Binding of human factor VIII to phospholipid vesicles

Factor VIII, a protein cofactor involved in blood coagulation, functions in vitro on a phospholipid membrane surface to greatly increase the rate of factor X activation by factor IXa. Using gel filtration, rapid sedimentation, and resonance energy transfer we have studied the interaction of recombinant-derived human factor VIII with small and large unilamellar phospholipid vesicles composed of phosphatidylserine and phosphatidylcholine. Resonance energy transfer, from intrinsic fluorophores in factor VIII to dansyl-phosphatidylethanolamine incorporated into vesicles, has been adapted for quantitative equilibrium measurements. Factor VIII binds rapidly and reversibly to small and large vesicles. At 8 degrees C the interaction of factor VIII with small vesicles fits a simple bimolecular model with a KD of 2 nM and a phospholipid binding site defined by 180 phospholipid monomers. At 25 degrees C the binding of factor VIII to small vesicles containing 20% phosphatidylserine can be described by an apparent KD of 4 nM; the phospholipid/protein ratio at saturation was 170. Binding to large vesicles was demonstrated with a KD of 2 nM and a phospholipid/protein ratio at saturation of 385. Binding was dependent upon the phosphatidylserine mole fraction and was nonlinear from 0 to 30% phosphatidylserine content. A direct comparison of factor VIII and factor V binding indicated that the affinity of factor V to phospholipid vesicles was equivalent to that of factor VIII and that the phosphatidylserine requirement was lower. A model is proposed to explain the nonlinear phosphatidylserine dependence of binding for factor VIII.

PADGEM protein: a receptor that mediates the interaction of activated platelets with neutrophils and monocytes

PADGEM (platelet activation dependent granule-external membrane protein) is an integral membrane protein of the alpha granules of platelets and Weibel-Palade bodies of endothelial cells that is expressed on the plasma membrane upon cell activation and granule secretion. Activated platelets, but not resting platelets, bind to neutrophils, monocytes, HL60 cells, and U937 cells. This interaction is inhibited by anti-PADGEM antibodies, PADGEM, and EDTA; anti-GPIIb-IIIa, anti-thrombospondin, anti-GPIV, and thrombospondin produce no effect. Neutrophils and U937 cells, in contrast to Jurkatt cells, contain PADGEM recognition sites, as shown by binding of PADGEM contained in phospholipid vesicles. These results indicate that PADGEM mediates adhesion of activated platelets to monocytes and neutrophils. Therefore, PADGEM shares not only structural but also functional homology with ELAM-1 and MEL-14, members of a new family of vascular cell adhesion molecules.

Vitamin K-dependent carboxylase: affinity purification from bovine liver by using a synthetic propeptide containing the gamma-carboxylation recognition site

The vitamin K-dependent carboxylase catalyzes the posttranslational modification of specific glutamic acid residues to form gamma-carboxyglutamic acid residues within the vitamin K-dependent proteins. This enzyme recognizes the gamma-carboxylation recognition site on the propeptide of the precursor forms of the vitamin K-dependent blood coagulation proteins. To purify this enzyme to homogeneity, the carboxylase from bovine liver microsomes was solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), the protein was fractionated with ammonium sulfate, and then the enzyme was isolated by affinity chromatography using a synthetic peptide based upon the structure of the prothrombin propeptide. Elution with 10 mM propeptide yielded a single major band on SDS gel electrophoresis with a molecular weight of 77,000. In the presence of high concentrations of propeptide, only minimal carboxylase activity was measurable. Antibodies to the protein inhibited the carboxylase activity in crude preparations. In an alternative affinity purification strategy the propeptide was coupled through an NH2-terminal cysteine to an activated thiol-Sepharose column. The carboxylase-propeptide complex was eluted at 25 degrees C by reductive cleavage of the enzyme-propeptide complex in the presence of detergent and phospholipids. The eluted protein (Mr, 77,000) contained both stable vitamin K-dependent carboxylase and vitamin K epoxidase activity. The protein, purified by either method, was detected as a single band (Mr, 77,000) in a Western blot using anti-carboxylase antibodies. A 10,000-fold purification of carboxylase activity from crude microsomes was estimated. Purified bovine liver vitamin K-dependent carboxylase should facilitate the study of its structure and of the mechanism of action of vitamin K as a cofactor in the reaction catalyzed by this enzyme.

