Deletion analysis of recombinant human factor V. Evidence for a phosphatidylserine binding site in the second C-type domain

Membrane binding and lipid-protein interaction of the C2 domain from coagulation factor V

Anchoring of coagulation factors to anionic regions of the membrane involves the C2 domain as a key player. The rate of enzymatic reactions of the coagulation factors is increased by several orders of magnitude upon membrane binding. However, the precise mechanisms behind the rate acceleration remain unclear, primarily because of a lack of understanding of the conformational dynamics of the C2-containing factors and corresponding complexes. We elucidate the membrane-bound form of the C2 domain from human coagulation factor V (FV-C2) by characterizing its membrane binding the specific lipid-protein interactions. Employing all-atom molecular dynamics simulations and leveraging the highly mobile membrane-mimetic (HMMM) model, we observed spontaneous binding of FV-C2 to a phosphatidylserine (PS)-containing membrane within 2-25 ns across twelve independent simulations. FV-C2 interacted with the membrane through three loops (spikes 1-3), achieving a converged, stable orientation. Multiple HMMM trajectories of the spontaneous membrane binding provided extensive sampling and ample data to examine the membrane-induced effects on the conformational dynamics of C2 as well as specific lipid-protein interactions. Despite existing crystal structures representing presumed "open" and "closed" states of FV-C2, our results revealed a continuous distribution of structures between these states, with the most populated structures differing from both "open" and "closed" states observed in crystal environments. Lastly, we characterized a putative PS-specific binding site formed by K23, Q48, and S78 located in the groove enclosed by spikes 1-3 (PS-specificity pocket), suggesting a different orientation of a bound headgroup moiety compared to previous proposals based upon analysis of static crystal structures.

The role of phosphatidylserine on the membrane in immunity and blood coagulation

The negatively charged aminophospholipid, phosphatidylserine (PtdSer), is located in the inner leaflet of the plasma membrane in normal cells, and may be exposed to the outer leaflet under some immune and blood coagulation processes. Meanwhile, Ptdser exposed to apoptotic cells can be recognized and eliminated by various immune cells, whereas on the surface of activated platelets Ptdser interacts with coagulation factors prompting enhanced production of thrombin which significantly facilitates blood coagulation. In the case where PtdSer fails in exposure or mistakenly occurs, there are occurrences of certain immunological and haematological diseases, such as the Scott syndrome and Systemic lupus erythematosus. Besides, viruses (e.g., Human Immunodeficiency Virus (HIV), Ebola virus (EBOV)) can invade host cells through binding the exposed PtdSer. Most recently, the Corona Virus Disease 2019 (COVID-19) has been similarly linked to PtdSer or its receptors. Therefore, it is essential to comprehensively understand PtdSer and its functional characteristics. Therefore, this review summarizes Ptdser, its eversion mechanism; interaction mechanism, particularly with its immune receptors and coagulation factors; recognition sites; and its function in immune and blood processes. This review illustrates the potential aspects for the underlying pathogenic mechanism of PtdSer-related diseases, and the discovery of new therapeutic strategies as well.

Severe thrombophilia in a factor V-deficient patient homozygous for the Ala2086Asp mutation (FV Besançon)

BACKGROUND

Coagulation factor V (FV), present in plasma and platelets, has both pro- and anticoagulant functions.

OBJECTIVE

We investigated an FV-deficient patient (FV:C 3%, FV:Ag 4%) paradoxically presenting with recurrent venous thrombosis (11 events) instead of bleeding.

METHODS/RESULTS

Thrombophilia screening revealed only heterozygosity for the F2 20210G>A mutation. Although thrombin generation in the patient's platelet-poor plasma was suggestive of a hypocoagulable state, thrombin generation in the patient's platelet-rich plasma (PRP) was higher than in control PRP and extremely resistant to activated protein C (APC). This was partially attributable to the complete abolition of the APC-cofactor activity of FV and a marked reduction of plasma tissue factor pathway inhibitor antigen and activity. The patient was homozygous for a novel missense mutation (Ala2086Asp, FVBesançon ) that favors a "closed conformation" of the C2 domain, predicting impaired binding of FV(a) to phospholipids. Recombinant FVBesançon was hardly secreted, indicating that this mutation is responsible for the patient's FV deficiency. Model system experiments performed using highly diluted plasma as a source of FV showed that, compared with normal FVa, FVaBesançon has slightly (≤1.5-fold) unfavorable kinetic parameters (Km , Vmax ) of prothrombin activation, but also a lower rate of APC-catalyzed inactivation in the presence of protein S.

CONCLUSIONS

FVBesançon induces a hypercoagulable state via quantitative (markedly decreased FV level) and qualitative (phospholipid-binding defect) effects that affect anticoagulant pathways (anticoagulant activities of FV, FVa inactivation, tissue factor pathway inhibitor α level) more strongly than the prothrombinase activity of FVa. A possible specific role of platelet FV cannot be excluded.

Blood coagulation factor Va's key interactive residues and regions for prothrombinase assembly and prothrombin binding

Blood coagulation factor Va serves an indispensable role in hemostasis as cofactor for the serine protease factor Xa. In the presence of an anionic phospholipid membrane and calcium ions, factors Va and Xa assemble into the prothrombinase complex. Following formation of the ternary complex with the macromolecular zymogen substrate prothrombin, the latter is rapidly converted into thrombin, the key regulatory enzyme of coagulation. Over the years, multiple binding sites have been identified in factor Va that play a role in the interaction of the cofactor with factor Xa, prothrombin, or the anionic phospholipid membrane surface. In this review, an overview of the currently available information on these interactive sites in factor Va is provided, and data from biochemical approaches and 3D structural protein complex models are discussed. The structural models have been generated in recent years and provide novel insights into the molecular requirements for assembly of both the prothrombinase and the ternary prothrombinase-prothrombin complexes. Integrated knowledge of functionally important regions in factor Va will allow for a better understanding of factor Va cofactor activity.

Factor Va alternative conformation reconstruction using atomic force microscopy

Protein conformational variability (or dynamics) for large macromolecules and its implication for their biological function attracts more and more attention. Collective motions of domains increase the ability of a protein to bind to partner molecules. Using atomic force microscopy (AFM) topographic images, it is possible to take snapshots of large multi-component macromolecules at the single molecule level and to reconstruct complete molecular conformations. Here, we report the application of a reconstruction protocol, named AFM-assembly, to characterise the conformational variability of the two C domains of human coagulation factor Va (FVa). Using AFM topographic surfaces obtained in liquid environment, it is shown that the angle between C1 and C2 domains of FVa can vary between 40° and 166°. Such dynamical variation in C1 and C2 domain arrangement may have important implications regarding the binding of FVa to phospholipid membranes.

Mechanism of Action of Thymosinα1: Does It Interact with Membrane by Recognition of Exposed Phosphatidylserine on Cell Surface? A Structural Approach

Thymosinα1 is a peptidic hormone with pleiotropic activity, which is used in the therapy of several diseases. It is unstructured in water solution and interacts with negative regions of micelles and vesicles assuming two tracts of helical conformation with a structural flexible break in between. The studies of the interaction of Thymosinα1 with micelles of mixed dipalmitoylphosphatidylcholine and sodium dodecylsulfate and vesicles with mixed dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine, the latter the negative component of the membranes, by (1)H and natural abundance (15)N NMR are herewith reported, reviewed, and discussed. The results indicate that the preferred interactions are those where the surface is negatively charged due to sodium dodecylsulfate or due to the presence of dipalmitoylphosphatidylserine exposed on the surface. In fact the unbalance of dipalmitoylphosphatidylserine on the cellular surface is an important phenomenon present in pathological conditions of cells. Moreover, the direct interaction of Thymosinα1 with K562 cells presenting an overexposure of phosphatidylserine as a consequence of resveratrol-induced apoptosis was carried out.

