Self-association of human protein S

The Kunitz-3 domain of TFPI-alpha is required for protein S-dependent enhancement of factor Xa inhibition

Protein S (PS) enhances the inhibition of factor Xa (FXa) by tissue factor pathway inhibitor-alpha (TFPI-alpha) in the presence of Ca(2+) and phospholipids. Altered forms of recombinant TFPI-alpha were used to determine the structures within TFPI-alpha that may be involved in this PS-dependent effect. Wild-type TFPI-alpha (TFPI(WT)), TFPI-alpha lacking the K3 domain (TFPI-(DeltaK3)), and TFPI-alpha containing a single amino acid change at the putative P1 residue of K3 (R199L, TFPI(K3P1)) produced equivalent FXa inhibition in the absence of PS, whereas the response in FXa inhibition produced by PS was reduced with TFPI(K3P1) (EC(50) 61.8 +/- 13.4nM vs 8.0 +/- 0.4nM for TFPI(WT)) and not detectable with TFPI-(DeltaK3). Ligand blotting and surface plasmon resonance experiments demonstrated that FXa bound TFPI(WT) and TFPI-(DeltaK3) but not the isolated K3 domain, whereas PS bound TFPI(WT) and the K3 domain but not TFPI-(DeltaK3). Addition of TFPI(WT), TFPI(K3P1), or TFPI-(DeltaK3) produced comparable prolongation of FXa-induced coagulation in PS-deficient plasma, but the anticoagulant effect of TFPI(WT) was substantially greater than that of TFPI(K3P1) > TFPI-(DeltaK3) in normal plasma and PS-deficient plasma reconstituted with PS. We conclude that the PS-mediated enhancement of FXa inhibition by TFPI-alpha involves an interaction between PS and TFPI-alpha, which requires the K3 domain of TFPI-alpha.

The thrombin-sensitive region of protein S mediates phospholipid-dependent interaction with factor Xa

To test the hypothesis that factor Xa (fXa) interacts with protein S, fXa was labeled active-site specifically with a dansyl (D) dye via a Glu-Gly-Arg (EGR) tether to yield DEGR-fXa(i). When protein S was added to phosphatidylcholine/phosphatidylserine (PC/PS, 4:1) vesicle-bound DEGR-fXa(i), the anisotropy of the dansyl moiety was altered from 0.219 +/- 0.002 to 0.245 +/- 0.003. This change in dansyl anisotropy was not observed when DEGR-Xa(i) was titrated with protein S in the absence of PC/PS vesicles, or in the presence of 100% PC vesicles, or when PC/PS vesicle-bound DEGR-fXa(i) was titrated with thrombin-cleaved protein S. The protein S-dependent dansyl fluorescence change was specific for fXa because it was not observed for two homologous and similarly labeled DEGR-fIXa(i) and DEGR-fVIIa(i). Furthermore, protein S specifically and saturably altered the fluorescence anisotropy of PC/PS-bound active site-labeled LWB-FPR-fXa(i) (Kd = 33 nm) and was photocross-linked to PC/PS-bound LWB-FPR-fXa(i) analog, independently confirming the above results. Chemically synthesized microprotein S, comprising residues 1-116 of protein S and including the gamma-carboxyglutamic-rich domain, the thrombin-sensitive region (TSR), and the first epidermal growth factor-like domain (EGF1) of protein S, altered the anisotropy of PC/PS-bound DEGR-fXa(i) from 0.219 to 0.242, similar to the effect of the protein S titration (Kd = 303 nm), suggesting that microprotein S binds to DEGR-fXa(i). To identify individual protein S domain(s) that binds DEGR-fXa(i), the EGF1 and TSR domains were chemically synthesized and studied. The TSR altered the anisotropy of DEGR-fXa(i) by approximately 16% (Kd = 3.9 microm), but the EGF1 domain had no effect on the signal. In controls, the TSR domain did not alter the anisotropy of DEGR-fIXa(i) and DEGR-fVIIa(i), respectively. These data demonstrate that membrane-bound fXa binding to protein S involves the TSR of protein S.

Citrate anticoagulation and the dynamics of thrombin generation

BACKGROUND

Sodium citrate has been used as an anticoagulant to stabilize blood and blood products for over 100 years, presumably by sequestering Ca(++) ions in vitro. Anticoagulation of blood without chelation can be achieved by inhibition of the contact pathway by corn trypsin inhibitor (CTI).