Vitamin K-dependent carboxylation. In vitro modification of synthetic peptides containing the gamma-carboxylation recognition site

Synthetic peptides including the gamma-carboxylation recognition site and acidic amino acids were compared as substrates for vitamin K-dependent gamma-carboxylation by bovine liver carboxylase. The 28-residue proPT28 (proprothrombin -18 to +10) and proFIX28 (pro-Factor IX -18 to +10) were carboxylated with a Km of 3 microM. The Vmax of proPT28 was 2-3 times greater than that of proFIX28. An analog of proFIX28 that contained the prothrombin propeptide had a Vmax 2-3-fold greater than an analog of proPT28 that contained the Factor IX propeptide. proFIX28/RS-1, based upon Factor IX Cambridge, proFIX28/RQ-4, based upon Factor IX Oxford 3, and proFIX28 had equivalent Km and Vmax values. Analogs of proPT28 containing Ala6-Glu7 or Glu6-Ala7 were carboxylated at equivalent rates. A peptide containing Asp6-Asp7 was carboxylated at a rate of about 1% of that of Glu carboxylation. Carboxylation of peptides containing Asp6-Glu7 and Glu6-Asp7 yielded results identical with peptides containing Ala6-Glu7 and Glu6-Ala7. Carboxymethylcysteine was not carboxylated when substituted for Glu6 in a peptide containing Asp7. These results indicate that the prothrombin propeptide is more efficient in the carboxylation process than is the Factor IX propeptide, but that both propeptides direct carboxylation; the gamma-carboxylation recognition site does not include residues -4 and -1; aspartic acid and carboxymethylcysteine are poor substrates for the carboxylase, but aspartic acid does not inhibit the carboxylation of adjacent glutamic acids.

Factor IX San Dimas. Substitution of glutamine for Arg-4 in the propeptide leads to incomplete gamma-carboxylation and altered phospholipid binding properties

DNA sequence analysis of the Factor IX gene from a hemophilia B patient (98% Factor IX antigen; less than 0.01 unit/ml clotting activity) has identified a point mutation in exon II. A guanine to adenine transition causes the substitution of a glutamine codon for an arginine codon at -4 in the propeptide of Factor IX. This variant, termed Factor IX San Dimas, circulates in the plasma as proFactor IX with a mutant 18-amino acid propeptide still attached. Like Factor IX Cambridge (Arg-1----Ser), Factor IX San Dimas is unable to express metal-induced epitopes recognized by conformation-specific polyclonal antibodies. Amino acid analysis of the alkaline hydrolysate indicates that purified Factor IX San Dimas contains a reduced number of gamma-carboxyglutamyl residues compared to Factor IX. However, this protein undergoes metal-induced quenching of the intrinsic fluorescence. In addition, Factor IX San Dimas is unable to interact with phospholipid vesicles. The absence of coagulant activity in Factor IX San Dimas can be attributed to impaired calcium-induced conformational changes and loss in the ability to bind phospholipid vesicles in the presence of calcium ions.

Amino acid sequence of a platelet-binding human anti-DNA monoclonal autoantibody

The complete amino acid sequences of the variable regions of the heavy and light chains of a human IgM monoclonal platelet-binding autoantibody have been determined. This antibody, HF2-1/17, produced by a human x human hybridoma prepared from lymphocytes of a patient with systemic lupus erythematosus and thrombocytopenia, is polyreactive with single-stranded DNA, synthetic polynucleotides, sulfated carbohydrates, and acidic glycolipids isolated from platelet membranes. The heavy chain is of the VHIII subgroup, and the light chain is of the VKI subgroup. The heavy chain is the expression product of the VH26 germline gene. The light chain bears significant homology to other immunoglobulins of known primary structure, including WEA, GAL, HAU, HK101, and DEE. These results suggest that HF2-1/17 may be an autoantibody derived with little or no modification from germline genes. A model of the antibody combining site suggests that arginine 24 and arginine 30 in the light chain (CDR1) interact with a surface defined by phosphate or sulfate groups of the antigen.