Platelet binding sites for factor VIII in relation to fibrin and phosphatidylserine

Thrombin-stimulated platelets expose very little phosphatidylserine (PS) but express binding sites for factor VIII (fVIII), casting doubt on the role of exposed PS as the determinant of binding sites. We previously reported that fVIII binding sites are increased three- to sixfold when soluble fibrin (SF) binds the αIIbβ3 integrin. This study focuses on the hypothesis that platelet-bound SF is the major source of fVIII binding sites. Less than 10% of fVIII was displaced from thrombin-stimulated platelets by lactadherin, a PS-binding protein, and an fVIII mutant defective in PS-dependent binding retained platelet affinity. Therefore, PS is not the determinant of most binding sites. FVIII bound immobilized SF and paralleled platelet binding in affinity, dependence on separation from von Willebrand factor, and mediation by the C2 domain. SF also enhanced activity of fVIII in the factor Xase complex by two- to fourfold. Monoclonal antibody (mAb) ESH8, against the fVIII C2 domain, inhibited binding of fVIII to SF and platelets but not to PS-containing vesicles. Similarly, mAb ESH4 against the C2 domain, inhibited >90% of platelet-dependent fVIII activity vs 35% of vesicle-supported activity. These results imply that platelet-bound SF is a component of functional fVIII binding sites.

Coagulation function and mechanisms in various clinical phenotypes of patients with acquired factor V inhibitors

BACKGROUND

The clinical phenotype of individuals with acquired factor V (A-FV) inhibitors varies from asymptomatic (non-B group) to life-threatening bleeding (B group), but the mechanism(s) underlying this variation in hemorrhagic phenotype are poorly understood.

OBJECTIVE

To investigate coagulation mechanistically in a range of patients with A-FV antibodies.

METHODS AND RESULTS

Ten cases of A-FV inhibitors in the non-B (n = 5) and B groups (n = 5) were studied. Thrombin generation assays in these plasmas revealed little thrombin generation, despite similar FV activity levels in both groups. However, prothrombin time-based clot waveform analysis revealed that the clot times were significantly prolonged and the maximum velocity and acceleration of coagulation were lower in the B group than in the non-B group, suggesting that this technique might be useful for predicting and monitoring hemorrhagic symptoms. A-FV inhibitors from the non-B group recognized predominantly the FV heavy chain, whereas those from the B group recognized the light chain. Purified anti-FV autoantibodies (autoAbs) from the B group inhibited FV binding to phospholipid by 60-90%, whereas there was little effect on this reaction in the non-B group. In addition, anti-FV autoAbs from the non-B group impaired the activated protein C (APC) cofactor activity of FV in FVIIIa inactivation mechanisms, and delayed APC-catalyzed cleavage of FVa at Arg306, but not at Arg506, indicating the presence of APC resistance in the non-B group.

CONCLUSIONS

The results suggest that the different hemorrhagic phenotypes in A-FV inhibitors depend on the specific epitope of anti-FV autoAbs, and appear to be associated with an imbalance of procoagulant and anticoagulant function.

Novel FV mutation (W1920R, FVNara) associated with serious deep vein thrombosis and more potent APC resistance relative to FVLeiden

FVNara (W1920R), associated with serious deep vein thrombosis, is more resistant to APC relative to FVLeiden (R506Q). This mechanism results from significant decreases in FVa susceptibility to APC and FV cofactor activity for APC.

New Fundamentals in Hemostasis

Hemostasis encompasses the tightly regulated processes of blood clotting, platelet activation, and vascular repair. After wounding, the hemostatic system engages a plethora of vascular and extravascular receptors that act in concert with blood components to seal off the damage inflicted to the vasculature and the surrounding tissue. The first important component that contributes to hemostasis is the coagulation system, while the second important component starts with platelet activation, which not only contributes to the hemostatic plug, but also accelerates the coagulation system. Eventually, coagulation and platelet activation are switched off by blood-borne inhibitors and proteolytic feedback loops. This review summarizes new concepts of activation of proteases that regulate coagulation and anticoagulation, to give rise to transient thrombin generation and fibrin clot formation. It further speculates on the (patho)physiological roles of intra- and extravascular receptors that operate in response to these proteases. Furthermore, this review provides a new framework for understanding how signaling and adhesive interactions between endothelial cells, leukocytes, and platelets can regulate thrombus formation and modulate the coagulation process. Now that the key molecular players of coagulation and platelet activation have become clear, and their complex interactions with the vessel wall have been mapped out, we can also better speculate on the causes of thrombosis-related angiopathies.

Conservative mutations in the C2 domains of factor VIII and factor V alter phospholipid binding and cofactor activity

Factor VIII and factor V share structural homology and bind to phospholipid membranes via tandem, lectin-like C domains. Their respective C2 domains bind via 2 pairs of hydrophobic amino acids and an amphipathic cluster. In contrast, the factor V-like, homologous subunit (Pt-FV) of a prothrombin activator from Pseudonaja textilis venom is reported to function without membrane binding. We hypothesized that the distinct membrane-interactive amino acids of these proteins contribute to the differing membrane-dependent properties. We prepared mutants in which the C2 domain hydrophobic amino acid pairs were changed to the homologous residues of the other protein and a factor V mutant with 5 amino acids changed to those from Pt-FV (FV(MTTS/Y)). Factor VIII mutants were active on additional membrane sites and had altered apparent affinities for factor X. Some factor V mutants, including FV(MTTS/Y), had increased membrane interaction and apparent membrane-independent activity that was the result of phospholipid retained during purification. Phospholipid-free FV(MTTS/Y) showed increased activity, particularly a 10-fold increase in activity on membranes lacking phosphatidylserine. The reduced phosphatidylserine requirement correlated to increased activity on resting and stimulated platelets. We hypothesize that altered membrane binding contributes to toxicity of Pt-FV.

X-linked juvenile retinoschisis: clinical diagnosis, genetic analysis, and molecular mechanisms

X-linked juvenile retinoschisis (XLRS, MIM 312700) is a common early onset macular degeneration in males characterized by mild to severe loss in visual acuity, splitting of retinal layers, and a reduction in the b-wave of the electroretinogram (ERG). The RS1 gene (MIM 300839) associated with the disease encodes retinoschisin, a 224 amino acid protein containing a discoidin domain as the major structural unit, an N-terminal cleavable signal sequence, and regions responsible for subunit oligomerization. Retinoschisin is secreted from retinal cells as a disulphide-linked homo-octameric complex which binds to the surface of photoreceptors and bipolar cells to help maintain the integrity of the retina. Over 190 disease-causing mutations in the RS1 gene are known with most mutations occurring as non-synonymous changes in the discoidin domain. Cell expression studies have shown that disease-associated missense mutations in the discoidin domain cause severe protein misfolding and retention in the endoplasmic reticulum, mutations in the signal sequence result in aberrant protein synthesis, and mutations in regions flanking the discoidin domain cause defective disulphide-linked subunit assembly, all of which produce a non-functional protein. Knockout mice deficient in retinoschisin have been generated and shown to display most of the characteristic features found in XLRS patients. Recombinant adeno-associated virus (rAAV) mediated delivery of the normal RS1 gene to the retina of young knockout mice result in long-term retinoschisin expression and rescue of retinal structure and function providing a 'proof of concept' that gene therapy may be an effective treatment for XLRS.

Membrane-binding properties of the Factor VIII C2 domain

Factor VIII functions as a cofactor for Factor IXa in a membrane-bound enzyme complex. Membrane binding accelerates the activity of the Factor VIIIa-Factor IXa complex approx. 100000-fold, and the major phospholipid-binding motif of Factor VIII is thought to be on the C2 domain. In the present study, we prepared an fVIII-C2 (Factor VIII C2 domain) construct from Escherichia coli, and confirmed its structural integrity through binding of three distinct monoclonal antibodies. Solution-phase assays, performed with flow cytometry and FRET (fluorescence resonance energy transfer), revealed that fVIII-C2 membrane affinity was approx. 40-fold lower than intact Factor VIII. In contrast with the similarly structured C2 domain of lactadherin, fVIII-C2 membrane binding was inhibited by physiological NaCl. fVIII-C2 binding was also not specific for phosphatidylserine over other negatively charged phospholipids, whereas a Factor VIII construct lacking the C2 domain retained phosphatidyl-L-serine specificity. fVIII-C2 slightly enhanced the cleavage of Factor X by Factor IXa, but did not compete with Factor VIII for membrane-binding sites or inhibit the Factor Xase complex. Our results indicate that the C2 domain in isolation does not recapitulate the characteristic membrane binding of Factor VIII, emphasizing that its role is co-operative with other domains of the intact Factor VIII molecule.

