The third Trp-Lys-Ser (WKS) tripeptide motif in tissue factor is associated with a function site

Tissue factor residues that putatively interact with membrane phospholipids

Blood clotting is initiated by the two-subunit enzyme consisting of the plasma protease, factor VIIa (the catalytic subunit), bound to the integral membrane protein, tissue factor (the regulatory subunit). Molecular dynamics simulations have predicted that certain residues in the tissue factor ectodomain interact with phosphatidylserine headgroups to ensure optimal positioning of the tissue factor/factor VIIa complex relative to its membrane-bound protein substrates, factors IX and X. In this study, we individually mutated to alanine all the putative phosphatidylserine-interactive residues in the tissue factor ectodomain and measured their effects on tissue factor cofactor function (activation of factors IX and X by tissue factor/factor VIIa, and clotting of plasma). Some tissue factor mutants exhibited decreased activity in all three assays, with the most profound defects observed from mutations in or near the flexible loop from Lys159 to Gly164. The decreased activity of all of these tissue factor mutants could be partially or completely overcome by increasing the phosphatidylserine content of tissue factor-liposomes. Additionally, yeast surface display was used to screen a random library of tissue factor mutants for enhanced factor VIIa binding. Surprisingly, mutations at a single amino acid (Lys165) predominated, with the Lys165→Glu mutant exhibiting a 3-fold enhancement in factor VIIa binding affinity. Our studies reveal the functional contributions of residues in the C-terminal half of the tissue factor ectodomain that are implicated in interacting with phosphatidylserine headgroups to enhance tissue factor cofactor activity, possibly by allosterically modulating the conformation of the adjacent substrate-binding exosite region of tissue factor.

Regulation of tissue factor procoagulant activity by post-translational modifications

Post-translational modification of amino acid residues is a common way to regulate localization, stability and ultimately the function of the protein. Tissue factor (TF), the major initiator of blood coagulation cascade, receives several post-translational modifications, like glycosylation, phosphorylation, palmitoylation and nitrosylation. Recent studies have demonstrated that these processes play important roles in modulating biological functions of TF. The present review highlights the mechanisms of several common protein post-translational modifications of TF with the special reference on the recent knowledge about their roles in regulation of trafficking, stability as well as procoagulant and signaling functions of TF.

Allosteric disulfide bonds in thrombosis and thrombolysis

Allosteric disulfide bonds control protein function by mediating conformational change when they undergo reduction or oxidation. The known allosteric disulfide bonds are characterized by a particular bond geometry, the -RHStaple. A number of thrombosis and thrombolysis proteins contain one or more disulfide bonds of this type. Tissue factor (TF) was the first hemostasis protein shown to be controlled by an allosteric disulfide bond, the Cys186-Cys209 bond in the membrane-proximal fibronectin type III domain. TF exists in three forms on the cell surface: a cryptic form that is inert, a coagulant form that rapidly binds factor VIIa to initiate coagulation, and a signaling form that binds FVIIa and cleaves protease-activated receptor 2, which functions in inflammation, tumor progression and angiogenesis. Reduction and oxidation of the Cys186-Cys209 disulfide bond is central to the transition between the three forms of TF. The redox state of the bond appears to be controlled by protein disulfide isomerase and NO. Plasmin(ogen), vitronectin, glycoprotein 1balpha, integrin beta(3) and thrombomodulin also contain -RHStaple disulfides, and there is circumstantial evidence that the function of these proteins may involve cleavage/formation of these disulfide bonds.

Evidence for activation of tissue factor by an allosteric disulfide bond

Tissue Factor (TF) is the mammalian plasma membrane cofactor responsible for initiation of blood coagulation. Binding of blood coagulation factor VIIa to TF activates the serine proteinase zymogens factors IX and X by limited proteolysis leading to the formation of a thrombin and fibrin meshwork that stabilizes the thrombus. TF on the plasma membrane of cells resides mostly in a cryptic configuration, which rapidly transforms into an active configuration in response to certain stimuli. The extracellular part of TF consists of two fibronectin type III domains. The disulfide bond in the membrane proximal domain (Cys186-Cys209) is atypical for domains of this type in that it links adjacent strands in the same beta sheet, what we have called an allosteric bond. Ablation of the allosteric disulfide by mutating both cysteine residues severely impairs procoagulant activity. The thiol-alkylating agents N-ethylmaleimide and methyl methanethiolsulfonate block TF activation by ionomycin, while the thiol-oxidizing agent HgCl2 and dithiol cross-linkers promote activation. TF activation could not be explained by exposure of phosphatidylserine on the outer leaflet of the plasma membrane. Cryptic TF contained unpaired cysteine thiols that were depleted upon activation, and de-encryption was associated with a change in the conformation of the membrane-proximal domain. These findings imply that the Cys186-Cys209 disulfide bond is reduced in the cryptic form of TF and that activation involves formation of the disulfide. It is likely that formation of this disulfide bond changes the conformation of the domain that facilitates productive binding of factors IX and X.