Molecular defects of factor IX Chicago-2 (Arg 145----His) and prothrombin Madrid (Arg 271----cys): arginine mutations that preclude zymogen activation

Factor IX Chicago-2 and prothrombin Madrid were purified from patients with hemophilia B and congenital dysprothrombinemia, respectively. Each protein displays defects in zymogen activation secondary to the failure to cleave one of the sessile bonds whose cleavage is necessary for full coagulant activity. These proteins were isolated by immunoaffinity chromatography using conformation-specific antibodies directed at either factor IX or prothrombin. Factor IX Chicago-2 is cleaved abnormally by factor XIa, yielding a pattern consistent with the failure to cleave the sessile bond between Arg 145 and Ala 146. Prothrombin Madrid is cleaved abnormally by factor Xa, yielding a pattern consistent with the failure to cleave the sessile bond between Arg 271 and Thr 272. Peptide mapping was performed on reduced and alkylated factor IX, factor IX Chicago-2, prothrombin, and prothrombin Madrid, and the hydrolysates were separated by high-performance liquid chromatography. The mutant peptide in factor IX Chicago-2 was identified by automated Edman degradation as residues 143 through 188 of factor IX, and had a histidine substituted for arginine at residue 145. The mutant peptide identified in prothrombin Madrid corresponds to residues 267 through 285 of prothrombin and has the substitution of cysteine for arginine at residue 271. These mutations, each occurring at arginines, are identical to those in factor IX Chapel Hill and prothrombin Barcelona. These results suggest that a limited repertoire of point mutations, many affecting arginine residues, may be responsible for hereditary defects of the vitamin K-dependent proteins in patients with normal antigen levels.

PADGEM (GMP140) is a component of Weibel-Palade bodies of human endothelial cells

PADGEM protein (PADGEM), also known as GMP140, is a platelet alpha-granule membrane protein that is translocated to the external membrane after platelet activation. Although the biosynthesis of this protein was originally thought to be confined to megakaryocytes, the synthesis of PADGEM in endothelial cells was recently demonstrated (McEver et al: Blood 70:1974a, 1987). We now describe the subcellular localization of this protein in endothelial cells. Immunofluorescence staining of permeabilized human umbilical vein endothelial cells with KC4, a well characterized monoclonal antibody to PADGEM, showed positively stained elongated structures similar in distribution and shape to Weibel-Palade bodies. Their identity as Weibel-Palade bodies was confirmed by double label immunofluorescence using KC4 and a polyclonal antiserum to von Willebrand factor (vWf), a protein known to be specifically stored in these organelles. All Weibel-Palade bodies were found to contain PADGEM. In contrast to strong perinuclear staining produced with anti-vWf antibodies, no significant perinuclear staining was obtained with KC4, indicating that relatively little PADGEM is present in the endoplasmic reticulum and in the Golgi apparatus. In endothelial cells treated with secretagogues that stimulate vWf release the elongated structures positive for PADGEM disappeared, further identifying these structures as Weibel-Palade bodies. This observation extends the parallels between Weibel-Palade bodies and alpha-granules and suggests a possible functional association between vWf and PADGEM.

PADGEM protein in human erythroleukemia cells

PADGEM protein, a platelet alpha granule membrane glycoprotein with a molecular weight of 140,000, is translocated to the plasma membrane during granule secretion and platelet activation. PADGEM protein is expressed on the surface of activated platelets but not on the surface of resting platelets. Human erythroleukemia (HEL) cells contain platelet alpha granule-like organelles, alpha granule proteins, and express platelet membrane glycoproteins GPIIb/IIIa and GPIb. We demonstrate that HEL cells express a protein that has a molecular weight identical to that of PADGEM and binds to anti-PADGEM antibodies. The exposure of HEL cells in culture to dimethylsulfoxide (DMSO) increased the number of cells expressing PADGEM. Fluorescence activated flow cytometric analysis demonstrated an increase in mean surface expression of PADGEM in DMSO-exposed cells compared to noninduced cells. Total cell content of PADGEM was increased 5.3-fold after DMSO exposure, as determined by radioimmunoassay. Direct binding experiments with the monoclonal anti-PADGEM antibody KC4 demonstrated specific, saturable, and time-dependent interaction of KC4 with HEL cells. A Kd of 7 nM was estimated. There were 14,000 surface binding sites per cell in noninduced cells and 24,000 surface binding sites per cell in DMSO-induced HEL cells. Surface expression of PADGEM protein on HEL cells was not increased with platelet agonists, including thrombin, epinephrine, ADP, nor cytokines, including IL-1, IL-2, tissue necrosis factor. The presence of PADGEM protein in HEL cells should facilitate the elucidation of the function of PADGEM protein.