Location of the multimerin 1 binding site in coagulation factor V: an update

Activated coagulation factor V (FVa) is an important cofactor that accelerates thrombin production. In human blood, 25% of the factor V (FV) is stored in platelets, complexed to the polymeric, FV binding protein multimerin 1 (MMRN1). The light chain of FV is required for MMRN1 binding, and its C2 domain contains a MMRN1 binding site that overlaps phospholipid binding residues essential for FVa procoagulant function. The homologous structures and roles of the FVa light chain C1 and C2 domains led us to investigate if the C1 domain also contains a MMRN1 binding site. The MMRN1 binding properties of FV constructs were tested by modified enzyme-linked immunoassays, before and after thrombin activation. The constructs tested included the combined C1 and C2 domain deleted FV, and B-domain deleted forms of FV containing C1 domain point mutations or combined C1 and C2 domain phospholipid binding site mutations. The MMRN1 binding site in FV/FVa was mapped to a large region that included the C1 domain phospholipid binding residues Y1956 and L1957. The FV construct with combined C1 and C2 domain phospholipid binding site mutations had no MMRN1 binding, highlighting the critical role of the FV C1 and C2 domain phospholipid binding residues in MMRN1 binding. Our data update the information on the structural features of FV and FVa important for MMRN1 binding, and suggest that the extended MMRN1 binding site in the C1 and C2 domains is important for the storage of FV-MMRN1 complexes in platelets.

Retinoschisin (RS1), the Protein Encoded by the X-linked Retinoschisis Gene, Is Anchored to the Surface of Retinal Photoreceptor and Bipolar Cells through Its Interactions with a Na/K ATPase-SARM1 Complex

Retinoschisin or RS1 is a discoidin domain-containing protein encoded by the gene responsible for X-linked retinoschisis (XLRS), an early onset macular degeneration characterized by a splitting of the retina. Retinoschisin, expressed and secreted from photoreceptors and bipolar cells as a homo-octameric complex, associates with the surface of these cells where it serves to maintain the cellular organization of the retina and the photoreceptor-bipolar synaptic structure. To gain insight into the role of retinoschisin in retinal cell adhesion and the pathogenesis of XLRS, we have investigated membrane components in retinal extracts that interact with retinoschisin. Unlike the discoidin domain-containing blood coagulation proteins Factor V and Factor VIII, retinoschisin did not bind to phospholipids or retinal lipids reconstituted into unilamellar vesicles or immobilized on microtiter plates. Instead, co-immunoprecipitation studies together with mass spectrometric-based proteomics and Western blotting showed that retinoschisin is associated with a complex consisting of Na/K ATPase (alpha3, beta2 isoforms) and the sterile alpha and TIR motif-containing protein SARM1. Double labeling studies for immunofluorescence microscopy confirmed the co-localization of retinoschisin with Na/K ATPase and SARM1 in photoreceptors and bipolar cells of retina tissue. We conclude that retinoschisin binds to Na/K ATPase on photoreceptor and bipolar cells. This interaction may be part of a novel SARM1-mediated cell signaling pathway required for the maintenance of retinal cell organization and photoreceptor-bipolar synaptic structure.

The factor VIII C1 domain contributes to platelet binding

Activated factor VIII (FVIIIa) forms a procoagulant complex with factor IXa on negatively charged membranes, including activated platelet surfaces. Membrane attachment involves the FVIII C2 domain; involvement of the adjacent C1 domain has not been established. Binding of recombinant FVIII C1C2 and C2 proteins to platelets was detected by flow cytometry using (1) anti-C2 monoclonal antibody ESH8 followed by a phycoerythrin-labeled secondary antibody; (2) biotinylated C1C2 detected by phycoerythrin-labeled streptavidin, and (3) C1C2 and C2 site-specifically labeled with fluorescein. Highest binding and lowest background were obtained using fluorescein-conjugated proteins. More than 90% of activated platelets bound C1C2, compared with approximately 50% for equimolar C2. Estimates using fluorescent microbeads indicated approximately 7,000 C1C2-binding sites per platelet, approximately 1,400 for C2, and approximately 3,000 for fluorescein-labeled FVIIIa. Unlike C2 or FVIII(a), C1C2 bound to approximately 700 sites/platelet before activation. C1C2 binding to activated platelets appeared independent of von Willebrand factor and was competed effectively by FVIII(a), but only partially by excess C2. Fluorescein-labeled FVIIIa was competed much more effectively by C1C2 than C2 for binding to activated platelets. Two monoclonal antibodies that inhibit C2 binding to membranes competed platelet binding of C2 more effectively than C1C2. Thus, the C1 domain of FVIII contributes to platelet-binding affinity.

Crystal structure of the bovine lactadherin C2 domain, a membrane binding motif, shows similarity to the C2 domains of factor V and factor VIII

Lactadherin, a glycoprotein secreted by a variety of cell types, contains two EGF domains and two C domains with sequence homology to the C domains of blood coagulation proteins factor V and factor VIII. Like these proteins, lactadherin binds to phosphatidylserine (PS)-containing membranes with high affinity. We determined the crystal structure of the bovine lactadherin C2 domain (residues 1 to 158) at 2.4 A. The lactadherin C2 structure is similar to the C2 domains of factors V and VIII (rmsd of C(alpha) atoms of 0.9 A and 1.2 A, and sequence identities of 43% and 38%, respectively). The lactadherin C2 domain has a discoidin-like fold containing two beta-sheets of five and three antiparallel beta-strands packed against one another. The N and C termini are linked by a disulfide bridge between Cys1 and Cys158. One beta-turn and two loops containing solvent-exposed hydrophobic residues extend from the C2 domain beta-sandwich core. In analogy with the C2 domains of factors V and VIII, some or all of these solvent-exposed hydrophobic residues, Trp26, Leu28, Phe31, and Phe81, likely participate in membrane binding. The C2 domain of lactadherin may serve as a marker of cell surface phosphatidylserine exposure and may have potential as a unique anti-thrombotic agent.

Factor V C2 domain contains a major thrombin-binding site responsible for thrombin-catalyzed factor V activation

Factor (F)V is converted into its active form, FVa, by limited proteolysis. Thrombin-catalyzed activation of FV is essential for its full cofactor activation. Previously, we reported that thrombin was bound to the C2 domain in the light chain of FVIII. As FV has a similar domain structure to FVIII, we focused on the FV C2 domain as a possible binding region for thrombin. Kinetic parameters, measured by surface plasmon resonance, revealed that the K(d) values of anhydro-thrombin for FV, FVa, and the FV C2 domain were 66, 240, and 670 nmol L(-1), respectively. FV activation was increased by approximately 9-fold by the addition of thrombin. In the presence of the FV C2 domain, this increase of the FV activation was inhibited. However, FV activation was not inhibited by the addition of the FVIII C2 domain. FV was cleaved into a 105-kDa heavy chain and a 71/74-kDa light chain by thrombin-catalyzed proteolysis at Arg709, Arg1018 and Arg1545. In the presence of the FV C2 domain, the cleavage was inhibited at all sites. Proteolysis was not affected by the addition of the FVIII C2 domain. These results indicated that the FV C2 domain contains a major binding site for thrombin and that this domain is necessary for the proteolysis at all cleavage sites. Furthermore, the present results also suggested that thrombin has an independent binding site for FV different from that for FVIII.