Exosite-driven substrate specificity and function in coagulation

Macromolecular substrate recognition and serine proteinase specificity lie at the heart of the tightly regulated hemostatic response. Mechanisms established for the less specific serine proteinases of digestion have played a dominant role in guiding investigations of the basis for the narrow specificities exhibited by the coagulation enzymes. These concepts have also dominated the development of specific inhibitors of coagulation for therapeutic purposes. Studies of the enzymology and physical biochemistry of prothrombinase challenge these prevailing ideas by establishing a principal role for exosites within the enzyme in determining substrate recognition and directing the action of the enzyme on its biological substrate. Mechanisms by which narrow protein substrate specificity is achieved by prothrombinase also apply to several other reactions of coagulation. These strategies are increasingly evident in the action of other families of enzymes that act with high specificity on protein substrates. Exosite-driven enzymic function probably represents a widely employed biological strategy for the achievement of high macromolecular substrate specificity.

Emerging anticoagulant drugs

The limitations of traditional anticoagulants, heparin and warfarin, have prompted the development of new anticoagulant drugs for prevention and treatment of both venous and arterial thromboembolism. After a brief review of thrombogenesis and its regulation, this study focuses on new anticoagulant agents in more advanced stages of clinical testing.

Probing the interface between factor Xa and tissue factor in the quaternary complex tissue factor-factor VIIa-factor Xa-tissue factor pathway inhibitor

Blood coagulation is triggered by the formation of a complex between factor VIIa (FVIIa) and its cofactor, tissue factor (TF). TF-FVIIa is inhibited by tissue factor pathway inhibitor (TFPI) in two steps: first TFPI is bound to the active site of factor Xa (FXa), and subsequently FXa-TFPI exerts feedback inhibition of TF-FVIIa. The FXa-dependent inhibition of TF-FVIIa activity by TFPI leads to formation of the quaternary complex TF-FVIIa-FXa-TFPI. We used site-directed fluorescence probing to map part of the region of soluble TF (sTF) that interacts with FXa in sTF-FVIIa-FXa-TFPI. We found that the C-terminal region of sTF, including positions 163, 166, 200 and 201, is involved in binding to FXa in the complex, and FXa, most likely via its Gla domain, is also in contact with the Gla domain of FVIIa in this part of the binding region. Furthermore, a region that includes the N-terminal part of the TF2 domain and the C-terminal part of the TF1 domain, i.e. the residues 104 and 197, participates in the interaction with FXa in the quaternary complex. Moreover, comparisons of the interaction areas between sTF and FX(a) in the quaternary complex sTF-FVIIa-FXa-TFPI and in the ternary complexes sTF-FVII-FXa or sTF-FVIIa-FX demonstrated large similarities.

Tissue factor alters the pK(a) values of catalytically important factor VIIa residues

Blood coagulation is triggered when the serine protease factor VIIa (fVIIa) binds to cell surface tissue factor (TF) to form the active enzyme-cofactor complex. TF binding to fVIIa allosterically augments the enzymatic activity of fVIIa toward macromolecular substrates and small peptidyl substrates. The mechanism of this enhancement remains unclear. Our previous studies have indicated that soluble TF (sTF; residues 1-219) alters the pH dependence of fVIIa amidolytic activity (Neuenschwander et al. (1993) Thromb. Haemostasis 70, 970), indicating an effect of TF on critical ionizations within the fVIIa active center. The pKa values and identities of these ionizable groups are unknown. To gain additional insight into this effect, we have performed a detailed study of the pH dependence of fVIIa amidolytic activity. Kinetic constants of Chromozym t-PA (MeSO(2)-D-Phe-Gly-Arg-pNA) hydrolysis at various pH values were determined for fVIIa alone and in complex with sTF. The pH dependence of both enzymes was adequately represented using a diprotic model. For fVIIa alone, two ionizations were observed in the free enzyme (pK(E1) = 7.46 and pK(E2) = 8.67), with at least a single ionization apparent in the Michaelis complex (pK(ES1) similar 7.62). For the fVIIa-TF complex, the pK(a) of one of the two important ionizations in the free enzyme was shifted to a more basic value (pK(E1) = 7.57 and pK(E2) = 9.27), and the ionization in the Michaelis complex was possibly shifted to a more acidic pH (pK(ES1) = 6.93). When these results are compared to those obtained for other well-studied serine proteases, K(E1) and K(ES1) are presumed to represent the ionization of the overall catalytic triad in the absence and presence of substrate, respectively, while K(E2) is presumed to represent ionization of the alpha-amino group of Ile(153). Taken together, these results would suggest that sTF binding to fVIIa alters the chemical environment of the fVIIa active site by protecting Ile(153) from deprotonation in the free enzyme while deprotecting the catalytic triad as a whole when in the Michaelis complex.