Thrombus imaging in a primate model with antibodies specific for an external membrane protein of activated platelets

The activated platelet is a potential target for the localization of thrombi in vivo since, after stimulation and secretion of granule contents, activated platelets are concentrated at sites of blood clot formation. In this study, we used antibodies specific for a membrane protein of activated platelets to detect experimental thrombi in an animal model. PADGEM (platelet activation-dependent granule-external membrane protein), a platelet alpha-granule membrane protein, is translocated to the plasma membrane during platelet activation and granule secretion. Since PADGEM is internal in unstimulated platelets, polyclonal anti-PADGEM and monoclonal KC4 antibodies do not bind to circulating resting platelets but do interact with activated platelets. Dacron graft material incubated with radiolabeled KC4 or anti-PADGEM antibodies in the presence of thrombin-activated platelet-rich plasma bound most of the antibody. Imaging experiments with 123I-labeled anti-PADGEM in baboons with an external arterial-venous Dacron shunt revealed rapid uptake in the thrombus induced by the Dacron graft; control experiments with 123I-labeled nonimmune IgG exhibited minimal uptake. Deep venous thrombi, formed by using percutaneous balloon catheters to stop blood flow in the femoral vein of baboons, were visualized with 123I-labeled anti-PADGEM. Thrombi were discernible against blood pool background activity without subtraction techniques within 1 hr. No target enhancement was seen with 123I-labeled nonimmune IgG. 123I-labeled anti-PADGEM cleared the blood pool with an initial half-disappearance time of 6 min and did not interfere with hemostasis. These results indicate that radioimmunoscintigraphy with anti-PADGEM antibodies can visualize thrombi in baboon models and is a promising technique for clinical thrombus detection in humans.

Fat-soluble vitamin nutriture in primary biliary cirrhosis

We measured serum levels of vitamins A, E, 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D, as well as levels of abnormal (des-gamma-carboxy) prothrombin, in 52 patients with primary biliary cirrhosis. Decreased serum levels of retinol (vitamin A) and 25-hydroxyvitamin D and elevated levels of abnormal prothrombin were common in these patients and correlated with the histologic stage of the disease and with the clinical severity of disease as judged by elevated serum bilirubin levels and decreased serum albumin levels. The increased levels of abnormal prothrombin were due primarily to vitamin K deficiency but also, in part, to the severity of the liver disease itself. Vitamin E deficiency was rare. Only 1 patient had clinical manifestations of fat-soluble vitamin deficiency, night blindness, and gastrointestinal bleeding related to a marked prolongation of the prothrombin time. Deficiencies of fat-soluble vitamins are most likely to be present in jaundiced patients with long-standing, severe cholestasis. We suggest that fat-soluble vitamin status be determined in all patients with primary biliary cirrhosis by appropriate blood tests and that vitamin supplements be given only to those patients who require them.

Vitamin K-dependent carboxylation. A synthetic peptide based upon the gamma-carboxylation recognition site sequence of the prothrombin propeptide is an active substrate for the carboxylase in vitro