Interactions of the C2 domain of human factor V with a model membrane

Activated coagulation Factor V is an important cofactor of the coagulation cascade that catalyzes the formation of the prothrombinase complex on the surface of membranes rich in phosphatidyl-L-serine (PS). Here we report molecular dynamics simulations of the two crystallographic structures (the open and closed conformations) of domain C2 of coagulation Factor V (FaVC2). The calculations were performed in water (1.5 ns for each conformation) and in the presence of a neutral phospholipid bilayer model (POPE; 10 ns for each conformation) in order to describe the dynamics of the free (plasma circulating) and membrane bound forms of FaVC2. Water simulations confirmed the hypothesis that the plasma circulating form is in the closed conformation. In contrast, the membrane simulations showed that both conformations are energetically compatible with membrane binding. We have investigated the mechanism, the dynamics, and the energetics of the binding process. Our data are consistent with published estimates of the immersion depth of the aromatic residues (W26 and W27), and with mutagenesis studies involving specific residues located on the spikes at the bottom of the FaVC2 structure. Electrostatic interactions between the phospholipid head groups and hydrophilic residues at the bottom of the structure play a key role in the binding process by creating a large number of hydrogen bonds that anchor the protein to the membrane. The simulations identified a stable phospholipid binding pocket reminiscent of a previously suggested PS interaction site. Our structural data could contribute to the design of potential inhibitors able to disrupt membrane association.

Role of the C2 domain of factor VIIIa in the assembly of factor-X activating complex on the platelet membrane

Optimal rates of factor X (FX) activation require binding of factor IXa (FIXa), factor VIII(a) [FVIII(a)], and FX to activated platelet receptors. To define the FVIIIa domains that mediate platelet interactions, albumin density gradient washed, gel-filtered platelets (3.5 x 10(8)/mL) activated by the thrombin receptor peptide, SFLLRN (25 microM), were incubated with 125I-labeled FVIII C2 domain, or 125I-FVIIIa, or 125I-FVIII((LC)), or peptides from the C2 domain region, with or without anti-C2 domain monoclonal antibodies (MoAb), ESH4 or ESH8. FVIIIa (Kd approximately 1.7 nM), FVIII((LC)) (Kd approximately 3 nM), and the C2 domain (Kd approximately 16 nM) all interacted with approximately 700-800 binding sites/platelet. Unlike FVIIIa, the C2 domain did not respond to the presence of excess EGR-FIXa (45 nM) and FX (1.5 microM) with enhanced binding stoichiometry and affinity. Both the MoAb ESH4 and a synthetic peptide corresponding to FVIII residues 2303-2332 (epitope for FVIII MoAb, ESH4) inhibited FVIIIa binding to platelets, whereas MoAb ESH8 and a C2 domain peptide corresponding to residues 2248-2285 (epitope for the FVIII MoAb, ESH8) failed to inhibit FVIIIa binding. Thus, a major platelet-binding site resides within residues 2303-2332 in the C2 domain of FVIIIa, and an additional site within residues 2248-2285 increases the stoichiometry and affinity of FVIIIa binding to activated platelets only in the presence of FIXa and FX but does not directly mediate FVIIIa binding to the platelet surface.

Mutation of hydrophobic residues in the factor Va C1 and C2 domains blocks membrane-dependent prothrombin activation

The binding of factor (FVa) to phosphatidylserine (PS) membranes regulates assembly of the prothrombinase complex. Two pairs of solvent-exposed amino acids, Tyr(1956)/Leu(1957) in the C1 domain and Trp(2063)/Trp(2064) in the C2 domain, each make significant contributions to the affinity of FVa for PS membranes, but individually neither pair of amino acids is required for prothrombinase assembly on 25% PS membranes. In this study we characterize a FVa mutant with alanine substitutions in both the C1 and C2 domains: (Y1956,L1957,W2063,W2064)A. We conclude that: (i) prothrombinase assembly on PS membranes requires Trp(2063)/Trp(2064) and/or Tyr(1956)/Leu(1957); (ii) combined mutation of Trp(2063)/Trp(2064) and Tyr(1956)/Leu(1957) results in only a modest 4-fold decrease in the rate of thrombin generation in the absence of membranes; (iii) the present data provide experimental support for the joint participation of the C1 and C2 domains in the binding of FVa to phospholipid membranes as suggested by the recently solved structure for FVai (A1/A3-C1-C2).

Lactadherin binds selectively to membranes containing phosphatidyl-L-serine and increased curvature

Lactadherin, a milk protein, contains discoidin-type lectin domains with homology to the phosphatidylserine-binding domains of blood coagulation factor VIII and factor V. We have found that lactadherin functions, in vitro, as a potent anticoagulant by competing with blood coagulation proteins for phospholipid binding sites [J. Shi and G.E. Gilbert, Lactadherin inhibits enzyme complexes of blood coagulation by competing for phospholipid binding sites, Blood 101 (2003) 2628-2636]. We wished to characterize the membrane-binding properties that correlate to the anticoagulant capacity. We labeled bovine lactadherin with fluorescein and evaluated binding to membranes of composition phosphatidylserine/phosphatidylethanolamine/phosphatidylcholine, 4:20:76 supported by 2 mum diameter glass microspheres. Lactadherin bound saturably with an apparent KD of 3.3+/-0.4 nM in a Ca++ -independent manner. The number of lactadherin binding sites increased proportionally to the phosphatidylserine content over a range 0-2% and less rapidly for higher phosphatidylserine content. Inclusion of phosphatidylethanolamine in phospholipid vesicles did not enhance the apparent affinity or number of lactadherin binding sites. The number of sites was at least 4-fold higher on small unilamellar vesicles than on large unilamellar vesicles, indicating that lactadherin binding is enhanced by membrane curvature. Lactadherin bound to membranes with synthetic dioleoyl phosphatidyl-L-serine but not dioleoyl phosphatidyl-D-serine indicating stereoselective recognition of phosphatidyl-L-serine. We conclude that lactadherin resembles factor VIII and V with stereoselective preference for phosphatidyl-L-serine and preference for highly curved membranes.

Identification of the MMRN1 Binding Region within the C2 Domain of Human Factor V

In platelets, coagulation cofactor V is stored in complex with multimerin 1 in alpha-granules for activation-induced release during clot formation. The molecular nature of multimerin 1 factor V binding has not been determined, although multimerin 1 is known to interact with the factor V light chain. We investigated the region in factor V important for multimerin 1 binding using modified enzyme-linked immunoassays and recombinant factor V constructs. Factor V constructs lacking the C2 region or entire light chain had impaired and absent multimerin 1 binding, respectively, whereas the B domain deleted construct had modestly reduced binding. Analyses of point mutated constructs indicated that the multimerin 1 binding site in the C2 domain of factor V partially overlaps the phosphatidylserine binding site and that the factor V B domain enhances multimerin 1 binding. Multimerin 1 did not inhibit factor V phosphatidylserine binding, and it bound to phosphatidylserine independently of factor V. There was a reduction in factor V in complex with multimerin 1 after activation, and thrombin cleavage significantly reduced factor V binding to multimerin 1. In molar excess, multimerin 1 minimally reduced factor V procoagulant activity in prothrombinase assays and only if it was added before factor V activation. The dissociation of factor V-multimerin 1 complexes following factor V activation suggests a role for multimerin 1 in delivering and localizing factor V onto platelets prior to prothrombinase assembly.

Surface-exposed hemophilic mutations across the factor VIII C2 domain have variable effects on stability and binding activities

Factor VIII (fVIII) is a plasma glycoprotein that functions as an essential cofactor in blood coagulation. Its carboxyl-terminal "C2" domain is responsible for binding to both activated platelet surfaces and von Willebrand factor. We characterized the effect of 20 hemophilia-associated missense mutations across this domain (that all occur in patients in vivo) on its stability and its binding activities. At least six of these mutations were severely destabilizing, and another four caused moderate destabilization and corresponding reductions in both binding functions. One mutant (A2201P) displayed a significant reduction in its membrane binding activity but normal von Willebrand factor binding, while two others (P2300S and R2304H) caused the opposite effect. Several mutations (including L2210P, V2223M, M2238V, and R2304C) displayed near wild-type stabilities and binding activities and may instead affect mRNA splicing or alternative properties or functions of the protein. This study demonstrated that von Willebrand factor and membrane binding activities can be uncoupled and uniquely disrupted by different mutations and that either effect can lead to similar reductions in clotting activity. It also illustrated how a heterogeneous genetic disorder causes diverse molecular phenotypes that result in similar disease states.