The tissue factor region that interacts with substrates factor IX and Factor X

The enzymatic activity of coagulation factor VIIa is controlled by its cellular cofactor tissue factor (TF). TF binds factor VIIa with high affinity and, in addition, participates in substrate interaction through its C-terminal fibronectin type III domain. We analyzed surface-exposed residues in the C-terminal TF domain to more fully determine the area on TF important for substrate activation. Soluble TF (sTF) mutants were expressed in E. coli, and their ability to support factor VIIa-dependent substrate activation was measured in the presence of phospholipid vesicles or SW-13 cell membranes. The results showed that factor IX and factor X interacted with the same TF region located proximal to the putative phospholipid surface. According to the degree of activity loss of the sTF mutants, this TF region can be divided into a main region (residues Tyr157, Lys159, Ser163, Gly164, Lys165, Lys166, Tyr185) forming a solvent-exposed patch of 488 A(2) and an extended region which comprises an additional 7-8 residues, including the distally positioned Asn199, Arg200, and Asp204. Some of the identified TF residues, such as Trp158 and those within the loop Lys159-Lys165, are near the factor VIIa gamma-carboxyglutamic acid (Gla) domain, suggesting that the factor VIIa Gla-domain may also participate in substrate interaction. Moreover, the surface identified as important for substrate interaction carries a net positive charge, suggesting that charge interactions may significantly contribute to TF-substrate binding. The calculated surface-exposed area of this substrate interaction region is about 1100 A(2), which is approximately half the size of the TF area that is in contact with factor VIIa. Therefore, a substantial portion of the TF surface (3000 A(2)) is engaged in protein-protein interactions during substrate catalysis.

A peptide sequence from mouse tissue factor inhibits human tissue factor dependent factor X activation

Synthetic peptides based on the putative factor X recognition site of human (Thr-Leu-Tyr-Tyr-Trp-Lys-Ser-Ser-Ser-Ser), rabbit (Thr-Leu-Tyr-Tyr-Trp-Arg-Ala-Ser-Ser-Thr), and murine tissue factor (Ile-Ile-Thr-Tyr-Arg-Lys-Gly-Ser-Ser-Thr) were dose-dependent inhibitors of human tissue factor/factor VIIa catalyzed factor X activation with IC50 values of 220, 17, and 33 microM, respectively. The mouse results were highly surprising given the low homology between the human and mouse sequence (40%) and that mouse tissue factor, in contrast with rabbit tissue factor, does not support the procoagulant activity of human factor VIIa on factor X. The inhibitory mechanism of the murine peptide was noncompetitive with respect to factor X but competitive with respect to tissue factor, indicating the peptide competes with tissue factor (or the tissue factor/factor VIIa complex) for binding to factor X. The peptide could be N-terminally truncated by two Ile without loss of inhibitory activity or changed inhibitory mechanism. Substitution of two Gly for the two Ile, which increased solubility, decreased IC50 to 17 microM whereas scrambling the peptide made it inactive.

Influence of mutations in tissue factor on the fine specificity of macromolecular substrate activation

The C-terminal fibronectin-type-III-like module of the tissue factor (TF) extracellular domain plays a requisite role in the activation of macromolecular substrates by factor VIIa (VIIa) in complex with TF. Unlike the mutations Lys165-->Ala, Lys166-->Ala in TF, which prevent efficient proteolysis of factor X, we found that the coagulant defect of a site-specific Trp158-->Arg, Ser160-->Gly replacement mutant of TF is largely attributable to the inability of TF to efficiently support the activation of the bound zymogen VII to the active protease VIIa. Binding studies demonstrated comparable affinity of binding of VIIa or VII by wild-type TF and TF(R158G160). In comparison with wild-type TF, the catalytic efficiency of factor X activation was reduced 56-fold with TF(A165A166) as the cofactor, but only 3.5-fold with TF(R165G160). The activation of VII bound to TF by factor Xa or VIIa was reduced 2-fold in the presence of TF(R158G160) and 7-8-fold with TF(A165A166). This suggests that the molecular recognition of VII in complex with TF by the enzymes TF-VIIa and factor Xa are similar. Generation of factor IXa by TF(R158G160)-VIIa was unaltered, but reduced 2-fold with TF(A165A166). In addition, the mutations affected the cleavage of the two scissile bonds of factor IX differently, providing further support for the idea that the cofactor, TF, influences the fine specificity of activation of macromolecular substrates by the TF-VIIa complex.