OBJECTIVE

To evaluate the influence of citrate anticoagulation on the performance of blood, platelet-rich and platelet-poor plasma assays.

METHODS

Blood was anticoagulated in three ways: by collection into citrate, CTI and citrate with CTI. Plasma was prepared using each anticoagulation regimen. Functional analyses included calibrated automated thrombography, thromboelastography, plasma clotting, the synthetic coagulation proteome and platelet aggregation. Coagulation reactions were initiated with tissue factor-phospholipid and Ca(++) (when indicated).

RESULTS

In all cases, citrate anticoagulation resulted in reaction dynamics significantly altered relative to blood or plasma stabilized with CTI alone. Subsequent experiments showed that calcium citrate itself impairs coagulation dynamics.

CONCLUSION

Coagulation analyses using blood that has been exposed to citrate and recalcified do not yield reliable depictions of the natural dynamics of blood coagulation processes.

Plasma contains protein S monomers and multimers with similar direct anticoagulant activity

BACKGROUND

Protein S (PS) has activated protein C-independent, direct anticoagulant activity (PS-direct). We reported that both multimers and monomers of affinity-purified PS have PS-direct similar to that in plasma, in contrast to another report.

OBJECTIVE

We extended our studies to establish the molecular forms and activity of plasma PS.

METHODS

Novel ELISAs were developed that could detect only multimeric, not monomeric, PS because they employed the same monoclonal antibody for capture and detection. PS forms were also examined on native PAGE immunoblots. A new activity assay for PS-direct was applied to plasma and gel-filtered plasma fractions.

RESULTS

Plasma PS multimers were clearly demonstrated using the ELISAs; 30-60% of free plasma PS appeared to be multimeric, a proportion similar to that of affinity-purified PS. On immunoblots, plasma PS multimers were more easily detected after gel filtration; plasma PS monomers and several apparent multimers comigrated with respective forms of affinity-purified PS. Antigen elution profiles after gel filtration of plasma revealed at least one major peak of apparent PS multimers (40-55% of free PS appeared multimeric). Biotin-factor Xa could bind to both plasma PS monomers and multimers. Strong plasma PS-direct was demonstrated, and plasma PS monomers, multimers, and PS-C4b-binding protein complexes each reconstituted PS-depleted plasma to similar levels of PS-direct.

CONCLUSION

Our data are in disagreement with a report that monomeric purified PS has little PS-direct and that only monomeric PS exists in plasma. We find that both affinity-purified and plasma PS exist as monomers and multimers with similar PS-direct.

Protein S multimers and monomers each have direct anticoagulant activity

BACKGROUND AND OBJECTIVES

Plasma protein S (PS) is an essential anticoagulant that has activated protein C-independent, direct anticoagulant activity (PS-direct). It was reported that monomeric purified PS has poor PS-direct and that a subpopulation of multimeric purified PS has high PS-direct and high affinity for phospholipids. We independently examined the relative PS-direct and affinity for phospholipids of monomeric and multimeric PS and we obtained contrasting results.

METHODS AND RESULTS

Unpurified recombinant protein S (rPS) was monomeric and had PS-direct potency similar to that of both PS in plasma and multimeric affinity-purified PS, as measured in plasma assays for PS-direct and in thrombin-generation assays. Multimers of unpurified rPS were not induced by ethylenediaminetetraacetic acid (EDTA), pH 2.5, NaSCN, or barium adsorption/elution. Multimers were induced by chromatography in the presence of EDTA and thus may be concentration-dependent. In contrast to a different report, monomers, dimers, trimers, and higher-order PS forms were clearly separated in sedimentation velocity experiments and multimers were not dissociated by adding Ca(2+). Active plasma-derived and recombinant immunoaffinity-purified PS were fractionated into monomers and multimers. On a mass basis, monomers and multimers had similar specific PS-direct and ability to compete with prothrombinase components (factors Xa/Va) for limiting phospholipids. FXa ligand blotted to both monomers and multimers.

CONCLUSIONS

Plasma PS-direct is similar to that of affinity-purified PS and unpurified rPS. Under our conditions, monomeric and multimeric PS have similar PS-direct and ability to compete for phospholipids. Discordant earlier findings are likely due to loss of PS-direct during conventional purification procedures.

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.