Disruption of Protein-Membrane Binding and Identification of Small-Molecule Inhibitors of Coagulation Factor VIII

Factor VIII is a critical member of the blood coagulation cascade. It binds to the membrane surfaces of activated platelets at the site of vascular injury via a highly specific interaction between factor VIII's carboxy-terminal C2 domain and their phosphatidylserine-rich lipid bilayer. We have identified small-molecule inhibitors of factor VIII's membrane binding activity that have IC50 values as low as 2.5 microM. This interaction is approximately 10(3)-fold tighter than that of free o-phospho-L-serine. These compounds also inhibit factor VIII-dependent activation of factor X, indicating that disruption of membrane lipid binding leads to inhibition of the intrinsic coagulation pathway. The tightest binding inhibitor is specific and does not prevent membrane binding by the closely related coagulation factor V. These results indicate that this and related compounds may be used as leads to develop novel antithrombotic agents.

The crystal structure of activated protein C-inactivated bovine factor Va: Implications for cofactor function

In vertebrate hemostasis, factor Va serves as the cofactor in the prothrombinase complex that results in a 300,000-fold increase in the rate of thrombin generation compared with factor Xa alone. Structurally, little is known about the mechanism by which factor Va alters catalysis within this complex. Here, we report a crystal structure of protein C inactivated factor Va (A1.A3-C1-C2) that depicts a previously uncharacterized domain arrangement. This orientation has implications for binding to membranes essential for function. A high-affinity calcium-binding site and a copper-binding site have both been identified. Surprisingly, neither shows a direct involvement in chain association. This structure represents the largest physiologically relevant fragment of factor Va solved to date and provides a new scaffold for the future generation of models of coagulation cofactors.

Trp2063 and Trp2064 in the Factor Va C2 Domain Are Required for High-Affinity Binding to Phospholipid Membranes but Not for Assembly of the Prothrombinase Complex

Interactions between factor Va and membrane phosphatidylserine (PS) regulate activity of the prothrombinase complex. Two solvent-exposed hydrophobic residues located in the C2 domain, Trp(2063) and Trp(2064), have been proposed to contribute to factor Va membrane interactions by insertion into the hydrophobic membrane bilayer. However, the prothrombinase activity of rHFVa W(2063, 2064)A was found to be significantly impaired only at low concentrations of PS (5 mol %). In this study, we find that 10-fold higher concentrations of mutant factor Va are required for half-maximal prothrombinase activity on membranes containing 25% PS. The ability of the mutant factor Va to interact with factor Xa on a membrane was also impaired since 4-fold higher concentrations of factor Xa were required for half-maximal prothrombinase activity. The interaction of factor Va with 25% PS membranes was also characterized using fluorescence energy transfer and surface plasmon resonance. We found that the affinity of mutant factor Va for membranes containing 25% PS was reduced at least 400-fold with a K(d) > 10(-7) M. The binding of mutant factor Va to 25% PS membranes was markedly enhanced in the presence of factor Xa, indicating stabilization of the factor Va-factor Xa-membrane complex. Our findings indicate that Trp(2063) and Trp(2064) play a critical role in the high-affinity binding of factor Va to PS membranes. It remains to be determined whether occupancy of this PS binding site in factor Va is also required for high-affinity binding to factor Xa.

Modulation of factor V levels in plasma by polymorphisms in the C2 domain

OBJECTIVE

Functional polymorphisms contributing to coagulation factor levels are preferential markers for association studies aimed at identifying prothrombic genetic components.

METHODS AND RESULTS

Factor V (FV) microsatellite genotypes were found to be associated with FV levels (P=0.003). Single nucleotide polymorphisms analysis and sequencing of the promoter and of coding regions identified two polymorphisms (Met2120Thr, Asp2194Gly) present in 20% of the population (n=1013) that are responsible for genotype-phenotype associations. The effect of the Met2120Thr polymorphism, both in plasma (mean reduction of FV level in the heterozygous condition: 25%) and in recombinant FV studies (34% reduction), was comparable to that of the Asp2194Gly change (20% and 34%, respectively). The study of 10 subjects with a rare genotype indicated that the Asp2194Gly substitution is the functional determinant of the reduced FV levels associated with the FVHR2 haplotype. Among Leiden carriers, the doubly heterozygous condition for FV2120Thr was found to be associated with a significantly increased activated protein-C resistance (APCR) (P<0.05), and the doubly heterozygous condition for FV2194Gly was found to be more frequent (P=0.009) in symptomatic than in asymptomatic subjects.

CONCLUSIONS

Extensive analysis of FV polymorphisms indicated that changes in the C2 domain modulate FV levels and might increase APCR and thrombotic risk in FV Leiden carriers through a pseudohomozygous mechanism.

Structural requirements for expression of factor Va activity

Thrombin activated factor Va (factor VIIa, residues 1-709 and 1546-2196) has an apparent dissociation constant (Kd,app) for factor Xa within prothrombinase of approximately 0.5 nM. A protease (NN) purified from the venom of the snake Naja nigricollis nigricollis, cleaves human factor V at Asp697, Asp1509, and Asp1514 to produce a molecule (factor VNN) that is composed of a Mr 100,000 heavy chain (amino acid residues 1-696) and a Mr 80,000 light chain (amino acid residues 1509/1514-2196). Factor VNN, has a Kd,app for factor Xa of 4 nm and reduced clotting activity. Cleavage of factor VIIa by NN at Asp697 results in a cofactor that loses approximately 60-80% of its clotting activity. An enzyme from Russell's viper venom (RVV) cleaves human factor V at Arg1018 and Arg1545 to produce a Mr 150,000 heavy chain and Mr 74,000 light chain (factor VRVV, residues 1-1018 and 1546-2196). The RVV species has affinity for factor Xa and clotting activity similar to the thrombin-activated factor Va. Cleavage of factor VNN at Arg1545 by alpha-thrombin (factor VNN/IIa) or RVV (factor VNN/RVV) leads to enhanced affinity of the cofactor for factor Xa (Kd,app approximately 0.5 nM). A synthetic peptide containing the last 13 residues from the heavy chain of factor Va (amino acid sequence 697-709, D13R) was found to be a competitive inhibitor of prothrombinase with respect to prothrombin. The peptide was also found to specifically interact with thrombin-agarose. These data demonstrate that 1) cleavage at Arg1545 and formation of the light chain of factor VIIa is essential for high affinity binding and function of factor Xa within prothrombinase and 2) a binding site for prothrombin is contributed by amino acid residues 697-709 of the heavy chain of the cofactor.

Factor V New Brunswick: Ala221Val associated with FV deficiency reproduced in vitro and functionally characterized

Factor V (FV) deficiency, also known as parahemophilia, is a rare bleeding disorder. Herein we investigate the first reported missense mutation associated with FV deficiency, Ala221Val, assigned as FV New Brunswick. To elucidate the molecular pathology associated with the Ala221Val substitution, the mutation was recreated in a recombinant system together with 3 FV mutants (Ala221Gly, Glu275Gln, and Cys220Ala/Cys301Ala) designed to help explain the Ala221Val phenotype. The expression pattern was analyzed by pulse-chase experiments and an FV-specific enzyme-linked immunosorbent assay (ELISA), the results suggesting the Ala221Val mutation not to interfere with the synthesis or secretion. The functional properties of the recombinant FV New Brunswick were evaluated in both plasma clotting and purified systems. The Ala221Val mutation did not affect the factor Xa (FXa) cofactor function; nor did it interfere with the activated protein C (APC)-mediated down-regulation of activated FV (FVa) activity. However, FV New Brunswick demonstrated reduced stability at 37 degrees C due to an increased rate of dissociation of light and heavy chains of FVa. In conclusion, this in vitro study of FV New Brunswick suggests the Ala221Val mutation not to impair synthesis and expression of procoagulant activity, indicating overall proper folding of the mutant molecule. Rather, the Ala221Val substitution appears to interfere with the stability of the activated FVa mutant, the reduced stability possibly explaining the deficiency symptoms associated with the mutation.

