Bordetella pertussis binds to human C4b-binding protein (C4BP) at a site similar to that used by the natural ligand C4b

Human complement regulators are important targets for pathogenic microorganisms. In one such interaction, Bordetella pertussis binds human C4b-binding protein (C4BP), a high-molecular-weight plasma protein that acts as inhibitor of the classical pathway of complement activation. At least two different B. pertussis surface components, one of which is the virulence factor filamentous hemagglutinin (FHA), contribute to the binding. We used a set of C4BP mutants and monoclonal antibodies to characterize the region in C4BP that binds B. pertussis and analyzed the salt sensitivity of the interaction. These studies indicated that positively charged residues at the interface between complement control protein modules 1-2 in the C4BP alpha-chain are important for binding, and that the site in C4BP that binds B. pertussis is very similar, but not identical, to the C4b-binding site. Bacteria-bound C4BP retained its complement regulatory function and B. pertussis selectively bound C4BP in human plasma, indicating that binding occurs also in vivo. Together, these findings indicate that B. pertussis exploits a site in C4BP, resembling that used by the natural ligand C4b.

Structural Requirements for the Complement Regulatory Activities of C4BP

C4b-binding protein (C4BP) is a regulator of the classical complement pathway C3 convertase (C4bC2a complex). It is a disulfide-linked polymer of seven alpha-chains and a unique beta-chain; the alpha- and beta-chains are composed of eight and three complement control protein (CCP) domains, respectively. To elucidate the importance of the polymeric nature of C4BP and the structural requirements for the interaction between C4b and the alpha-chain, 19 recombinant C4BP variants were created. Six truncated monomeric variants, nine polymeric variants in which individual CCPs were deleted, and finally, four variants in which double alanine residues were introduced between CCPs were functionally characterized. The smallest truncated C4BP variant still active in regulating fluid phase C4b comprised CCP1-3. The monomeric variants were less efficient than polymeric C4BP in degrading C4b on cell surfaces. All three N-terminal CCP domains contributed to the binding of C4b and were important for full functional activity; CCP2 and CCP3 were the most important. The spatial arrangements of the first CCPs were found to be important, as introduction of alanine residues between CCPs 1 and 2, CCPs 2 and 3, and CCPs 3 and 4 resulted in functional impairment. The results presented here elucidate the structural requirements of individual CCPs of C4BP, as well as their spatial arrangements within and between subunits for expression of full functional activity.

Structural and energetic characteristics of the heparin-binding site in antithrombotic protein C

Human activated protein C (APC) is a key component of a natural anticoagulant system that regulates blood coagulation. In vivo, the catalytic activity of APC is regulated by two serpins, alpha1-antitrypsin and the protein C inhibitor (PCI), the inhibition by the latter being stimulated by heparin. We have identified a heparin-binding site in the serine protease domain of APC and characterized the energetic basis of the interaction with heparin. According to the counter-ion condensation theory, the binding of heparin to APC is 66% ionic in nature and comprises four to six net ionic interactions. To localize the heparin-binding site, five recombinant APC variants containing amino acid exchanges in loops 37, 60, and 70 (chymotrypsinogen numbering) were created. As demonstrated by surface plasmon resonance, reduction of the electropositive character of loops 37 and 60 resulted in complete loss of heparin binding. The functional consequence was loss in heparin-induced stimulation of APC inhibition by PCI, whereas the PCI-induced APC inhibition in the absence of heparin was enhanced. Presumably, the former observations were due to the inability of heparin to bridge some APC mutants to PCI, whereas the increased inhibition of certain APC variants by PCI in the absence of heparin was due to reduced repulsion between the enzymes and the serpin. The heparin-binding site of APC was also shown to interact with heparan sulfate, albeit with lower affinity. In conclusion, we have characterized and spatially localized the functionally important heparin/heparan sulfate-binding site of APC.

