Studies of the Interaction between Human Protein S and Human C4b-Binding Protein Using Deletion Variants of Recombinant Human Protein S

Anticoagulant protein S-New insights on interactions and functions

Protein S is a critical regulator of coagulation that functions as a cofactor for the activated protein C (APC) and tissue factor pathway inhibitor (TFPI) pathways. It also has direct anticoagulant functions, inhibiting the intrinsic tenase and prothrombinase complexes. Through these functions, protein S regulates coagulation during both its initiation and its propagation phases. The importance of protein S in hemostatic regulation is apparent from the strong association between protein S deficiencies and increased risk for venous thrombosis. This is most likely because both APC and TFPIα are inefficient anticoagulants in the absence of any cofactors. The detailed molecular mechanisms involved in protein S cofactor functions remain to be fully clarified. However, recent advances in the field have greatly improved our understanding of these functions. Evidence suggests that protein S anticoagulant properties often depend on the presence of synergistic cofactors and the formation of multicomponent complexes on negatively charged phospholipid surfaces. Their high affinity binding to negatively charged phospholipids helps bring the anticoagulant proteins to the membranes, resulting in efficient and targeted regulation of coagulation. In this review, we provide an update on protein S and how it functions as a critical hemostatic regulator.

Streptococcal pyrogenic exotoxin B inhibits apoptotic cell clearance by macrophages through protein S cleavage

Clearance of apoptotic cells by macrophages plays an important role in maintaining tissue homeostasis. Previous study indicated that streptococcal pyrogenic exotoxin B (SPE B) reduces phagocytic activity in group A streptococcus (GAS) infection. Here, we demonstrate that SPE B causes an inhibitory effect on protein S-mediated phagocytosis. In the presence of SPE B, serum- and purified protein S-mediated phagocytosis of apoptotic cells were significantly inhibited. The binding abilities of protein S to apoptotic cells were decreased by treatment with SPE B. Bacterial culture supernatants from GAS NZ131 strain also caused a reduction of protein S binding to apoptotic cells, but speB mutant strain did not. SPE B directly cleaved protein S in vitro and in vivo, whereas a lower level of cleavage occurred in mice infected with a speB isogenic mutant strain. SPE B-mediated initial cleavage of protein S caused a disruption of phagocytosis, and also resulted in a loss of binding ability of protein S-associated C4b-binding protein to apoptotic cells. Taken together, these results suggest a novel pathogenic role of SPE B that initiates protein S degradation followed by the inhibition of apoptotic cell clearance by macrophages.

Understanding the functional difference between growth arrest-specific protein 6 and protein S: an evolutionary approach

Although protein S (PROS1) and growth arrest-specific protein 6 (GAS6) proteins are homologous with a high degree of structural similarity, they are functionally different. The objectives of this study were to identify the evolutionary origins from which these functional differences arose. Bioinformatics methods were used to estimate the evolutionary divergence time and to detect the amino acid residues under functional divergence between GAS6 and PROS1. The properties of these residues were analysed in the light of their three-dimensional structures, such as their stability effects, the identification of electrostatic patches and the identification potential protein-protein interaction. The divergence between GAS6 and PROS1 probably occurred during the whole-genome duplications in vertebrates. A total of 78 amino acid sites were identified to be under functional divergence. One of these sites, Asn463, is involved in N-glycosylation in GAS6, but is mutated in PROS1, preventing this post-translational modification. Sites experiencing functional divergence tend to express a greater diversity of stabilizing/destabilizing effects than sites that do not experience such functional divergence. Three electrostatic patches in the LG1/LG2 domains were found to differ between GAS6 and PROS1. Finally, a surface responsible for protein-protein interactions was identified. These results may help researchers to analyse disease-causing mutations in the light of evolutionary and structural constraints, and link genetic pathology to clinical phenotypes.

Amino acid residues in the laminin G domains of protein S involved in tissue factor pathway inhibitor interaction

