosides.

Ganglioside GM3 inhibits epidermal growth factor (EGF)-dependent cell proliferation in a variety of cell lines. Both in vitro and in vivo, this glycosphingolipid inhibits the kinase activity of the EGF receptor (EGFR). Furthermore, membrane preparations containing EGFR can bind to GM3-coated surfaces. These data suggest that GM3 may interact directly with the EGFR. In this study, the interaction of gangliosides with the extracellular domain (ECD) of the EGFR was investigated. The purified human recombinant ECD from insect cells bound directly to ganglioside GM3. The ganglioside interaction site appears to be distinct from the EGF-binding site. In agreement with previous reports on the effects of specific gangliosides on EGFR kinase activity, the ECD preferentially interacted with GM3. The order of relative binding of other gangliosides investigated was as follows: GM3 GM2, GD3, GM4 > GM1, GD1a, GD1b, GT1b, GD2, GQ1b > lactosylceramide. These data suggest that NeuAc-lactose is essential for binding and that any sugar substitution reduces binding. In agreement with the specificity of soluble ECD binding to gangliosides, GM3 specifically inhibited EGFR autophosphorylation. Identification of a ganglioside interaction site on the ECD of the EGFR is consistent with the hypothesis that endogenous GM3 may function as a direct modulator of EGFR activity.     

EGF induces increased ligand binding affinity and dimerization of soluble epidermal growth factor (EGF) receptor extracellular domain.

The binding of epidermal growth factor (EGF) to its cell surface receptor (EGF-R) results in a number of intracellular responses including the activation of the receptor intracellular tyrosine kinase. Receptor oligomerization induced by ligand binding has been suggested to play an important role in signal transduction. However, the mechanisms involved in oligomerization and signal transduction are poorly understood. We have produced and purified several milligrams of recombinant extracellular domain of the EGF receptor (EGF-Rx) using the baculovirus/insect cell expression system. The baculovirus-generated EGF-Rx is glycosylated, has had its signal peptide correctly cleaved, and exhibits a dissociation constant for EGF similar to that for solubilized full-length receptor, of about 100 nM. The binding of EGF to EGF-Rx leads to the formation of receptor dimers and higher oligomerization states which are irreversibly captured using the covalent cross-linking agent disuccinimidyl suberate. Interestingly, purified receptor monomers and dimers, stabilized by the cross-linker in the presence of EGF, exhibit increased binding affinity toward EGF as compared with receptor monomers which have not been exposed to EGF. It appears that the high affinity state of receptor can be maintained by the covalent cross-linking agent. These results indicate that in addition to ligand binding, the extracellular domain of EGF receptor possesses the inherent ability to undergo ligand-induced dimerization and that the low affinity state is converted to a high affinity state by EGF.     

Factor VIIa-induced p44/42 mitogen-activated protein kinase activation requires the proteolytic activity of factor VIIa and is independent of the tissue factor cytoplasmic domain.

Signal transduction induced by activated factor VII (FVIIa) was studied with baby hamster kidney (BHK) cells transfected with human tissue factor (TF). FVIIa induced phosphorylation of p44/42 mitogen-activated protein kinase (MAPK) in cells expressing TF, BHK(+TF), but not in wild-type BHK(-TF) cells. BHK(+TF) cells responded to FVIIa in a dose-dependent manner, with detectable phosphorylation above 10-20 nM FVIIa. BHK cells transfected with a cytoplasmic domain-deleted version of TF, (des248-263)TF, or a C245S substitution variant of TF also supported FVIIa-induced MAPK activation. Experiments with active site-inhibited FVIIa, thrombin, factor Xa, and hirudin confirmed that the catalytic activity of FVIIa was mandatory for p44/42 MAPK activation. Furthermore, a high concentration of FVIIa in complex with soluble TF induced p44/42 MAPK phosphorylation in BHK(-TF) cells. These data suggest that TF was not directly involved in FVIIa-induced p44/42 MAPK phosphorylation but rather served to localize the action of FVIIa to the cell surface, potentially to cleave a cell surface receptor. Desensitization experiments with sequential addition of proteases suggested that the p44/42 MAPK response induced by FVIIa was distinctly different from the thrombin response, possibly involving a novel member of the protease-activated receptor family.     

Substitution of the Gla domain in factor X with that of protein C impairs its interaction with factor VIIa/tissue factor: lack of comparable effect by similar substitution in factor IX

We previously reported that the first epidermal growth factor-like (EGF1) domain in factor X (FX) or factor IX (FIX) plays an important role in the factor VIIa/tissue factor (FVIIa/TF)-induced coagulation. To assess the role of gamma-carboxyglutamic acid (Gla) domains of FX and FIX in FVIIa/TF induced coagulation, we studied four new and two previously described replacement mutants: FX(PCGla) and FIX(PCGla) (Gla domain replaced with that of protein C), FX(PCEGF1) and FIX(PCEGF1) (EGF1 domain replaced with that of protein C), as well as FX(PCGla/EGF1) and FIX(PCGla/EGF1) (both Gla and EGF1 domains replaced with those of protein C). FVIIa/TF activation of each FX mutant and the corresponding reciprocal activation of FVII/TF by each FXa mutant were impaired. In contrast, FVIIa/TF activation of FIX(PCGla) was minimally affected, and the reciprocal activation of FVII/TF by FIXa(PCGla) was normal; however, both reactions were impaired for the FIX(PCEGF1) and FIX(PCGla/EGF1) mutants. Predictably, FXIa activation of FIX(PCEGF1) was normal, whereas it was impaired for the FIX(PCGla) and FIX(PCGla/EGF1) mutants. Molecular models reveal that alternate interactions exist for the Gla domain of protein C such that it is comparable with FIX but not FX in its binding to FVIIa/TF. Further, additional interactions exist for the EGF1 domain of FX, which are not possible for FIX. Importantly, a seven-residue insertion in the EGF1 domain of protein C prevents its interaction with FVIIa/TF. Cumulatively, our data provide a molecular framework demonstrating that the Gla and EGF1 domains of FX interact more strongly with FVIIa/TF than the corresponding domains in FIX.     

Probing the structural changes in the light chain of human coagulation factor VIIa due to tissue factor association.

The crystallographic structure of human coagulation factor VIIa/tissue factor complex bound with calcium ions was used to model the solution structure of the light chain of factor VIIa (residues 1-142) in the absence of tissue factor. The Amber force field in conjunction with the particle mesh Ewald summation method to accommodate long-range electrostatic interactions was used in the trajectory calculations. The estimated TF-free solution structure was then compared with the crystal structure of factor VIIa/tissue factor complex to estimate the restructuring of factor VIIa due to tissue factor binding. The solution structure of the light chain of factor VIIa in the absence of tissue factor is predicted to be an extended domain structure similar to that of the tissue factor-bound crystal. Removal of the EGF1-bound calcium ion is shown by simulation to lead to minor structural changes within the EGF1 domain, but also leads to substantial relative reorientation of the Gla and EGF1 domains.