Severe factor V deficiency: exon skipping in the factor V gene causing a partial deletion of the C1 domain

BACKGROUND

Severe factor V (FV) deficiency is a rare coagulation disorder, characterized by very low or unmeasurable plasma levels of functional and immunoreactive FV. Among rare inherited coagulopathies, FV deficiency is the least characterized from a molecular point of view (only 12 mutations have been reported).

OBJECTIVES

The aim of this work was to investigate, at the molecular level, the pathogenetic mechanisms responsible for a case of severe FV deficiency.

PATIENTS AND METHODS

A 19-year-old Iranian man showing unmeasurable FV activity and severely reduced FV antigen level in plasma was studied. Mutation screening was performed by sequencing. The effect of the identified mutation was investigated both at the mRNA and at the protein level.

RESULTS

Molecular analysis of the factor V (FV) gene identified a novel homozygous A-->T transversion at position + 3 of the donor splice site of intron 19 (IVS19 + 3A-->T). Production of mutant mRNA in HeLa cells demonstrated that this mutation causes the entire exon 19 to be skipped from the FV mRNA. The mutant processed transcript codes for a deleted FV, lacking the first 24 amino acids of the C1 domain. Expression of the mutant FV protein in COS-1 cells showed that the deleted protein was synthesized but not secreted; moreover, the intracellular amount of deleted FV was reduced compared to wild type, suggesting intracellular degradation of mutant FV.

CONCLUSIONS

This work reports the molecular characterization of the first mutation causing a partial deletion in the FV molecule, resulting in a severe impairment of protein secretion.

Binding of factor VIII inhibitors to discrete regions of the factor VIII C2 domain disrupt phospholipid binding

We characterized seven factor VIII inhibitors with epitopes in the C2 domain of factor VIII using a series of factor V C2 domain chimeras that substituted exon-sized fragments of the C2 domain of factor VIII for the corresponding regions of factor V. All inhibited co-factor activity of factor VIII and six inhibited binding of factor VIII to phosphatidylserine. Inhibitors Hz, JN and GK32 bound epitopes within amino acids S2173-K2281; inhibitors GK24 and TO bound epitopes within amino acids V2223-Y2332; and inhibitors UNC11 and UNC12 bound epitopes throughout the C2 domain (amino acids S2173-Y2332). Inhibitors Hz, JN and UNC12 inhibited the co-factor activity of chimera 5A, which substituted amino acids S2173-Q2222 of factor VIII for the corresponding region of factor V, in a prothrombinase assay. This inhibition could be partially reversed by pre-incubation of chimera 5A with phospholipid vesicles, suggesting that these antibodies interfered with phospholipid binding. Inhibitors UNC11 and UNC12, on the other hand, did not inhibit the binding of chimera 1 A to phosphatidylserine, suggesting that binding to the segment spanning amino acids V2282-Y2332 does not necessarily block phospholipid binding. These results agree with the model of the phospholipid-binding site determined by crystal structure of the C2 domain of factor VIII.

Anti-factor V auto-antibody in the plasma and platelets of a patient with repeated gastrointestinal bleeding

Development of autoantibody against coagulation factor V (FV) is a rare clinical condition with hemorrhagic complications of varying severity. The aim of this study was to establish the pathomechanism of an acquired FV deficiency and characterize the FV inhibitor responsible for the clinical symptoms. A 78-year-old female was admitted to hospital with severe gastrointestinal bleeding. General clotting tests and determination of clotting factors were performed by standard methods. FV antigen and FV containing immune complexes were measured by ELISA. The FV molecule was investigated by Western blotting and by sequencing the f5 gene. The binding of patient's IgG to FV and activated FV (FVa) was demonstrated in an ELISA system and its effect on the procoagulant activity of FVa was tested in clotting tests and in a chromogenic prothrombinase assay. Localization of the epitope for the antibody was performed by blocking ELISA. FV activity was severely suppressed both in plasma and platelets. FV antigen levels were normal by ELISA using polyclonal anti-FV antibody or monoclonal antibody against the connecting region of FV, but depressed when HV1 monoclonal antibody against the C2 domain in the FV light-chain was used as capture antibody. The FV molecule was found intact. An IgG reacting with both FV and FVa was present in the patient's plasma and its binding to FV was inhibited by HV1 antibody. FV-containing immune complexes were detected in the patient's plasma and platelet lysate. The patient's IgG inhibited the procoagulant function of FVa. An anti-FV IgG was present in the patient's plasma and platelets. The autoantibody reacted with an epitope in the C2 domain of FV light chain and neutralized the procoagulant function of FVa.

Lactadherin inhibits enzyme complexes of blood coagulation by competing for phospholipid-binding sites

Lactadherin, a glycoprotein of the milk-fat globule membrane, contains tandem C domains with homology to discoidin-type lectins and to membrane-binding domains of blood-clotting factors V and VIII. We asked whether the structural homology confers the capacity to compete for the membrane-binding sites of factor VIII and factor V and to function as an anticoagulant. Our results indicate that lactadherin competes efficiently with factor VIII and factor V for binding sites on synthetic phosphatidylserine-containing membranes with half-maximal displacement at lactadherin concentrations of 1 to 4 nM. Binding competition correlated to functional inhibition of factor VIIIa-factor IXa (factor Xase) enzyme complex. In contrast to annexin V, lactadherin was an efficient inhibitor of the prothrombinase and the factor Xase complexes regardless of the degree of membrane curvature and the phosphatidylserine content. Lactadherin also inhibited the factor VIIa-tissue factor complex efficiently whereas annexin V was less effective. Because the inhibitory concentration of lactadherin was proportional to the phospholipid concentration, and because lactadherin was not an efficient inhibitor in the absence of phospholipid, the major inhibitory effect of lactadherin relates to blocking phospholipid sites rather than forming inhibitory protein-protein complexes. Lactadherin was also an effective inhibitor of a modified whole blood prothrombin time assay in which clotting was initiated by dilute tissue factor; 60 nM lactadherin prolonged the prothrombin time 150% versus 20% for 60 nM annexin V. These results indicate that lactadherin can function as a potent phospholipid-blocking anticoagulant.

Arg2074Cys missense mutation in the C2 domain of factor V causing moderately severe factor V deficiency: molecular characterization by expression of the recombinant protein

Factor V (FV) deficiency is a rare bleeding disorder whose genetic basis has been described in a relatively small number of cases. Among a total of 12 genetic defects reported in severely or moderately severe deficient patients, 3 were missense mutations and in no case was the mechanism underlying the deficiency explored at the molecular level. In this study, a homozygous missense mutation at cDNA position 6394 in exon 23 of the FV gene was identified in a 22-year-old Italian patient. This mutation causes the replacement of arginine 2074 with a cysteine residue (Arg2074Cys) in the C2 domain of the protein. The effect of the Arg2074Cys mutation on FV secretion, stability, and activity was investigated. Site-directed mutagenesis of FV cDNA was used to introduce the identified mutation, and wild-type as well as mutant FV proteins were expressed by transient transfection in COS-1 cells. An enzyme immunoassay detected low FV antigen levels both in the conditioned media of cells expressing the mutant protein and in cell lysates. Metabolic labeling and pulse-chase experiments confirmed that the mutation caused an impaired secretion of FV associated with rapid intracellular degradation. In addition, evaluation of wild-type and mutant coagulant activity demonstrated that the FV molecules carrying the Arg2074Cys mutation have reduced activity. These findings, beside confirming the structural and functional importance of the arginine 2074 residue, demonstrate that its substitution with a cysteine impairs both FV secretion and activity.