Protein S functions as a cofactor for tissue factor pathway inhibitor (TFPI) and activated protein C (APC). The sex hormone binding globulin (SHBG)-like region of protein S, consisting of two laminin G-like domains (LG1 and LG2), contains the binding site for C4b-binding protein (C4BP) and TFPI. Furthermore, the LG-domains are essential for the TFPI-cofactor function and for expression of full APC-cofactor function. The aim of the current study was to localise functionally important interaction sites in the protein S LG-domains using amino acid substitutions. Four protein S variants were created in which clusters of surface-exposed amino acid residues within the LG-domains were substituted. All variants bound normally to C4BP and were fully functional as cofactors for APC in plasma and in pure component assays. Two variants, SHBG2 (E612A, I614A, F265A, V393A, H453A), involving residues from both LG-domains, and SHBG3 (K317A, I330A, V336A, D365A) where residues in LG1 were substituted, showed 50-60 % reduction in enhancement of TFPI in FXa inhibition assays. For SHBG3 the decreased TFPI cofactor function was confirmed in plasma based thrombin generation assays. Both SHBG variants bound to TFPI with decreased affinity in surface plasmon resonance experiments. The TFPI Kunitz 3 domain is known to contain the interaction site for protein S. Using in silico analysis and protein docking exercises, preliminary models of the protein S SHBG/TFPI Kunitz domain 3 complex were created. Based on a combination of experimental and in silico data we propose a binding site for TFPI on protein S, involving both LG-domains.

Molecular diversity and thrombotic risk in protein S deficiency: The PROSIT study

The Protein S Italian Team (PROSIT) enrolled 79 protein S (PS) deficient families and found 38 PROS1 variations (19 novel) in 53 probands. Of these, 23 variants were selected for expression in'vitro, to evaluate their role as possible causative variants. Transient expression showed high secretion levels (>75%) for three variants, which were considered neutral. Seven missense and five nonsense variants showed low (<or=11%) expression levels and were classified as severe defects. Intermediate expression was observed for eight variants, which were evaluated by factor Va inactivation assay in order to be globally classified as severe or intermediate. Based on the cumulative data, the hazard ratio associated with causative variants was 4.9 (95% CI: 1.4-17.7) for deep vein thrombosis and/or pulmonary embolism, 5.1 (95% CI: 1.1-23.9) for superficial thrombophlebitis, and 4.8 (95% CI: 1.8-13.0) for any venous thrombosis. The hazard ratio for deep vein thrombosis and/or pulmonary embolism in carriers of severe defects only was 7.4 (95% CI: 1.6-24.1). PROSIT showed that dysfunctional variants causing PS deficiency are more common than expected and confirmed that PS deficiency is associated with increased thrombotic risk, although risk assessment is complicated by molecular heterogeneity.

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.

Functional properties of the sex-hormone-binding globulin (SHBG)-like domain of the anticoagulant protein S

Protein S (PS) possesses a sex-hormone-binding globulin (SHBG)-like domain in place of the serine-protease domain found in other vitamin K-dependent plasma proteins. This SHBG-like domain is able to bind a complement fraction, C4b-binding protein (C4b-BP). To establish whether the PS SHBG-like domain can fold normally in the absence of other domains, and to obtain information on the specific functions of this region, we expressed the PS SHBG-like domain alone or together with its adjacent domain EGF4. The folding of the two recombinant modules was studied by analyzing their binding to C4b-BP. The apparent dissociation constants of this interaction indicated that both recombinant modules adopted the conformation of native PS, indicating that the PS SHBG-like region is an independent folding unit. We also obtained the first direct evidence that the SHBG-like domain alone is sufficient to support the interaction with C4b-BP. In addition, both recombinant modules were able to bind Ca2+ directly, as shown by the migration shift in agarose gel electrophoresis in the presence of Ca2+, together with the results of equilibrium dialysis and the functional effect of Ca2+ on the C4b-BP/PS interaction, confirming the presence of one Ca2+ binding site within the SHBG-like domain. Neither recombinant module exhibited activated protein C (aPC) cofactor activity in a clotting assay, suggesting that the PS SHBG-like region must be part of the intact molecule for it to contribute to aPC cofactor activity, possibly by constraining the different domains in a conformation that permits optimal interaction with aPC.

Structural requirements of anticoagulant protein S for its binding to the complement regulator C4b-binding protein

The vitamin K-dependent anticoagulant protein S binds with high affinity to C4b-binding protein (C4BP), a regulator of complement. Despite the physiological importance of the complex, we have only a patchy view of the C4BP-binding site in protein S. Based on phage display experiments, protein S residues 447-460 were suggested to form part of the binding site. Several experimental approaches were now used to further elucidate the structural requirements for protein S binding to C4BP. Peptides comprising residues 447-460, 451-460, or 453-460 of protein S were found to inhibit the protein S-C4BP interaction, whereas deletion of residues 459-460 from the peptide caused complete loss of inhibition. In recombinant protein S, each of residues 447-460 was mutated to Ala, and the protein S variants were tested for binding to C4BP. The Y456A mutation reduced binding to C4BP approximately 10-fold, and a peptide corresponding to residues 447-460 of this mutant was less inhibitory than the parent peptide. A further decrease in binding was observed using a recombinant variant in which a site for N-linked glycosylation was moved from position 458 to 456 (Y456N/N458T). A monoclonal antibody (HPSf) selective for free protein S reacted poorly with the Y456A variant but reacted efficiently with the other variants. A second antibody, HPS 34, which partially inhibited the protein S-C4BP interaction, reacted poorly with several of the Ala mutants, suggesting that its epitope was located in the 451-460 region. Phage display analysis of the HPS 34 antibody further identified this region as its epitope. Taken together, our results suggest that residues 453-460 of protein S form part of a more complex binding site for C4BP. A recently developed three-dimensional model of the sex hormone-binding globulin-like region of protein S was used to analyze available experimental data.

Sex hormone binding globulins and atherosclerotic risk in systemic lupus

Sex hormone binding globulin (SHBG) is a transport protein in human plasma which regulates the bioavailability of sex hormones, mediates membrane receptor signaling and may affect inflammatory processes, suggesting a regulatory role for this protein in the prevention of atherosclerosis. The current report summarizes literature implicating several members of the SHBG family in the regulation of hormonal and inflammatory processes which may be pertinent to the accelerated atherosclerosis seen in systemic lupus.

C-terminal residues 621-635 of protein S are essential for binding to factor Va

Protein S is anticoagulant in the absence of activated protein C because of direct interactions with coagulation Factors Xa and Va. Synthetic peptides corresponding to amino acid sequences of protein S were tested for their ability to inhibit prothrombinase activity. The peptide containing the C-terminal sequence of protein S, residues 621-635 (PSP14), reversibly inhibited prothrombinase activity in the presence but not in the absence of Factor Va (K(i) approximately 2 microM). PSP14 inhibition of prothrombinase was independent of phospholipids but could be competitively overcome by increasing Factor Xa concentrations, suggesting that the C-terminal region of protein S may compete for a Factor Xa binding site on Factor Va. Studies using peptides with amino acid substitutions suggested that lysines 630, 631, and 633 were critical residues. PSP14 inhibited Factor Va activity in Factor Xa-one-stage clotting assays. PSP14 inhibited protein S binding to immobilized Factor Va. When preincubated with protein S, antibodies raised against PSP14 inhibited binding of protein S to Factor Va and blocked inhibition of prothrombinase activity by protein S. These results show that the C-terminal region of protein S containing residues 621-635 is essential for binding of protein S to Factor Va and that this interaction contributes to anticoagulant action.

Both G-type domains of protein S are required for the high-affinity interaction with C4b-binding protein

Anticoagulant protein S interacts with the complement regulatory protein C4b-binding protein (C4BP) via its sex-hormone-binding globulin (SHB6)-like region, which contains two globular (G) domains. Similar G domains are found in Gas6, a protein homologous to protein S, which is not known to bind C4BP or to have any anticoagulant activity. To determine the relative importance of the two G domains in protein S for C4BP protein binding, three recombinant protein S chimeras were produced having either of the two globular domains, or the whole SHB6-like globulin region, replaced by corresponding parts from Gas6. The chimeras were tested for binding to immobilized C4BP using surface-plasmon-resonance technology and microtiter plate-based assays. In both systems, chimeras containing either only globular domains G1 or G2 from protein S were found to bind C4BP. Binding was stimulated by Ca2+ in a manner similar to that found for wild-type protein S. The affinities for C4BP of both chimeras containing individual G domains from protein S, were lower than that of wild-type protein S. Chimera II, containing the G1 domain from protein S, consistently bound C4BP more efficiently than chimera I, which had the protein S-derived G2 domain. The chimera containing the whole SHB6-like globulin region from Gas6 interacted considerably more weakly with C4BP. Our results demonstrate that both G domains of protein S are involved in the interaction between protein S and C4BP and that full affinity binding is dependent on contributions from both domains.

The molecular genetics of familial venous thrombosis

In the past few years, important advances have been made in the identification of factors predisposing to familial thrombophilia. Particular attention has been paid to the characterization of known inherited defects and their genotype-phenotype relationship, and to studying the interaction between single or multiple inherited conditions and acquired risk factors for venous thrombosis. The recent discovery of 'new' and very common genetic lesions predisposing to thrombosis has greatly expanded the interest in this field. Hereditary predisposition to venous thrombosis may be related to lesions in one or more of 10-15 genes encoding antithrombin, Protein C, Protein S, Factor V, prothrombin, enzymes of the homocysteine metabolic pathway, fibrinogen, heparin cofactor II, plasminogen and thrombomodulin. About 500 different gene lesions (substitutions, deletions, insertions) have so far been reported to affect these genes in patients with thrombotic disease. Because there are potentially multiple interactions between genetic and environmental factors, familial thrombophilia is now considered to be a multifactorial disease. The aim of this chapter is to review aspects of the molecular genetics of familial thrombophilia. In particular, those gene/protein defects for which there is convincing evidence of an association with familial thrombosis will be examined in detail.

Interaction between protein S and complement C4b-binding protein (C4BP). Affinity studies using chimeras containing c4bp beta-chain short consensus repeats

Human C4b-binding protein (C4BP) is a regulator of the complement system and plays an important role in the regulation of the anticoagulant protein C pathway. C4BP can bind anticoagulant protein S, resulting in a decreased cofactor function of protein S for activated protein C. C4BP is a multimeric protein containing several identical alpha-chains and a single beta-chain (C4BPbeta), each chain being composed of short consensus repeats (SCRs). Previous studies have localized the protein S binding site to the NH2-terminal SCR (SCR-1) of C4BPbeta. To further localize the protein S binding site, we constructed chimeras containing C4BPbeta SCR-1, SCR-2, SCR-3, SCR-1+2, SCR-1+3, and SCR-2+3 fused to tissue-type plasminogen activator. Binding assays of protein S with these chimeras indicated that SCR-2 contributes to the interaction of protein S with SCR-1, since the affinity of protein S for SCR-1+2 was up to 5-fold higher compared with SCR-1 and SCR-1+3. Using an assay that measures protein S cofactor activity, we showed that cofactor activity was decreased due to binding to constructs that contain SCR-1. SCR-1+2 inhibited more potently than SCR-1 and SCR-1+3. SCR-3 had no additional effect on SCR-1, and therefore the effect of SCR-2 was specific. In conclusion, beta-chain SCR-2 contributes to the interaction of C4BP with protein S.

The complement regulator C4b-binding protein analyzed by molecular modeling, bioinformatics and computer-aided experimental design

Molecular modeling and bioinformatics have gained recognition as scientific disciplines of importance in the field of biomedical research. Molecular modeling not only allows to predict the three-dimensional structure of a protein but also helps to define its function. Careful incorporation of the experimental findings in the structural/theoretical data provides means to understand molecular mechanisms for highly complex biological systems. C4b-binding protein (C4BP) is composed of one beta-chain and seven alpha-chains essentially built from three- and eight-complement control protein (CCP) modules, respectively, followed by a non-repeat carboxy-terminal region involved in polymerization of the chains. C4BP is involved in the regulation of the complement system and interacts with many molecules such as C4b, Arp, protein S and heparin. Here, we report experimental and computer data obtained for C4BP. Protein modeling together with site directed mutagenesis indicate that R39, R64 and R66 from the C4BP alpha-chain form a key binding site for heparin, suggesting that this region could be of major importance for interaction with C4b. We also propose that the first CCP of the C4BP beta-chain displays a key hydrophobic surface of major importance for the interaction with the coagulation cofactor protein S.

Involvement of amino acid residues 423-429 of human protein S in binding to C4b-binding protein

Human protein S binds to C4b-binding protein (C4BP) both in plasma and in a system using purified proteins. Amino acid residues 420-434 of the first disulfide loop of the sex hormone binding globulinlike domain of protein S are involved in the interaction of protein S with C4BP. To define the involvement of specific polar amino acids within residues 420-434, we studied in parallel synthetic protein S peptides and recombinant protein S variants containing the same amino acid replacements, K423E, E424K, Q427E and K429E. Synthetic peptide analogs of peptide PSP-420 (residues 420-434) were assayed for binding C4BP and as inhibitors of complex formation. The PSP-420 peptide and the analogous peptide with the substitution E424K, but not the peptides containing the substitutions K423E and K429E, were able to bind C4BP. Recombinant proteins with mutations of K423E, Q427E and K429E showed reduced affinity for C4BP compared to plasma protein S, recombinant wild type protein S, or E424K-protein S. These results suggest that Lys-423, Gln-427 and Lys-429 of protein S are important for normal binding to C4BP. The anti-protein S monoclonal antibody LJ-56, raised against peptide PSP-420, recognizes only free protein S and inhibits complex formation with C4BP. Antibody LJ-56 recognized the E424K and Q427E peptides but not the K423E or K429E peptides. Similarly, the E424K and Q427E protein S mutants were recognized by LJ-56, whereas the K423E and K429E protein S mutants were not recognized. This suggests that both in the peptide PSP-420 and in protein S, Lys-423 and Lys-429 significantly contribute to binding to antibody LJ-56. These results demonstrate that protein S residues 423, 427 and 429, but not residue 424, are involved in binding to both the antibody LJ-56 and to C4BP. When peptides PSP 420 and SL-6 (residues 447-460) with carboxyterminal amide or carboxylate moieties were compared to their ability to inhibit C4BP-protein S complexation, PSP-420-amide was the most potent. This finding together with the other results described here supports the hypothesis that the residues 420 and 434 in protein S provides a major binding site for C4BP.

Sequence and functional relationships between androgen-binding protein/sex hormone-binding globulin and its homologs protein S, Gas6, laminin, and agrin

Androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) is an extracellular binding protein that regulates the bioavailability of sex steroids. ABP/SHBG is closely related to the globular (G) domain of vitamin K-dependent protein S family of proteins and more distantly related to the G domains of several extracellular matrix proteins. ABP/SHBG appears to have evolved from the fusion of two ancestral G domains. Expanding evidence suggests that ABP/SHBG has other functions that are mediated through membrane binding, including signal transduction; however, the types of binding proteins (receptors) have not been identified. Sequence comparisons of ABP/SHBG with G domains of its homologs protein S, Gas6, laminin, and agrin have identified regions of ABP/SHBG that may bind receptors related to homolog receptors. These membrane receptors include beta-integrins, alpha-dystroglycan, and receptor tyrosine kinases. The G domains of laminin and related proteins have clearly evolved from a common ancestor to interact with specific receptors and binding proteins. It remains to be determined if ABP/SHBG followed this evolutionary pathway.

A region of vitamin K-dependent protein S that binds to C4b binding protein (C4BP) identified using bacteriophage peptide display libraries

Vitamin K-dependent protein S, a blood coagulation inhibitor, interacts with the C4b-binding protein (C4BP) in human plasma with high affinity (KD = 0.1 nM). Identification of a portion of protein S that binds to C4BP has been approached using random libraries of 6- and 15-mer peptides displayed on bacteriophage surfaces. Bacteriophage binding to the beta-chain of C4BP were selected in several rounds of affinity purification with intervening amplification in E. coli. Homology searches of the affinity purified peptide sequences against protein S led to the identification of four regions in protein S that were similar to several of the selected peptides. These regions were synthesized as linear peptides and tested in inhibition experiments. Only one distinct peak (around position 450) was observed when the homology scores versus human protein S sequence were averaged over all affinity purified peptides. A synthetic peptide comprising residues 439-460 in human protein S was found to inhibit protein S binding to C4BP. The same result was found with two overlapping peptides (residues 447-468 and 435-468, respectively) in a second set of synthetic peptides. Direct binding of the peptides to C4BP was inferred from titrations monitored by recording the near UV circular dichroism spectra or the polarization of tryptophan fluorescence. The results suggest that residues 447-460 constitute a portion of protein S that is important for the interaction with C4BP. These findings may have implications for patients suffering from thrombosis, due to the lack of free protein S, by directing the design of drugs that disrupt protein S binding to C4BP.

Expression and functional characterization of chimeras between human and bovine vitamin-K-dependent protein-S-defining modules important for the species specificity of the activated protein C cofactor activity

Vitamin-K-dependent protein S is an anticoagulant plasma protein functioning as a cofactor to activated protein C (APC) in the degradation of factors Va and VIIIa. The APC-cofactor function of protein S is species specific, as human protein S potentiates the anticoagulant activity of human but not that of bovine APC, whereas bovine protein S is a cofactor to APC from both species. To elucidate which modules in protein S determine the species specificity, in vitro mutagenesis was used to construct six recombinant chimeric molecules between human and bovine protein S. Wild-type human and bovine protein S and the chimeras were expressed in 293 cells and the recombinant proteins purified by monoclonal antibody affinity chromatography. The recombinant proteins were found to be post-translationally modified, they bound C4b-binding protein and were functionally active as cofactors to APC. Chimeras having both the thrombin-sensitive region (TSR) and the first epidermal-growth-factor-(EGF)-like module of bovine origin expressed APC-cofactor activity similar to that of bovine protein S. Those chimeras, in which TSR or EGF1 derived from different species, manifested APC-cofactor activity similar to that of human protein S, i.e. they did not express cofactor activity to bovine APC. These data indicate that sequence differences in the TSR and EGF1 of human and bovine protein S cause the species specificity of the APC-cofactor activity. The data support the concept that these two modules of protein S interact with APC on the surface of negatively charged phospholipids.