Environments of the four tryptophans in the extracellular domain of human tissue factor: comparison of results from absorption and fluorescence difference spectra of tryptophan replacement mutants with the crystal structure of the wild-type protein

The local environments of the four tryptophan residues of the extracellular domain of human tissue factor (sTF) were assessed from difference absorption and fluorescence spectra. The difference spectra were derived by subtracting spectra from single Trp-to-Phe or Trp-to-Tyr replacement mutants from the corresponding spectrum of the wild-type protein. Each of the mutants was capable of enhancing the proteolytic activity of factor VIIa showing that the mutations did not introduce major structural changes, although the mutants were more susceptible to denaturation by guanidinium chloride. The difference spectra indicate that the Trp residues are buried to different extents within the protein matrix. This evaluation was compared with the x-ray crystal structure of sTF. There is excellent agreement between predictions from the difference spectra and the environments of the Trp residues observed in the x-ray crystal structure, demonstrating that difference absorption and particularly fluorescence spectra derived from functional single-Trp replacement mutants can be used to obtain information about the local environments of individual Trp residues in multi-tryptophan proteins.

Identification of the factor VIIa binding site on tissue factor by homologous loop swap and alanine scanning mutagenesis

Tissue factor (TF) is a membrane-bound glycoprotein that functions as a cofactor for coagulation factor VIIa (VIIa) and initiates blood coagulation at sites of vascular injury. On the basis of sequence alignments, TF was predicted to be a member of the cytokine receptor superfamily. Utilizing the structural information available for the cytokine receptor superfamily, we have used site-directed mutagenesis to identify the binding site on TF for VIIa. The predicted loop regions in TF were systematically replaced with the homologous loops from the gamma-interferon receptor (gamma-IFN-R), the protein most related to TF in the superfamily of cytokine receptors. Six discontinuous regions (residues 16-20, 40-46, 60-69, 101-111, 129-151, 193-207) were identified that are required for interaction with VIIa and enhancement of activity. Individual substitution of 68 residues within these loops with alanine revealed eight residues (K20, D44, W45, K46, Q110, R135, F140, V207) that are required for cofactor activity. These residues fall into two groups, those that are required only for interactions with VIIa (K46, Q110, R135, F140, V207) and those that are also required to induce the conformational change in VIIa required for enhanced activity (K20, D44, W45). The discontinuous regions of TF required for interactions with VIIa form a single binding surface for VIIa that is analogous to the interface defined by the crystal structure of the complex between growth hormone and its receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue factor pathway

Blood coagulation is initiated in response to vessel damage in order to preserve the integrity of the mammalian vascular system. The coagulation cascade can also be initiated by mediators of the inflammatory response, and fibrin deposition has been noted in a variety of pathological states. The cascade of coagulation zymogen activations which leads to clot formation is initiated by exposure of flowing blood to tissue factor (TF), the cellular receptor and cofactor for factor VII (FVII). FVII binds to the receptor in a 1:1 stoichiometric complex and is rapidly activated. FVIIa undergoes an active site transition upon binding TF in the presence of calcium which enhances the fundamental properties of the enzyme. This results in rapid autocatalytic activation of FVII to VIIa thereby amplifying the response by generating more TF-VIIa complexes. The TF-VIIa activates both FIX and FX. Further FXa generation by the IXa-VIIIa-Ca(2+)-phospholipid complex is required to sustain the coagulation mechanism, since the TF-VIIa complex is rapidly inactivated. Structure and function studies have identified a number of regions on both TF and FVII involved in this interaction. It is clear, however, that the molecular structures of TF, FVII and the TF-VII complex will have to be solved before we fully understand this complex interaction. The activity of the TF-VIIa complex is controlled by two inhibitors:tissue factor pathway inhibitor (TFPI) and antithrombin III (AT-III). TFPI circulates in plasma, is associated with vascular cell surface and is released from platelets following stimulation by thrombin. TFPI requires the formation of an active TF-VIIa complex and FXa generation before inhibition can occur. Similarly, AT-III which is unable to inhibit circulating FVIIa requires the formation of the TF-VIIa complex. TFPI prevents further participation of TF in the coagulation process by forming a stable quaternary complex, TF-VIIa-Xa-TFPI. In contrast, the AT-III-VIIa complex is thought to dissociate from TF allowing it to interact with additional FVII-VIIa. TFPI has been considered the primary regulator of TF-VIIa activity during haemostasis. Whether AT-III in the presence of glycosaminoglycans on cell surfaces expressing TF can function as an auxiliary second physiological regulator is not known.