The vitamin K-dependent blood-clotting proteins contain a gamma-carboxylation recognition site in the propeptide, between the signal peptide and the mature protein, that directs gamma-carboxylation of specific glutamic acid residues. To develop a better substrate for the in vitro assay of the vitamin K-dependent gamma-carboxylase and to understand the substrate recognition requirements of the carboxylase, we prepared synthetic peptides based upon the structure of human proprothrombin. These peptides were employed as substrates for in vitro carboxylation using a partially purified form of the bovine liver carboxylase. A 28-residue peptide (HVFLAPQQARSLLQRVRRANTFLEEVRK), based on residues -18 to +10 in proprothrombin, includes the complete propeptide and the first 10 residues of acarboxyprothrombin. Carboxylation of this peptide is characterized by a Km of 3.6 microM. In contrast, FLEEL is carboxylated with a Km of about 2200 microM. A 10-residue peptide (ANTFLEEVRK), based on residues +1 to +10 in prothrombin, and a 20-residue peptide (ARSLLQRVRRANTFLEEVRK), based on residues -10 to +10 in proprothrombin, are also poor substrates for the carboxylase. Replacement of phenylalanine with alanine at residue 3 (equivalent to position -16 in proprothrombin) in the 28-residue peptide significantly alters the Km to 200 microM. A synthetic propeptide (HVFLAPQQARSLLQRVRRY), homologous to residues -18 to -1 in proprothrombin, inhibited carboxylation of the 28-residue peptide substrate with a Ki of 3.5 microM, but modestly stimulated the carboxylation of the 5- and 10-residue peptide substrates. These results indicate that an intact carboxylation recognition site is required for efficient in vitro carboxylation and that this site includes critical residues in region -18 to -11 of proprothrombin. The carboxylation recognition site in the propeptide binds directly to the carboxylase or to a closely associated protein.

Effect of propeptide mutations on post-translational processing of factor IX. Evidence that beta-hydroxylation and gamma-carboxylation are independent events

Post-translational processing of Factor IX includes glycosylation, cleavage of the signal peptide and propeptide, vitamin K-dependent carboxylation of specific glutamic acid residues to form gamma-carboxyglutamic acid, and beta-hydroxylation of aspartic acid at residue 64 to form beta-hydroxyaspartic acid. The human Factor IX cDNA coding sequence was modified in the propeptide region (residue -18 to -1) using oligonucleotide-directed site-specific mutagenesis, and the altered Factor IX cDNA was expressed in Chinese hamster ovary cells. The effects of the mutations on proteolytic processing, gamma-carboxylation, and beta-hydroxylation were assessed by direct structural analysis. After purification, the molecular weight of each of the recombinant Factor IX species and its NH2-terminal amino acid sequence were shown to be identical to those of plasma Factor IX. gamma-Carboxyglutamic acid and beta-hydroxyaspartic acid analyses revealed that recombinant wild-type Factor IX contained 9.2 gamma-carboxyglutamic acid and 0.3 beta-hydroxyaspartic acid residues/molecule compared with 11.4 gamma-carboxyglutamic acid and 0.39 beta-hydroxyaspartic acid residues in plasma Factor IX. When the 18-residue propeptide was deleted or when the cells were grown in the presence of sodium warfarin, secreted Factor IX contained no detectable gamma-carboxyglutamic acid but 0.36 and 0.40 residues of beta-hydroxyaspartic acid, respectively. Point mutations leading to substitution of alanine for phenylalanine at residue -16 or glutamic acid for alanine at residue -10 contained 0.2 and 1.7 gamma-carboxyglutamic acid residues, respectively, and 0.2 residues of beta-hydroxyaspartic acid. These data confirm that the propeptide mutations made do not interfere with proteolytic processing and that the Factor IX propeptide contains a recognition site that designates the adjacent glutamic acid-rich domain for gamma-carboxylation. In contrast, beta-hydroxylation of aspartic acid 64 is an independent process which does not require vitamin K and is mediated through a hydroxylation recognition site in the mature Factor IX, not in the propeptide.

Separation of human plasma factor IX from HTLV-I or HIV by immunoaffinity chromatography using conformation-specific antibodies

Immunoaffinity chromatography using conformation-specific antibodies yields pure factor IX from human plasma in a single rapid, facile purification step. We evaluated this technique to determine whether factor IX can be separated from human T cell leukemia virus-I (HTLV-I) and human immunodeficiency virus (HIV) in plasma supplemented with these viruses. Viral content was determined with an enzyme-linked immunosorbent (ELISA) assay sensitive to 50 ng viral protein. Both HTLV-I and HIV coeluted with unbound protein. Neither HTLV-I nor HIV was detected in purified factor IX. We conclude that, to the limits of detection, factor IX purified by this method is free of viral contamination.

The factor IX phospholipid-binding site is required for calcium-dependent activation of factor IX by factor XIa

To determine the functional role of the metal-dependent conformational changes in Factor IX, two populations of conformation-specific anti-Factor IX antibodies were prepared. Anti-Factor IX X Mg(II) antibodies bind to Factor IX in the presence of Mg(II) and other metal ions, but not in the absence of metal ions. Anti-Factor IX X Ca(II)-specific antibodies bind to Factor IX in the presence of Ca(II) and Sr(II), but not in the presence of Mn(II), Mg(II), and Ba(II). In the presence of a metal ion that induces the conformational transition recognized by the anti-Factor IX X Mg(II) antibodies, the concentrations of CaCl2 and SrCl2 needed for the half-maximal binding of the anti-Factor IX X Ca(II)-specific antibodies to Factor IX were reduced 3- and 20-fold, respectively. Factor IX binding to phospholipid vesicles was inhibited by the Fab fragments of the anti-Factor IX X Ca(II)-specific antibodies, but was not inhibited by the Fab fragments of the anti-Factor IX X Mg(II) antibodies. Factor XIa activation of Factor IX was also inhibited by the Fab fragments of the anti-Factor IX X Ca(II)-specific antibodies, but not by the anti-Factor IX X Mg(II) antibodies. These results support the hypothesis that Factor IX undergoes two metal-dependent conformational transitions: FIX----FIX'----FIX*. The first transition (FIX----FIX') is metal-dependent but cation-nonselective; the second transition (FIX'----FIX*) is metal-selective for Ca(II) or Sr(II). The second transition results in the expression of conformational determinants necessary for membrane binding and the Ca(II)-dependent activation of Factor IX by Factor XIa. These results suggest chemical similarity between a surface of a domain of Factor XIa and phospholipid vesicles, both of which interact with Factor IX in the presence of Ca(II).

Expression of completely gamma-carboxylated recombinant human prothrombin

Human prothrombin cDNA has been expressed in mammalian cells to yield biologically active, fully gamma-carboxylated prothrombin. A 2.0-kilobase cDNA encoding full-length prothrombin was isolated from a human fetal liver library using a cDNA fragment recovered from a lambda gt11 human hepatoma expression library. Prothrombin cDNA was cloned into a mammalian expression vector and transfected into Chinese hamster ovary cells. Selection for expression of dihydrofolate reductase yielded cell lines secreting up to 0.55 microgram/ml of prothrombin. Recombinant prothrombin synthesized in the presence of vitamin K was quantitatively recovered from tissue culture medium by affinity chromatography using conformation-specific antibodies directed against the metal-stabilized, gamma-carboxylated conformer. The purified material migrated as a single band on denaturing polyacrylamide gels with an electrophoretic mobility equivalent to that of plasma-derived human prothrombin. Automated Edman degradation of recombinant prothrombin revealed a single amino-terminal sequence identical to that of plasma-derived prothrombin. Recombinant and plasma-derived prothrombin interacted similarly with antibodies specific for total prothrombin, abnormal des-gamma-carboxyprothrombin, and two metal-stabilized conformers of prothrombin. Recombinant prothrombin exhibited a specific coagulant activity equivalent to that of plasma-derived prothrombin. The gamma-carboxyglutamic acid analysis of recombinant prothrombin demonstrated 9.9 +/- 0.4 mol of gamma-carboxyglutamic acid/mol of prothrombin. These results represent the first description of the expression of a recombinant vitamin K-dependent protein in which all of the expressed protein is gamma-carboxylated.

Recognition site directing vitamin K-dependent gamma-carboxylation resides on the propeptide of factor IX

Posttranslational processing of vitamin K-dependent proteins includes gamma-carboxylation of specific glutamic acid residues to form gamma-carboxyglutamic acids. To determine whether carboxylation is directed by the propeptide sequence, homologous among the precursors of these proteins, alterations were made in the Factor IX propeptide cDNA. The extent of gamma-carboxylation of recombinant Factor IX was assessed using conformation-specific antibodies directed against the gamma-carboxyglutamic acid-dependent, metal-stabilized structure. Deletion of the propeptide (residues -18 to -1) abolished carboxylation, but not secretion, of Factor IX. Substitution of alanine for phenylalanine -16 or glutamic acid for alanine -10 also impaired carboxylation. These results indicate that the Factor IX propeptide participates in defining a recognition site that designates an adjacent glutamic acid-rich domain for gamma-carboxylation. The association of the propeptide with the gamma-carboxylation recognition site provides the first demonstration of a specific function served by a propeptide in posttranslational protein processing.