Identification of a binding site for blood coagulation factor Xa on the heavy chain of factor Va. Amino acid residues 323-331 of factor V represent an interactive site for activated factor X

We have recently shown that amino acid region 307-348 of factor Va heavy chain (42 amino acids, N42R) is critical for cofactor activity and may contain a binding site for factor Xa and/or prothrombin [(2001) J. Biol. Chem. 276, 18614-18623]. To ascertain the importance of this region for factor Va cofactor activity, we have synthesized eight overlapping peptides (10 amino acid each) spanning amino acid region 307-351 of the heavy chain of factor Va and tested them for inhibition of prothrombinase activity. The peptides were also tested for the inhibition of the binding of factor Va to membrane-bound active site fluorescent labeled Glu-Gly-Arg human factor Xa ([OG488]-EGR-hXa). Factor Va binds specifically to membrane-bound [OG488]-EGR-hXa (10nM) with half-maximum saturation reached at approximately 6 nM. N42R was also found to interact with [OG488]-EGR-hXa with half-maximal saturation observed at approximately 230 nM peptide. N42R was found to inhibit prothrombinase activity with an IC50 of approximately 250 nM. A nonapeptide containing amino acid region 323-331 of factor Va (AP4') was found to be a potent inhibitor of prothrombinase. Kinetic analyses revealed that AP4' is a noncompetitive inhibitor of prothrombinase with respect to prothrombin, with a K(i) of 5.7 microM. Thus, the peptide interferes with the factor Va-factor Xa interaction. Displacement experiments revealed that the nonapeptide inhibits the direct interaction of factor Va with [OG488]-EGR-hXa (IC50 approximately 7.5 microM). The nonapeptide was also found to bind directly to [OG488]-EGR-hXa and to increase the catalytic efficiency of factor Xa toward prothrombin in the absence of factor Va. In contrast, a peptadecapeptide from N42R encompassing amino acid region 337-351 of factor Va (P15H) had no effect on either prothrombinase activity or the ability of the cofactor to interact with [OG488]-EGR-hXa. Our data demonstrate that amino acid sequence 323-331 of factor Va heavy chain contains a binding site for factor Xa.

Structural and functional aspects of three major glycoproteins of the human milk fat globule membrane

The MUC1 mucin, lactadherin, and butyrophilin are 3 major components of the human milk fat globule membrane. The mucin inhibits binding of S-fimbriated Escherichia coli to buccal epithelial cells, and lactadherin prevents symptomatic rotavirus infection in breast-fed infants. Butyrophilin has been suggested to be a structural component of the human milk fat globule (HMFG) membrane and to have receptor functions, but has no known anti-infective activity. These HMFG glycoproteins also are present in skimmed milk, possibly associated with phospholipid micelles, while mucin is also in a soluble form. Mucin and lactadherin resist digestion in the stomach of milk-fed infants, while butyrophilin is rapidly degraded. The MUC1 mucin is an extended rod-like structure forming part of the glycocalyx on the surface of many epithelial cells and membranes of milk, and may act as a decoy for binding of infective agents. The extracellular segment of butyrophilin has homology to Ig superfamily receptors and an intracellular domain with homology to developmentally regulated proteins. Lactadherin is a laterally mobile cell adhesion molecule that interacts with integrins and has a novel means of membrane-association involving specific binding to phosphatidylserine. The structural and functional aspects of these glycoproteins are discussed with regard to their role in human milk for breast-fed infants.

Phosphatidylserine binding alters the conformation and specifically enhances the cofactor activity of bovine factor Va

Factor V(a) is a cofactor for the serine protease factor X(a) that activates prothrombin to thrombin in the presence of Ca(2+) and a platelet membrane surface. A platelet membrane lipid, phosphatidylserine (PS), regulates the proteolytic activity of factor X(a) as well as the structure of prothrombin. Here we ask whether PS also regulates the structure and cofactor activity of factor V(a), which is a heterodimer composed of one heavy chain (A1-A2 domains) and one light chain (A3-C1-C2 domains). We use fluorescence, circular dichroism, equilibrium dialysis, and activity measurements to demonstrate the following: (1) Factor V(a) has four sites for dicaproyl-sn-glycero-3-phospho-L-serine (C(6)PS, a soluble form of PS); the heavy and light chains each bind two C(6)PS molecules. (2) In the absence of Ca(2+), only two sites remain, one in the heavy chain and another in the light chain. (3) Binding to these sites causes conformational changes evidenced by changes in intrinsic fluorescence and in CD spectra and changes in cofactor activity. (4) At least some of the four lipid binding sites are nonspecific with respect to soluble lipid species, but the site(s) that regulate(s) cofactor activity is (are) specific for C(6)PS, phosphatidic acid, or phosphatidyl(homo)serine and produce a response comparable to that seen with a PS-containing membrane. (5) Like Ca(2+), C(6)PS also mediates the interaction between factor V(a) heavy (V(a)-HC) and light (V(a)-LC) chains. We conclude that PS regulates both the cofactor and the enzyme of the prothrombin-activating complex.

Four hydrophobic amino acids of the factor VIII C2 domain are constituents of both the membrane-binding and von Willebrand factor-binding motifs

Factor VIII binds to phospholipid membranes and to von Willebrand factor (vWf) via its second C domain, which has lectin homology. The crystal structure of the C2 domain has prompted a model in which membrane binding is mediated by two hydrophobic spikes, each composed of a pair of residues displayed on a beta-hairpin turn, and also by net positive charge and specific interactions with phospho-l-serine. To test this model, we prepared 16 factor VIII mutants in which single or multiple amino acids were changed to alanine. Mutants at Arg(2215), Arg(2220), Lys(2227), Lys(2249), Gln(2213), Asn(2217), and Phe(2196)/Thr(2197) had specific activities that were >70% of the wild type. Mutants at Arg(2209), Lys(2227), Trp(2313), and Arg(2320) were degraded within the cell. Hydrophobic spike mutants at Met(2199)/Phe(2200), Leu(2251)/Leu(2252), and Met(2199)/Phe(2200)/Leu(2251)/Leu(2252) (4-Ala) exhibited 43, 59, and 91% reduction in specific activity in the activated partial thromboplastin time assay. In a phospholipid-limiting factor Xa activation assay, these mutants had a 65, 85, and 96% reduction in specific activity. Equilibrium binding of fluorescent, sonicated phospholipid vesicles to mutants immobilized on Superose beads was measured by flow cytometry. The affinities for phospholipid were reduced approximately 20-, 30-, and >35-fold for 2199/2200, 2251/2252, and 4-Ala, respectively. A dimeric form of mature vWf bound to immobilized factor VIII and the same mutants, but the affinities of the mutants were reduced approximately 5-, 10-, and >20-fold, respectively. In a competition, solution phase enzyme-linked immunosorbent assay, plasma vWf bound factor VIII and the same mutants with the affinities for the mutants reduced >5-, >5-, and >50-fold, respectively. We conclude that the two hydrophobic spikes are constituents of both the phospholipid-binding and vWf-binding motifs. In plasma, vWf apparently binds the inherently sticky membrane-binding motif, preventing nonspecific interactions.

Severe coagulation factor V deficiency caused by 2 novel frameshift mutations: 2952delT in exon 13 and 5493insG in exon 16 of factor 5 gene

A male infant with severe bleeding tendency had undetectable factor V activity. Sequence analysis of the proband's DNA revealed one base deletion in exon 13 (2952delT) and one base insertion in exon 16 (5493insG) in heterozygous form. Both mutations introduced a frameshift and a premature stop at codons 930 and 1776, respectively. The proband's father and mother were heterozygous for 2952delT and for 5493insG, respectively. Both mutations would result in the synthesis of truncated proteins lacking complete light chain or its C-terminal part. In the patient's plasma, no factor V light chain was detected by enzyme-linked immunosorbent assay. The N-terminal portion of factor V containing the heavy chain, and the connecting B domain was severely reduced but detectable (1.7%). A small amount of truncated factor V-specific protein with a molecular weight ratio of 236 kd could be immunoprecipitated from the plasma and detected by Western blotting. This protein, factor V(Debrecen), corresponds to the translated product of exon 16 mutant allele.

Soluble phosphatidylserine binds to a single identified site in the C2 domain of human factor Va

Factor V(a) (FV(a)) is a cofactor for the serine protease factor X(a) that activates prothrombin to thrombin in the presence of Ca(2+) and a membrane surface. FV(a) is a heterodimer composed of one heavy chain (A1 and A2 domains) and one light chain (A3, C1, and C2 domains). We use fluorescence, circular dichroism, and equilibrium dialysis to demonstrate that (1) the FV C2 domain expressed in Sf9 cells binds one molecule of C6PS with a k(d) of approximately 2 microM, (2) stabilizing changes occur in the FV C2 domain upon C6PS binding, (3) the C6PS binding site in the FV C2 domain is located near residue Cys(2113), which reacts with DTNB, and (4) binding to a PS-containing membrane is an order of magnitude tighter than that to soluble C6PS. Coupled with a recently published crystal structure of the C2 domain, these results support a model for the mechanism of C2-membrane interaction.

Mechanism of factor Va inactivation by plasmin. Loss of A2 and A3 domains from a Ca2+-dependent complex of fragments bound to phospholipid

The coagulation cofactor Va (FVa) is a noncovalent heterodimer consisting of a heavy chain (FVaH) and a light chain (FVaL). Previously, the fibrinolytic effector plasmin (Pn) has been shown to inhibit FVa function. To understand this mechanism, the fragmentation profile of human FVa by Pn and the noncovalent association of the derived fragments were determined in the presence of Ca(2+) using anionic phospholipid (aPL)-coated microtiter wells and large (1 microm) aPL micelles as affinity matrices. Following Pn inactivation of aPL-bound FVa, a total of 16 fragments were observed and their NH(2) termini sequenced. These had apparent molecular weights and starting residues as follows (single letter abbreviation is used): 50(L1766), 48(L1766), 43(Q1828), 40(Q1828), 30(S1546), 12(T1657), and 7(S1546) kDa from FVaL; and 65(A1), 50(A1), 45(A1), 34(S349), 30(L94), 30(M110), and 3 small <5(W457, W457, and K365) kDa from FVaH. Of these, 50(L1766), 48(1766), 43(Q1828), and 40(Q1828) spanning the C1/C2 domains, and 30(L94), but not the similar 30(M110), positioned within the A1 domain remained associated with aPL. These were detected antigenically during Pn- or tissue plasminogen activator-mediated lysis of fibrin clot formed in plasma. Chelation by EDTA dissociated the 30(L94)-kDa fragment, which was observed to associate with intact FVaL upon recalcification, indicating that the Leu-94 to Lys-109 region of the A1 domain plays a critical role in the FVaL and FVaH Ca(2+)-dependent association. By using domain-specific monoclonal antibodies and an assay for thrombin generation, loss of FVa prothrombinase function was coincident with proteolysis at sites in the A2 and A3 domains resulting in their dissociation. Inactivation of FV or FVa by Pn was independent of the thrombophilic R506Q mutation. These results identify the molecular composition of Pn-cleaved FVa that remains bound to membrane as largely A1-C1/C2 in the presence of Ca(2+) and suggest that Pn inhibits FVa by a process involving A2 and A3 domain dissociation.

The factor V HR2 haplotype: prevalence and association of the A4070G and A6755G polymorphisms

Recently, a polymorphism was identified in exon 25 of the factor V gene that is possibly a functional candidate for the HR2 haplotype. This haplotype is characterized by a single base substitution named R2 (A4070G) in the B domain of the protein. A mutation (A6755G; 2194Asp-->Gly) located near the C terminus has been hypothesized to influence protein folding and glycosylation, and might be responsible for the shift in factor V isoform (FV1 / FV2) ratio. This study investigated the prevalence of these two factor V HR2 haplotype polymorphisms in a cohort of normal blood donors, patients with osteoarthritis and women with complications during pregnancy, and in families of factor V Leiden individuals. A high allele frequency for the two polymorphisms was found in the blood donor group (6.2% R2, 5.6% A6755G). No significant difference in allele frequency was observed in the clinical groups (obstetric complications and osteoarthritis, 4.1-4.9% for the two polymorphisms) when compared with that of healthy blood donors. We confirm that the factor V A6755G polymorphism shows strong linkage to the R2 allele, although it is not exclusively inherited with the exon 13 A4070G variant and can occur independently.

Structural and functional characterization of platelet receptor-mediated factor VIII binding

Optimal rates of factor X (FX) activation require occupancy of receptors for factor IXa (FIXa), factor VIII (FVIII), and FX on the activated platelet surface. The presence of FVIII and FX increases 5-fold the affinity of FIXa for the surface of activated platelets, and the presence of FVIII or FVIIIa generates a high affinity, low capacity specific FX-binding site on activated platelets. We have now examined the effects of FX and active site-inhibited FIXa (EGR-FIXa) on the binding of both FVIII and FVIIIa to activated platelets and show the following: (a) von Willebrand factor inhibits FVIII binding (K(i) = 0.54 nM) but not FVIIIa binding; (b) thrombin and the thrombin receptor activation peptide (SFLLRN amide) are the most potent agonists required for FVIII-binding site expression, whereas ADP is inert; (c) FVa does not compete with FVIIIa or FVIII for functional platelet-binding sites; and (d) Annexin V is a potent inhibitor of FVIIIa binding (IC(50) = 10 nM) to activated platelets. The A2 domain of FVIII significantly increases the affinity and stoichiometry of FVIIIa binding to platelets and contributes to the stability of the FX-activating complex. Both FVIII and FVIIIa binding were specific, saturable, and reversible. FVIII binds to specific, high affinity receptors on activated platelets (n = 484 +/- 59; K(d) = 3.7 +/- 0.31 nM) and FVIIIa interacts with an additional 300-500 sites per platelet with enhanced affinity (K(d) = 1.5 +/- 0.11 nM). FVIIIa binding to activated platelets in the presence of FIXa and FX is closely coupled with rates of F-X activation. The presence of EGR-FIXa and FX increases both the number and the affinity of binding sites on activated platelets for both FVIII and FVIIIa, emphasizing the validity of a three-receptor model in the assembly of the F-X-activating complex on the platelet surface.

Identification of functionally important amino acid residues within the C2-domain of human factor V using alanine-scanning mutagenesis

We have previously determined that the C2-domain of human factor V (residues 2037-2196) is required for expression of cofactor activity and binding to phosphatidylserine (PS)-containing membranes. Naturally occurring factor V inhibitors and a monoclonal antibody (HV-1) recognized epitopes in the amino terminus of the C2-domain (residues 2037-2087) and blocked PS binding. We have now investigated the function of individual amino acids within the C2-domain using charge to alanine mutagenesis. Charged residues located within the C2-domain were changed to alanine in clusters of 1-3 mutations per construct. In addition, mutants W2063A, W2064A, (W2063, W2064)A, and L2116A were constructed as well. The resultant 30 mutants were expressed in COS cells using a B-domain deleted factor V construct (rHFV des B). All mutants were expressed efficiently based on the polyclonal antibody ELISA. The charged residues, Arg(2074), Asp(2098), Arg(2171), Arg(2174), and Glu(2189) are required for maintaining the structural integrity of the C2-domain of factor V. Four of these residues (Arg(2074), Asp(2098), Arg(2171), and Arg(2174)) correspond to positions in the factor VIII C-type domains that have been identified as point mutations in patients with hemophilia A. The epitope for the inhibitory monoclonal antibody HV-1 has been localized to Lys(2060) through Glu(2069) in the factor V C2-domain. The epitope for the inhibitory monoclonal antibody 6A5 is composed of amino acids His(2128) through Lys(2137). The PS-binding site in the factor V C2-domain includes amino acid residues Trp(2063) and Trp(2064). This site overlaps with the epitope for monoclonal antibody HV-1. These factor V C2-domain mutants should provide valuable tools for further defining the molecular interactions responsible for factor V binding to phospholipid membranes.