C4bp binding to porin mediates stable serum resistance of Neisseria gonorrhoeae

Screening of 29 strains of Neisseria gonorrhoeae revealed that 16/21 serum resistant strains and 0/8 serum sensitive strains bound C4bp, suggesting that C4bp binding to gonococci could contribute to serum resistance. C4bp bound to gonococci retained cofactor (C4b-degrading) function. Using allelic exchange to construct strains with hybrid Por1A/B molecules, we demonstrate that the N-terminal loop (loop 1) of Por1A is required for C4bp binding. Serum resistant Por1B gonococcal strains also bind C4bp via their Por molecule. Using allelic exchange and site-directed mutagenesis, we have shown that loops 5 and 7 together form a negatively charged C4bp binding domain. C4bp-Por1B interactions are ionic in nature (inhibited by high salt as well as by heparin), while the C4bp-Por1A bond is hydrophobic. mAbs directed against SCR1 of the alpha-chain of C4bp inhibit C4bp binding to both Por1A and Por1B. Furthermore, only recombinant C4bp mutant molecules that contain alpha-chain SCR1 bind both Por1A and Por1B gonococci, confirming that SCR1 contains Por binding sites. C4bp alpha-chain monomers do not bind strains with either Por molecule, suggesting that the polymeric form of C4bp is required for binding to gonococci. Inhibition of C4bp binding to serum resistant Por1A and Por1B strains in a serum bactericidal assay using fAb fragments against C4bp SCR1 results in complete killing at 30 min of otherwise fully serum resistant strains in only 10% normal serum, underscoring the role of C4bp in mediating gonococcal serum resistance.

Secondary substrate-binding exosite in the serine protease domain of activated protein C important for cleavage at Arg-506 but not at Arg-306 in factor Va

Proteolytic inactivation of activated factor V (FVa) by activated protein C (APC) is a key reaction in the regulation of hemostasis. We now demonstrate the importance of a positive cluster in loop 37 of the serine protease (SP) domain of APC for the degradation of FVa. Lysine residues in APC at positions 37, 38, and 39 form a secondary binding site for FVa, which is important for cleavage of FVa at Arg-506 while having no effect on Arg-306 cleavage. In contrast, topological neighbors Lys-62, Lys-63, and Arg-74 in APC appear of minor importance in FVa degradation. This demonstrates that secondary binding exosites of APC specifically guide the proteolytic action of APC, resulting in a more favorable degradation of the 506-507 peptide bond as compared with the 306-307 bond.

Proposed lipocalin fold for apolipoprotein M based on bioinformatics and site-directed mutagenesis

Apolipoprotein M (apoM) is a novel apolipoprotein that is predominantly present in high-density lipoprotein. Sensitive sequence searches, threading and comparative model building experiments revealed apoM to be structurally related to the lipocalin protein family. In a 3D model, characterized by an eight-stranded anti-parallel beta-barrel, a segment including Asn135 could adopt a closed or open conformation. Using site-directed mutagenesis, we demonstrated Asn135 in wild-type apoM to be glycosylated, suggesting that the segment is solvent exposed. ApoM displays two strong acidic patches of potential functional importance, one around the N-terminus and the other next to the opening of the beta-barrel.

A novel interaction between type IV pili of Neisseria gonorrhoeae and the human complement regulator C4B-binding protein

C4b-binding protein (C4BP) is an important plasma inhibitor of the classical pathway of complement activation. Several bacterial pathogens bind C4BP, which may contribute to their virulence. In the present report we demonstrate that isolated type IV pili from Neisseria gonorrhoeae bind human C4BP in a dose-dependent and saturable manner. C4BP consists of seven identical alpha-chains and one beta-chain linked together with disulfide bridges. We found that pili bind to the alpha-chain of C4BP, which is composed of eight homologous complement control protein (CCP) domains. From the results of an inhibition assay with C4b and a competition assay in which we tested mutants of C4BP lacking individual CCPs, we concluded that the binding area for pili is localized to CCP1 and CCP2 of the alpha-chain. The binding between pili and C4BP was abolished at 0.25 M NaCl, implying that it is based mostly on ionic interactions, similarly to what have been observed for C4b-C4BP binding. Furthermore, the N-terminal part of PilC, a structural component of pili, appeared to be responsible for binding of C4BP. Membrane cofactor protein, previously shown to be a receptor for pathogenic N. gonorrhoeae on the surface of epithelial cells, competed with C4BP for binding to pili only at high concentrations, suggesting that different parts of pili are involved in these two interactions. Accordingly, high concentrations of C4BP were required to inhibit binding of N. gonorrhoeae to Chang conjunctiva cells, and no inhibition of binding was observed with cervical epithelial cells.

Three-dimensional model of the SHBG-like region of anticoagulant protein S: new structure-function insights

Protein S (PS) is a vitamin K-dependent glycoprotein that consists of several modules including a C-terminal sex hormone-binding globulin (SHBG)-like domain that has been subdivided into two laminin LG-type domains. The SHBG-like region of PS is known to bind to a complement regulator molecule, C4b-binding protein (C4BP), coagulation factor Va (FVa) and receptor tyrosine kinases. Inherited PS deficiency has been associated with thromboembolic disease. Yet, study of the mechanisms by which the SHBG-like region of PS serves its essential functions has so far been hampered because of the lack of structural information. Recently, the three-dimensional (3D) structure of LG domains from plasma SHBG, laminin and neurexin have been reported and were found related to the pentraxin family. We used these X-ray structures to build homology models of the SHBG-like region of human PS. We then analyzed previously reported experimental/clinical data in the light of the predicted structures. A potential calcium-binding site is found in the first LG domain of PS and D292 could play a role in this process. This region is close to the interface between the two LG domains and is also surrounded by segments that have been suggested by synthetic peptide studies to be important for C4BP or FVa binding. The 39 point mutations linked to PS deficiencies or reported as neutral variants were rationalized in the 3D structure. Proteins 2001;43:203-216.

Binding of C4b-binding protein to porin: a molecular mechanism of serum resistance of Neisseria gonorrhoeae

We screened 29 strains of Neisseria gonorrhoeae and found 16/21 strains that resisted killing by normal human serum and 0/8 serum sensitive strains that bound the complement regulator, C4b-binding protein (C4bp). Microbial surface-bound C4bp demonstrated cofactor activity. We constructed gonococcal strains with hybrid porin (Por) molecules derived from each of the major serogroups (Por1A and Por1B) of N. gonorrhoeae, and showed that the loop 1 of Por1A is required for C4bp binding. Por1B loops 5 and 7 of serum-resistant gonococci together formed a negatively charged C4bp-binding domain. C4bp-Por1B interactions were ionic in nature (inhibited by high salt or by heparin), whereas the C4bp-Por1A bond was hydrophobic. Only recombinant C4bp mutant molecules containing the NH2-terminal alpha-chain short consensus repeat (SCR1) bound to both Por1A and Por1B gonococci, suggesting that SCR1 contained Por binding sites. C4bp alpha-chain monomers did not bind gonococci, indicating that the polymeric form of C4bp was required for binding. Using fAb fragments against C4bp SCR1, C4bp binding to Por1A and Por1B strains was inhibited in a complement-dependent serum bactericidal assay. This resulted in complete killing of these otherwise fully serum resistant strains in only 10% normal serum, underscoring the importance of C4bp in mediating gonococcal serum resistance.

Deficiency or inhibition of Gas6 causes platelet dysfunction and protects mice against thrombosis

The growth arrest-specific gene 6 product (Gas6) is a secreted protein related to the anticoagulant protein S but its role in hemostasis is unknown. Here we show that inactivation of the Gas6 gene prevented venous and arterial thrombosis in mice, and protected against fatal collagen/epinephrine-induced thrombo embolism. Gas6-/- mice did not, however, suffer spontaneous bleeding and had normal bleeding after tail clipping. In addition, we found that Gas6 antibodies inhibited platelet aggregation in vitro and protected mice against fatal thrombo embolism without causing bleeding in vivo. Gas6 amplified platelet aggregation and secretion in response to known agonists. Platelet dysfunction in Gas6-/- mice resembled that of patients with platelet signaling transduction defects. Thus, Gas6 is a platelet-response amplifier that plays a significant role in thrombosis. These findings warrant further evaluation of the possible therapeutic use of Gas6 inhibition for prevention of thrombosis.

Screening the molecular surface of human anticoagulant protein C: a search for interaction sites

Protein C (PC), a 62 kDa multi-modular zymogen, is activated to an anticoagulant serine protease (activated PC or APC) by thrombin bound to thrombomodulin on the surface of endothelial cells. PC/APC interacts with many proteins and the characterisation of these interactions is not trivial. However, molecular modelling methods help to study these complex biological processes and provide basis for rational experimental design and interpretation of the results. PC/APC consists of a Gla domain followed by two EGF modules and a serine protease domain. In this report, we present two structural models for full-length APC and two equivalent models for full-length PC, based on the X-ray structures of Gla-domainless APC and of known serine protease zymogens. The overall elongated shape of the models is further cross-validated using size exclusion chromatography which allows evaluation of the Stokes radius (rs for PC = 33.15 A; rs for APC = 34.19 A), frictional ratio and axial ratio. We then propose potential binding sites at the surface of PC/APC using surface hydrophobicity as a determinant of the preferred sites of intermolecular recognition. Most of the predicted binding sites are consistent with previously reported experimental data, while some clusters highlight new regions that should be involved in protein-protein interactions.

[Use the entire "analysis package" when investigating patients with venous thrombosis! Don't forget the relatives...]

During the last few years several genetic markers have been discovered that contribute to an increased risk of venous thrombosis. Patients who have several genetic markers are at a considerably higher risk of being affected than patients with only one marker. Recommendations are made as to which patients should be investigated, with appropriate laboratory analyses, when an increased risk of venous thrombosis is suspected. Most of the analyses can be done even if the patient is undergoing warfarin treatment.

Protein S Thr103Asn mutation associated with type II deficiency reproduced in vitro and functionally characterised

Protein S functions as a cofactor to activated protein C (APC) in the degradation of FVa and FVIIIa. In protein S, the thrombin sensitive region (TSR) and the first EGF-like domain are important for expression of the APC cofactor activity. A naturally occurring Thr103Asn (T103N) mutation in the first EGF-like domain of protein S has been associated with functional (type II) protein S deficiency. To elucidate the functional consequences of the T103N mutation, recombinant protein S mutant was expressed in mammalian cells and functionally characterised. The expression level of protein S T103N from transiently transfected COS 1 cells was equal to that of wild type protein S. The mutant protein S and wild type protein S were also expressed in 293 cells after stable transfection, and the recombinant proteins purified. In APTT- and PT-based coagulation assays, the mutant protein demonstrated approximately 50% lower anticoagulant activity as compared to wild type protein S. The functional defect was further investigated in FVa- and FVIIIa-degradation assays. The functional defect of mutant protein S was attenuated at increasing concentrations of APC. The results demonstrate the region around residue 103 of protein S to be of functional importance, possibly through a direct interaction with APC.

The second laminin G-type domain of protein S is indispensable for expression of full cofactor activity in activated protein C-catalysed inactivation of factor Va and factor VIIIa

Vitamin K-dependent protein S is a cofactor to the anticoagulant serine protease activated protein C (APC) in the proteolytic inactivation of the procoagulant, activated factor V (FVa) and factor VIII (FVIIIa). In the FVa degradation, protein S selectively accelerates the cleavage at Arg306, having no effect on the Arg506 cleavage. In the FVIIIa inactivation, the APC-cofactor activity of protein S is synergistically potentiated by FV, which thus has the capacity to function both as a pro- and an anticoagulant protein. The SHBG-like region of protein S, containing two laminin G-type domains, is required for the combined action of protein S and FV. To elucidate whether both G domains in protein S are needed for expression of APC-cofactor activities, chimeras of human protein S were created in which the individual G domains were replaced by the corresponding domain of the homologous Gas6, which in itself has no anticoagulant activity. In a plasma-based assay, chimera I (G1 from Gas6) was as efficient as wild-type recombinant protein S, whereas chimera II (G2 from Gas6) was less effective. The synergistic cofactor activity with FV in the inactivation of FVIIIa was lost by the replacement of the G2 domain in protein S (chimera II). However, chimera I did not exert full APC-cofactor activity in the FVIIa degradation, indicating involvement of both G domains or the entire SHBG-like region in this reaction. Chimera I was fully active in the degradation of FVa in contrast to chimera II, which exhibited reduced cofactor activity compared to protein S. In conclusion, by using protein S-Gas6 chimeric proteins, we have identified the G2 domain of protein S to be indispensable for an efficient inactivation of both FVIIa and FVa, whereas the G1 domain was found not to be of direct importance in the FVa-inactivation experiments.

Deficient APC-cofactor activity of protein S Heerlen in degradation of factor Va Leiden: a possible mechanism of synergism between thrombophilic risk factors

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa-degradation system. In contrast, protein S Heerlen functioned equally well as an APC cofactor in the degradation of factor Va as wild-type protein S did. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, protein S Heerlen was found to be a poor APC cofactor as compared with wild-type protein S. These in vitro results suggest a possible mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits. (Blood. 2000;96:523-531)

Positively charged amino acids at the interface between alpha-chain CCP1 and CCP2 of C4BP are required for regulation of the classical C3-convertase

C4b-binding protein (C4BP) is an abundant and potent down-regulator of complement activation. In its presence, the assembly of the classical pathway C3-convertase is prevented and its natural decay is accelerated. C4BP also acts as a cofactor to the serine proteinase factor I in the cleavage of C4b. C4BP contains repeats of small structural domains: complement control protein (CCP) modules. Previously, we constructed and purified nine recombinant C4BP molecules in which solvent exposed positively charged amino acids at the interface between CCP1 and CCP2 were mutated to polar glutamines. Several of these mutants showed lower binding ability for C4b. In the present investigation, the collection of mutants was tested with functional assays and we found a correlation between changes in the apparent affinity of C4BP mutants for C4b and their ability to down regulate the C3-convertase. Mutagenesis of R(39), K(63), R(64) and particularly H(67) resulted in impaired C4b binding paralleled by lost ability of the C4BP mutants to prevent C3-convertase assembly and to increase the decay rate of the C3-convertase. Furthermore, these amino acids were found to be crucial for the factor I cofactor activity of C4BP in fluid phase degradation of C4b. In conclusion, a cluster of positively charged amino acid residues at the CCP1-CCP2 interface is identified as functionally important in the regulation of the C3-convertase by C4BP.

Human C4b-binding protein has overlapping, but not identical, binding sites for C4b and streptococcal M proteins

Many strains of Streptococcus pyogenes bind C4b-binding protein (C4BP), an inhibitor of complement activation. The binding is mediated by surface M proteins in a fashion that has been suggested to mimic the binding of C4b. We have previously shown that a positively charged cluster at the interface between complement control protein domains 1 and 2 of C4BP alpha-chain is crucial for the C4b-C4BP interaction. To extend this observation, and to investigate the interaction with M proteins, we constructed and characterized a total of nine mutants of C4BP. We identified a key recognition surface for M proteins that overlaps with the C4b binding site because substitution of R64 and H67 by Gln dramatically reduces binding to both ligands. However, the analysis of all mutants indicates that the binding sites for C4b and M proteins are only overlapping, but not identical. Furthermore, M proteins were able to displace C4BP from immobilized C4b, whereas C4b only weakly affected binding of C4BP to immobilized M proteins. We found that the molecular mechanisms involved in these two interactions differ because the binding between M proteins and C4BP is relatively insensitive to salt in contrast to the C4BP-C4b binding. In addition, six mAbs directed against the alpha-chain interfered with C4b-C4BP interaction, whereas only two of them efficiently inhibited binding of C4BP to M proteins. Collectively, our results suggest that binding between C4b and C4BP is governed mostly by electrostatic interactions, while additional noncovalent forces cause tight binding of C4BP to streptococcal M proteins.

Blood coagulation

Under normal circumstances, the coagulation system is balanced in favour of anticoagulation. Thrombin is the key effector enzyme of the clotting cascade. Antagonists of vitamin K inhibit a vitamin-K-dependent post-translational modification of several coagulation proteins, which is required for these proteins to attain a phospholipid-binding conformation. Heparin stimulates the activity of antithrombin, a serine-protease inhibitor. Analysis of knock-out mice has shown the relative importance of the coagulation factors in vivo. Gene therapy may soon be a therapeutic option for inherited deficiencies of factors VIII and IX.

Factor V R506Q mutation (activated protein C resistance) is an additional risk factor for early renal graft loss associated with acute vascular rejection

BACKGROUND

The factor V R506Q mutation (FV R506Q, FV:Q506, or FV Leiden) resulting in activated protein C (APC) resistance is the most common inherited risk factor for venous thrombosis, including in renal transplant recipients. We investigated a possible association between the FV mutation and early renal graft loss, and the prevalence of macro- and microvascular thrombosis, endothelialitis, and fibrinoid vascular necrosis by FV genotype.

METHOD

One hundred and nine renal allograft recipients were genotyped for FV mutation. A vascular rejection subgroup of patients (n=29) had experienced at least one episode of vascular rejection, or graft thrombosis. A second group of patients (n=80) had experienced no acute rejection and retained a well-functioning graft.

RESULTS

The prevalence of APC resistance was numerically but not statistically significantly higher in the vascular rejection group (17.2%) compared with the group without rejection episodes (7.5%) (P=0.16). There was a significant association between the presence or absence of FV mutation and graft survival, with a 55.6% 1-year graft survival rate versus a 76.4% rate, respectively (P=0.02). The prevalence of vascular rejection, as evidenced by endothelialitis or fibrinoid vascular necrosis, was significantly associated with APC resistance but macro- or microvascular thrombosis were not.

CONCLUSION

Renal transplant recipients who are carriers of the FV:Q506 allele have an increased risk of early graft loss. Vascular rejection changes including endothelialitis and fibrinoid vascular necrosis were more common in this group, and therefore an association between the hypercoagulable state, which entails an up-regulation of the mitogenic and proinflammatory enzyme thrombin, and the immunological challenge to the endothelium may be the cause of inferior prognosis in these patients.

Tracking structural features leading to resistance of activated protein C to alpha 1-antitrypsin

Activated protein C (APC) is a multi-modular anticoagulant serine protease, which degrades factor V/Va and factor VIIIa. Human APC (hAPC) is inhibited by human alpha 1-antitrypsin (AAT), while the bovine enzyme (bAPC) is fully resistant to this serpin. Structural features in the catalytic domains between the two species cause this difference, but detailed knowledge about the causal molecular difference is missing. To gain insight into the APC-AAT interaction and to create a human protein C resistant to AAT inhibition, we have used molecular modeling and site-directed mutagenesis. First, a structural model for bAPC based on the Gla-domainless X-ray structure of hAPC was built. Screening the molecular surface of the human and bovine APC enzymes suggested that a hAPC molecule resistant to AAT inhibition could be constructed by substituting only a few amino acids. We thus produced recombinant hAPC molecules with a single mutation (S173E, the numbering follows the chymotrypsinogen nomenclature), two mutations (E60aS/S61R) or a combination of all these substitutions (E60aS/S61R/S173E). Amidolytic and anticoagulant activities of the three mutant APC molecules were similar to those of wild-type hAPC. Inhibition of wild-type hAPC by AAT was characterized by a second-order rate constant (k2) of 2.71 M-1 s-1. The amino acid substitution at position 173 (S173E mutant) led to partial resistance to AAT (k2 = 0.84 M-1 s-1). The E60aS/S61R mutant displayed mild resistance to AAT inhibition (k2 = 1.70 M-1 s-1), whereas the E60aS/S61R/S173E mutant was inefficiently inactivated by AAT (k2 = 0.40 M-1 s-1). Inhibition of recombinant APC molecules by the serpin protein C inhibitor (PCI) in the presence and absence of heparin was also investigated.

The first laminin G-type domain in the SHBG-like region of protein S contains residues essential for activation of the receptor tyrosine kinase sky

Vitamin K-dependent protein S and the product of growth-arrest-specific gene 6 (Gas6) both possess the ability to phosphorylate members of the Axl/Sky subfamily of receptor tyrosine kinases. However, Gas6 appears to be the bona fide ligand for these receptors in man, as human protein S has been demonstrated to activate murine Sky but not the human orthologue. In contrast, bovine protein S is able to stimulate human Sky despite its high degree of sequence identity with human protein S. The domain organisations of protein S and Gas6 are virtually identical and the C-terminal SHBG-like region, containing two globular (G) domains, has been shown to play a crucial role in the receptor stimulation. In order to further localise the area responsible for the interaction, a number of protein chimeras were used to stimulate human Sky. Each chimera had one part of the human protein S SHBG-like region replaced by the corresponding part of bovine protein S or human Gas6. We found that human protein S may indeed activate human Sky but only above physiological plasma concentrations. The human-bovine protein S chimeras provided new information implying that the first G domain contains critical residues for the interaction with the Sky receptor. Moreover, these residues do not seem to be clustered but rather to be distributed at various positions in the first G domain.

Mutations in a potential phospholipid binding loop in the C2 domain of factor V affecting the assembly of the prothrombinase complex

Activated factor V (FVa) serves as a cofactor to activated factor X in the prothrombinase complex. FVa is homologous to activated factor VIII (FVIIIa), the light chains of both proteins being formed by similar domains (A3-C1-C2). Interaction of FVa and FVIIIa with negatively charged phospholipid membranes is crucial for the function of both cofactors. In both proteins, the C2 domains are important for membrane binding but a detailed understanding of the binding mechanisms is missing. Recently, knowledge has been gained into the three-dimensional structures of the C domains facilitating studies of structure-function relationships. Structural analysis of the C2 domain in FVa predicted a surface-exposed loop (K2060, K2061, S2062, W2063, W2064) to be involved in membrane binding. Three double mutants were created, K2060Q-K2061Q, W2063Y-W2064Y and W2063A-W2064A, and expressed in a transient expression system. In addition, a FV variant in which all four residues were mutated, K2060Q-K2061Q-W2063A-W2064A, was produced. Mutagenesis of the two lysines showed no functional consequences, whereas mutagenesis of the two tryptophanes yielded FVa with impaired ability to interact with the phospholipid, as demonstrated by a poor functional activity at limiting phospholipid concentrations. A molecular model of FVa, anchored at the surface of a phospholipid membrane, was developed and used as a template for the interpretation of the mutagenesis experiments.

Protein S secretion differences of missense mutants account for phenotypic heterogeneity

To elucidate the molecular background for the heterogeneity in protein S plasma concentrations observed in protein S deficient individuals, the in vitro synthesis of recombinant protein S missense mutants was investigated. Six different naturally occurring mutations identified in the protein S gene (PROS1) of thrombosis patients were reproduced in protein S cDNA by site directed mutagenesis. Two mutants, G441C and Y444C (group A), were associated with low total plasma concentration of protein S. Modestly low protein S was found in families with R520G and P626L (group B) mutants. T57S and I518M (group C), which was associated with marginally low protein S, did not segregate with protein S deficiency in the respective families, raising doubts as to whether they were causative mutations or rare neutral variants. The 6 protein S mutants were transiently expressed in COS 1 cells. The Y444C mutant showed the lowest level of secretion (2.5%) followed by the G441C mutant (40%). Group B demonstrated around 50% reduction in secretion, whereas group C mutants showed normal secretion. Pulse-chase experiments demonstrated impaired protein S processing with intracellular degradation and decreased secretion into the culture media of group A and B mutants. Interestingly, there was a good correlation between in vitro secretion and the concentration of free protein S in the plasma of heterozygous carriers. These results demonstrate impaired protein S secretion to be an important mechanism underlying hereditary protein S deficiency and that variations in protein secretion is a major determinant of the phenotypic heterogeneity observed in protein S deficiency. (Blood. 2000;95:173-179)