A gamma-carboxyglutamic acid (gamma) variant (gamma 6D, gamma 7D) of human activated protein C displays greatly reduced activity as an anticoagulant

The structure-function relationship of activated protein C

Protein C is the central enzyme of the natural anticoagulant pathway and its activated form APC (activated protein C) is able to proteolyse non-active as well as active coagulation factors V and VIII. Proteolysis renders these cofactors inactive, resulting in an attenuation of thrombin formation and overall down-regulation of coagulation. Presences of the APC cofactor, protein S, thrombomodulin, endothelial protein C receptor and a phospholipid surface are important for the expression of anticoagulant APC activity. Notably, APC also has direct cytoprotective effects on cells: APC is able to protect the endothelial barrier function and expresses anti-inflammatory and anti-apoptotic activities. