Characterization of recombinant epidermal growth factor (EGF)-like modules from vitamin-K-dependent protein S expressed in Spodoptera cells--the cofactor activity depends on the N-terminal EGF module in human protein S

Antigenic binding sites of anti-protein S autoantibodies in patients with recurrent pregnancy loss

BACKGROUND

Protein S (PS) deficiency is a risk factor for adverse pregnancy outcomes including recurrent pregnancy loss. Several studies have shown that the presence of anti-PS autoantibodies (anti-PS) leads to an acquired PS deficiency. Hence, an epitope mapping study was conducted to know the pathogenesis of anti-PS in patients with recurrent pregnancy loss.

METHODS

PS was treated with thrombin to divide the protein into γ-carboxyglutamic acid (Gla) domain and Gla-domain free PS. For the preparation of fragments of epidermal growth factor (EGF)-like domains (EGF1-4), PS was subjected to proteolysis using lysyl endopeptidase. The epitopes were identified in immunoblot. Whether anti-PS recognized EGF family proteins in anti-PS-positive patients was also examined.

RESULTS

Anti-PS recognized Gla-domain free PS, especially the three fragments of EGF-like domains, EGF1-2, EGF3-4, and EGF1-4. Anti-PS recognized recombinant human EGF. Anti-PS and polyclonal antibodies to recombinant human EGF recognized PS in the absence of Ca2+ but not in the presence of Ca2+. In competitive inhibition studies, polyclonal antibodies to recombinant mouse EGF blocked anti-PS binding to PS in a concentration-dependent manner.

CONCLUSIONS

These results suggest that anti-PS in patients with recurrent pregnancy loss recognize EGF-like domains in PS. Interestingly, anti-PS also recognized EGF family proteins. Anti-PS in patients with recurrent pregnancy loss may be associated with not only thrombophilia but also the disruption of the EGF system.

The anticoagulant function of coagulation factor V

Almost two decades ago an anticoagulant function of factor V (FV) was discovered, as an anticoagulant cofactor for activated protein C (APC). A natural mutant of FV in which the R506 inactivation site was mutated to Gln (FVLeiden) was inactivated slower by APC, but also could not function as anticoagulant cofactor for APC in the inactivation of activated factor VIII (FVIIIa). This mutation is prevalent in populations of Caucasian descent, and increases the chance of thrombotic events in carriers. Characterisation of the FV anticoagulant effect has elucidated multiple properties of the anticoagulant function of FV: 1) Cleavage of FV at position 506 by APC is required for anticoagulant function. 2) The C-terminal part of the FV B domain is required and the B domain must have an intact connection with the A3 domain of FV. 3) FV must be bound to a negatively charged phospholipid membrane. 4) Protein S also needs to be present. 5) FV acts as a cofactor for inactivation of both FVa and FVIIIa. 6) The prothrombotic function of FVLeiden is a function of both reduced APC cofactor activity and resistance of FVa to APC inactivation. However, detailed structural and mechanistic properties remain to be further explored.

Activated protein C cofactor function of protein S: a critical role for Asp95 in the EGF1-like domain

Protein S has an established role in the protein C anticoagulant pathway, where it enhances the factor Va (FVa) and factor VIIIa (FVIIIa) inactivating property of activated protein C (APC). Despite its physiological role and clinical importance, the molecular basis of its action is not fully understood. To clarify the mechanism of the protein S interaction with APC, we have constructed and expressed a library of composite or point variants of human protein S, with residue substitutions introduced into the Gla, thrombin-sensitive region (TSR), epidermal growth factor 1 (EGF1), and EGF2 domains. Cofactor activity for APC was evaluated by calibrated automated thrombography (CAT) using protein S-deficient plasma. Of 27 variants tested initially, only one, protein S D95A (within the EGF1 domain), was largely devoid of functional APC cofactor activity. Protein S D95A was, however, gamma-carboxylated and bound phospholipids with an apparent dissociation constant (Kd(app)) similar to that of wild-type (WT) protein S. In a purified assay using FVa R506Q/R679Q, purified protein S D95A was shown to have greatly reduced ability to enhance APC-induced cleavage of FVa Arg306. It is concluded that residue Asp95 within EGF1 is critical for APC cofactor function of protein S and could define a principal functional interaction site for APC.

Plasma protein S contains zinc essential for efficient activated protein C-independent anticoagulant activity and binding to factor Xa, but not for efficient binding to tissue factor pathway inhibitor

Protein S (PS) is a cofactor for activated protein C (APC), which inactivates coagulation factors (F) Va and VIIIa. Deficiency of protein C or PS is associated with risk of thrombosis. We found that PS also has APC-independent anticoagulant activity (PS-direct) and directly inhibits thrombin generated by FXa/FVa (prothrombinase complex). Here we report that PS contains Zn(2+) that is required for PS-direct and that is lost during certain purification procedures. Immunoaffinity-purified PS contained 1.4 +/- 0.6 Zn(2+)/mol, whereas MonoQ-purified and commercial PS contained 0.15 +/- 0.15 Zn(2+)/mol. This may explain the controversy regarding the validity of PS-direct. Zn(2+) content correlated positively with PS-direct in prothrombinase assays and clotting assays, but APC-cofactor activity of PS was independent of Zn(2+) content. PS-direct and Zn(2+) were restored to inactive PS under mildly denaturing conditions. Conversely, o-phenanthroline reversibly impaired the PS-direct of active PS. Zn(2+)-containing PS bound FXa more efficiently (K(d)(app)=9.3 nM) than Zn(2+)-deficient PS (K(d)(app)=110 nM). PS bound TFPI efficiently, independently of Zn(2+) content (K(d)(app)=21 nM). Antibodies that block PS-direct preferentially recognized Zn(2+)-containing PS, suggesting conformation differences at or near the interface of 2 laminin G-like domains near the PS C terminus. Thus, Zn(2+) is required for PS-direct and efficient FXa binding and may play a role in stabilizing PS conformation.

Functional Analysis of the Factor IX Epidermal Growth Factor-Like Domain Mutation Ile66Thr Associated with Mild Hemophilia B

The present study focused on the functional role of the mutation Ile66Thr located in the N-terminal epidermal growth factor-like domain of coagulation factor IX (FIX). This mutation causes mild hemophilia B with approximately 25% FIX coagulant activity and FIX antigen levels of around 90% of normal. In the 3-dimensional structure of porcine FIXa and in the subsequent 3-dimensional model of human FIXa that we have previously developed, residue 66 is exposed to the solvent and can be replaced by many amino acids, including Thr, without affecting the major folding/stability of the molecule. This is consistent with the basically normal antigen levels observed. We found that the FIX Ile66Thr mutant was activated to a normal extent by FVIIa/TF and FXIa. However, the ability of FIX Ile66Thr to activate FX was impaired in both the presence and absence of FVIIIa, indicating that Ile66 is not directly involved in the binding of FIX to FVIIIa.

Binding of calcium to anticoagulant protein S: role of the fourth EGF module

Protein S is an anticoagulant protein containing a Gla (enclosing gamma-carboxyglutamic acids) module, a TSR (thrombin sensitive region) module, four EGF (epidermal growth factor)-like modules, and a SHBG (sex hormone binding globulin)-like region. Protein S is a cofactor to activated protein C (APC) in the degradation of coagulation factors Va and VIIIa but also has APC-independent activities. The function of the fourth EGF module (EGF4) in protein S has so far not been clear. We have now investigated this module through studies of recombinant wild-type protein S and a naturally occurring mutant (Asn217Ser). The mutant has essentially normal APC anticoagulant activity and a previously reported secretion defect. In the wild-type protein, Asn217 is normally beta-hydroxylated. The binding of calcium to wild-type protein S is characterized by four high-affinity binding sites with K(D) values ranging from 10(-)(7) to 10(-)(9) M. Three of these binding sites are located in EGF modules. Using surface plasmon resonance, competition with a calcium chelator, and antibody-based methods, we found that one high-affinity binding site for calcium was lost in protein S Asn217Ser but that the mutation also affected the calcium-dependent conformation of EGF1. We conclude that binding of calcium to EGF4 of protein S, involving Asn217, is important for the maintenance of the structure of protein S. Also, the abolition of binding of calcium to EGF4, related to Asn217, impairs both the structure and function of EGF1.

Multifunctional specificity of the protein C/activated protein C Gla domain

Activated protein C (APC) has potent anticoagulant and anti-inflammatory properties that are mediated in part by its interactions with its cofactor protein S and the endothelial cell protein C receptor (EPCR). The protein C/APC Gla domain is implicated in both interactions. We sought to identify how the protein C Gla domain enables specific protein-protein interactions in addition to its conserved role in phospholipid binding. The human prothrombin Gla domain, which cannot bind EPCR or support protein S cofactor activity, has 22/45 residues that are not shared with the human protein C Gla domain. We hypothesized that the unique protein C/APC Gla domain residues were responsible for mediating the specific interactions. To assess this, we generated 13 recombinant protein C/APC variants incorporating the prothrombin residue substitutions. Despite anticoagulant activity similar to wild-type APC in the absence of protein S, APC variants APC(PT33-39) (N33S/V34S/D35T/D36A/L38D/A39V) and APC(PT36/38/39) (D36A/L38D/A39V) were not stimulated by protein S, whereas APC(PT35/36) (D35T/D36A) exhibited reduced protein S sensitivity. Moreover, PC(PT8/10) (L8V/H10K) displayed negligible EPCR affinity, despite normal binding to anionic phospholipid vesicles and factor Va proteolysis in the presence and absence of protein S. A single residue variant, PC(PT8), also failed to bind EPCR. Factor VIIa, which also possesses Leu-8, bound soluble EPCR with similar affinity to wild-type protein C, collectively confirming Leu-8 as the critical residue for EPCR recognition. These results reveal the specific Gla domain residues responsible for mediating protein C/APC molecular recognition with both its cofactor and receptor and further illustrate the multifunctional potential of Gla domains.

Solution structure of the Ca2+-Binding EGF3-4 pair from vitamin K-dependent protein S: identification of an unusual fold in EGF3

Vitamin K-dependent protein S is a cofactor of activated protein C, a serine protease that regulates blood coagulation. Deficiency of protein S can cause venous thrombosis. Protein S has four EGF domains in tandem; domains 2-4 bind calcium with high affinity whereas domains 1-2 mediate interaction with activated protein C. We have now solved the solution structure of the EGF3-4 fragment of protein S. The linker between the two domains is similar to what has been observed in other calcium-binding EGF domains where it provides an extended conformation. Interestingly, a disagreement between NOE and RDC data revealed a conformational heterogeneity within EGF3 due to a hinge-like motion around Glu186 in the Cys-Glu-Cys sequence, the only point in the domain where flexibility is allowed. The dominant, bent conformation of EGF3 in the pair has no precedent among calcium-binding EGF domains. It is characterized by a change in the psi angle of Glu186 from 160 degrees +/- 40 degrees , as seen in ten other EGF domains, to approximately 0 degrees +/- 15 degrees . NOESY data suggest that Tyr193, a residue not conserved in other calcium-binding EGF domains (except in the homologue Gas6), induces the unique fold of EGF3. However, SAXS data, obtained on EGF1-4 and EGF2-4, showed a dominant, extended conformation in these fragments. This may be due to a counterproductive domain-domain interaction between EGF2 and EGF4 if EGF3 is in a bent conformation. We speculate that the ability of EGF3 to adopt different conformations may be of functional significance in protein-protein interactions involving protein S.

The γ-carboxyglutamic acid domain of anticoagulant protein S is involved in activated protein C cofactor activity, independently of phospholipid binding

We expressed 2 chimeras between human protein S (PS) and human prothrombin (FII) in which the prothrombin γ-carboxyglutamic acid (Gla) domain replaced the PS Gla domain in native PS (GlaFII-PS) or in PS deleted of the thrombin-sensitive region (TSR) (GlaFII-ΔTSR-PS). Neither PS/FII chimera had activated protein C (APC) cofactor activity in plasma clotting assays or purified systems, but both bound efficiently to phospholipids. This pointed to a direct involvement of the PS Gla domain in APC cofactor activity through molecular interaction with APC. Using computational methods, we identified 2 opposite faces of solvent-exposed residues on the PS Gla domain (designated faces 1 and 2) as potentially involved in this interaction. Their importance was supported by functional characterization of a PS mutant in which the face 1 and face 2 PS residues were reintroduced into GlaFII-PS, leading to significant APC cofactor activity, likely through restored interaction with APC. Furthermore, by characterizing PS mutants in which PS face 1 and PS face 2 were individually replaced by the corresponding prothrombin faces, we found that face 1 was necessary for efficient phospholipid binding but that face 2 residues were not strictly required for phospholipid binding and were involved in the interaction with APC.

N-Terminal Domain Linkage Modulates the Folding Properties of Protein S Epidermal Growth Factor-like Modules

Protein S interacts with activated protein C to play a crucial role in blood anticoagulation, and protein S deficiency is associated with increased risk of thrombosis. Despite the large volume of functional data available for this protein, no atomic resolution structure data have yet been reported. This is due at least in part to difficulties encountered when trying to produce fragments dissected from the intact protein; however, a few successful strategies have been described. In this research we have expressed a number of constructs containing protein S epidermal growth factor-like (EGF) domains 1 and 2 in Escherichia coli and Pichia pastoris. None of the proteins produced was stably folded as assayed by solution nuclear magnetic resonance spectroscopy. We therefore constructed a series of non-native protein S EGF concatemers to investigate the role of pairwise domain linkage in domain folding. Our results demonstrate that N-terminal domain linkage can either positively or negatively impact on the refolding of an adjacent domain. Furthermore, analysis of the NMR data for EGF3-4 reveals the expected interdomain NOEs that are characteristic of an extended arrangement of calcium-binding EGF domains and a similar average [(1)H]-(15)N heteronuclear NOE value for each of the two domains. These results provide the first data in support of protein S EGF3-4 adopting the same extended domain orientation as observed for the functionally distinct proteins fibrillin-1 and the low-density lipoprotein receptor. The results also have important implications for future studies, particularly when a dissection approach is used, of tandem EGF domains from protein S and other proteins.

Coagulation, inflammation, and apoptosis: different roles for protein S and the protein S-C4b binding protein complex

Protein S (PS) has an established role as an important cofactor to activated protein C (APC) in the degradation of coagulation cofactors Va and VIIIa. This anticoagulant role is evident from the consequences of its deficiency, when there is an increased risk of venous thromboembolism. In human plasma, PS circulates approximately 40% as free PS (FPS) and 60% in complex with C4b-binding protein (C4BP). Formation of this complex results in loss of PS cofactor function, and C4BP can then modulate the anticoagulant activity of APC. It had long been predicted that the complex could act as a bridge between coagulation and inflammation due to the involvement of C4BP in regulating complement activation. This prediction was recently supported by the demonstration of binding of the PS-C4BP complex to apoptotic cells. This review aims to summarize recent findings on the structure and functions of PS, the basis and importance of its deficiency, its interaction with C4BP, and the possible physiologic and pathologic importance of the PS-C4BP interaction.

Functional analysis of the EGF-like domain mutations Pro55Ser and Pro55Leu, which cause mild hemophilia B

We studied the functional role of two mutations, Pro55Ser and Pro55Leu, located in the N-terminal Epidermal Growth Factor-like domain (EGF1) of coagulation factor (F) IX. Both mutations cause mild hemophilia B with habitual FIX coagulant activities of 10-12% and FIX antigen levels of 50%. We found that activation by FVIIa/TF and FXIa was normal for FIXPro55Ser, but resulted in proteolysis of FIXPro55Leu at Arg318-Ser319 with a concomitant loss of amidolytic activity, suggesting intramolecular communication between EGF1 and the serine protease domain in FIX. This was further supported by experiments using an anti-EGF1 monoclonal antibody. Activation of FX by FIXaPro55Ser was impaired in both the presence and the absence of phospholipid or FVIIIa, indicating that Pro55 is not directly involved in binding to FVIIIa. We also studied the effect of the two Pro55 mutations on Ca2+ affinity and found only small changes. Thus, the Pro55Ser mutation causes hemophilia primarily through to an impaired ability to activate FX whereas at least in vitro the Pro55Leu defect interferes with the activation of FIX.

Deletion or replacement of the second EGF-like domain of protein S results in loss of APC cofactor activity

Human protein S (PS), a cofactor of anticoagulant-activated protein C (APC), is a modular protein containing 4 epidermal growth factor (EGF)-like domains. EGF1 appears to mediate PS interaction with APC, but the roles of EGFs 2, 3, and 4 are less clear. We synthesized PS variants lacking single EGF domains (EGF2, 3, or 4) and assessed their APC cofactor activity in a factor Va inactivation assay. The variant lacking EGF2 (variant 134) showed the most dramatic loss of activity (approximately 10% of recombinant wild-type PS activity). Replacement of EGF2 by an additional EGF3 (variant 1334) resulted in a comparable loss of activity, suggesting that the loss of a specific rather than "spacer" function of EGF2 was responsible. We confirmed that the variant 134 had a functional gamma-carboxyglutamic acid (Gla) domain and that EGF1 was correctly folded. This is the first clear evidence that EGF2 is required for the expression of PS activity.

The N-terminal epidermal growth factor-like domain of coagulation factor IX. Probing its functions in the activation of factor IX and factor X with a monoclonal antibody

The absence or reduced activity of coagulation factor IX (FIX) causes the severe bleeding disorder hemophilia B. FIX contains an N-terminal Gla domain followed by two epidermal growth factor-like (EGF) domains and a serine protease domain. In this study, the epitope of monoclonal antibody AW, which is directed against the C-terminal part of the first EGF domain in human FIX, was defined, and the antibody was used to study interactions between the EGF domain of FIX and other coagulation proteins. Antibody AW completely blocks activation of FIX by activated factor XI, but activation by activated factor FVII-tissue factor is inhibited only slightly. The antibody also causes a marginal reduction in the apparent k(cat) for factor X both in the presence and absence of activated factor VIII. Based on these results, we produced a preliminary model of the structure of the activated factor IX-activated factor VIII-AW complex on the surface of phospholipid. The model suggests that in the Xase complex, EGF1 of activated factor IX is not involved in direct binding to activated factor VIII. Studies of the interaction of antibody AW with a mutated FIX molecule (R94D) also suggest that the Glu(78)-Arg(94) salt bridge is not important for maintaining the structure of FIX.

An anti-EGF monoclonal antibody that detects intramolecular communication in factor IX

Coagulation factor IX contains a gamma-carboxyglutamic acid (Gla) module, two epidermal growth factor-like (EGF) modules, and a serine protease region. We have characterized a mouse monoclonal antibody that binds the N-terminal EGF-like module of human factor IX with high affinity. Studies of recombinant factor IX mutants indicated that the epitope is located in the C-terminal end of the EGF-like module, which is consistent with the binding being non-Ca(2+)-dependent. The antibody bound factor IXa (K(D) = 7.6 x 10(-10) M) with about 10-fold higher affinity than factor IX (K(D) = 6.2 x 10(-9) M). Binding of the antibody to factor IXa did not affect the amidolytic activity of the protein, nor was binding affected by active site inhibition of factor IXa. These results are consistent with long-range interactions between the serine protease region and the N-terminal EGF-like module in factor IX.

Characterization of the EGF-like module pair 3-4 from vitamin K-dependent protein S using NMR spectroscopy reveals dynamics on three separate time scales and extensive effects from calcium binding

Protein S, a cofactor of anticoagulant activated protein C, exhibits three high-affinity Ca(2+)-binding sites in a region comprising four EGF modules. The EGF 3-4 module pair constitutes the smallest fragment that retains one high-affinity Ca(2+)-binding site and is therefore useful for investigation of the structural basis of the unusually high-affinity Ca(2+) binding compared to other EGF-containing proteins characterized so far. Extensive chemical shift effects caused by Ca(2+) binding to the EGF 3-4 module pair are observed, particularly from Ca(2+) binding to the high-affinity site in EGF 4. Ca(2+) binding to the high-affinity site in EGF 4 and the low-affinity site in EGF 3 is associated with slow and fast exchange on the NMR time-scale, respectively. We show the presence of two isoforms, characterized by a cis or trans Lys 167-Pro 168 peptide bond, that do not convert on time scales that were accessible to the experiments (k(ex) < 0.2 s(-1)). Both conformers have similar Ca(2+) affinities and backbone dynamics. Further, broadening of (1)H resonances involving residues in the major beta-sheet of EGF 3 and (15)N exchange terms, primarily in the N-terminal part of the protein, indicate the presence of slow exchange on a microsecond to millisecond time scale. (15)N spin relaxation data suggest that the module pair has a well-defined relative orientation between EGF modules 3 and 4 and has a significantly anisotropic rotational diffusion tensor in solution.

Conformational changes in activated protein C caused by binding of the first epidermal growth factor-like module of protein S

The first epidermal growth factor-like module of human plasma protein S (EGF1, residues 76-116) was chemically synthesized and tested for its ability to inhibit the anticoagulant cofactor activity of protein S for the anticoagulant protease, activated protein C (APC). EGF1 completely inhibited the stimulation of APC activity by protein S in plasma coagulation assays, with 50% inhibition at approx. 1 microM+ EGF1, suggesting direct binding of EGF1 to APC. To investigate a direct interaction between EGF1 and APC, fluorescence resonance energy transfer (FRET) experiments were employed. APC labelled in the active site with fluorescein as the donor, and phospholipid vesicles containing octadecylrhodamine as the acceptor, showed that EGF1 association with APC caused an increase in energy transfer consistent with a relocation of the active site of APC from 94 A (9.4 nm) to 85 A above the phospholipid surface (assuming kappa(2)=2/3). An identical increase in energy transfer between the APC active site-bound fluorescein and phospholipid-bound rhodamine was obtained upon association of protein S or protein S-C4b-binding protein complex with APC. The latter suggests the presence of a ternary complex of protein S-C4b-binding protein with APC on the phospholipid surface. To confirm a direct interaction of EGF1 with APC, rhodamine was covalently attached to the alpha-N-terminus of EGF1, and binding of the labelled EGF1 to APC was directly demonstrated using FRET. The data suggested a separation between the active site of APC and the N-terminus of EGF1 of 76 A (kappa(2)=2/3), placing the APC-bound protein S-EGF1 close to, but above, the phospholipid surface and near the two EGF domains of APC. Thus we provide direct evidence for binding of protein S-EGF1 to APC and show that it induces a conformational change in APC.

Calcium-binding EGF-like modules in coagulation proteinases: function of the calcium ion in module interactions

Epidermal growth factor (EGF)-like modules are involved in protein-protein interactions and are found in numerous extracellular proteins and membrane proteins. Among these proteins are enzymes involved in blood coagulation, fibrinolysis and the complement system as well as matrix proteins and cell surface receptors such as the EGF precursor, the low density lipoprotein receptor and the developmentally important receptor, Notch. The coagulation enzymes, factors VII, IX and X and protein C, all have two EGF-like modules, whereas the cofactor of activated protein C, protein S, has four EGF-like modules in tandem. Certain of the cell surface receptors have numerous EGF modules in tandem. A subset of EGF modules bind one Ca(2+). The Ca(2+)-binding sequence motif is coupled to a sequence motif that brings about beta-hydroxylation of a particular Asp/Asn residue. Ca(2+)-binding to an EGF module is important to orient neighboring modules relative to each other in a manner that is required for biological activity. The Ca(2+) affinity of an EGF module is often influenced by its N-terminal neighbor, be it another EGF module or a module of another type. This can result in an increase in Ca(2+) affinity of several orders of magnitude. Point mutations in EGF modules that involve amino acids which are Ca(2+) ligands result in the biosynthesis of biologically inactive proteins. Such mutations have been identified, for instance, in factor IX, causing hemophilia B, in fibrillin, causing Marfan syndrome, and in the low density lipoprotein receptor, causing hypercholesterolemia. In this review the emphasis will be on the coagulation factors.

EGF-like module pair 3-4 in vitamin K-dependent protein S: modulation of calcium affinity of module 4 by module 3, and interaction with factor X

Calcium-binding epidermal growth factor (EGF)-like modules are found in numerous extracellular and membrane proteins involved in such diverse processes as blood coagulation, lipoprotein metabolism, determination of cell fate, and cell adhesion. Vitamin K-dependent protein S, a cofactor of the anticoagulant enzyme activated protein C, has four EGF-like modules in tandem with the three C-terminal modules each harbouring a Ca(2+)-binding consensus sequence. Recombinant fragments containing EGF modules 1-4 and 2-4 have two Ca(2+)-binding sites with dissociation constants ranging from 10(-8) to 10(-5) M. Module-module interactions that greatly influence the Ca(2+) affinity of individual modules have been identified. As a step towards an analysis of the structural basis of the high Ca(2+) affinity, we expressed the Ca(2+)-binding EGF pair 3-4 from human protein S. Correct folding was shown by (1)H NMR spectroscopy. Calcium-binding properties of the C-terminal module were determined by titration with chromophoric chelators; binding to the low-affinity N-terminal site was monitored by (1)H-(15)N NMR spectroscopy. At physiological pH and ionic strength, the dissociation constants for Ca(2+) binding were 1.0x10(-6) M and 4. 8x10(-3) M for modules 4 and 3, respectively, i.e. the calcium affinity of the C-terminal site was about 5000-fold higher than that of the N-terminal site. Moreover, the Ca(2+) affinity of EGF 4, in the pair 3-4, was about 9000-fold higher than that of synthetic EGF 4. The EGF modules in protein S are known to mediate the interaction with factor Xa. We have now found modules 3-4 to be involved in this interaction. However, the individual modules 3 and 4 manifested no measurable activity.

Amino acid residues in thrombin-sensitive region and first epidermal growth factor domain of vitamin K-dependent protein S determining specificity of the activated protein C cofactor function

Human protein S (PS) potentiates the anticoagulant activity of human but not bovine activated protein C (APC), whereas bovine PS is a cofactor to APC from both species. The structural requirements for the specificity of the APC cofactor function of human PS are located in its thrombin-sensitive region (TSR) and the first epidermal growth factor (EGF1)-like module. To elucidate which residues in these two modules determine the specificity of the APC cofactor activity, 41 human PS mutants were expressed. All mutants were cofactors to human APC and some also to bovine APC. Residues in TSR (positions 49 and 52) and EGF1 (residues 97 and 106) together determined the specificity of the APC cofactor function, whereas substitution of individual residues did not change specificity. Bovine PS, and mutants expressing cofactor activity to bovine APC, stimulated phospholipid binding of bovine APC. In contrast, human PS and mutants lacking cofactor activity to bovine APC failed to support binding of bovine APC to phospholipids. These data indicate that residues in TSR and EGF1 cause the specificity of the APC cofactor activity and support the concept that key residues in these two modules interact with APC on the phospholipid surface.

The SHBG-like region of protein S is crucial for factor V-dependent APC-cofactor function

Activated protein C (APC) regulates blood coagulation by degrading factor Va (FVa) and factor VIIIa (FVIIIa). Protein S is a cofactor to APC in the FVa degradation, whereas FVIIIa degradation is potentiated by the synergistic APC-cofactor activity of protein S and factor V (FV). To elucidate the importance of the sex-hormone-binding globulin (SHBG)-like region in protein S for expression of anticoagulant activity, a recombinant protein S/Gas6 chimera was constructed. It comprised the amino-terminal half of protein S and the SHBG-like region of Gas6, a structurally similar protein having no known anticoagulant properties. The protein S/Gas6 chimera expressed 40-50%, APC-cofactor activity in plasma as compared to wild-type protein S. In the degradation of FVa by APC, the protein S/Gas6 chimera was only slightly less efficient than wild-type protein S. In contrast, the protein S/Gas6 chimera expressed no FV-dependent APC-cofactor activity in a FVIIIa-degradation system. This demonstrates the SHBG-like region to be important for expression of APC-cofactor activity of protein S and suggests that the SHBG-like region of protein S interacts with FV during the APC-mediated inactivation of FVIIIa.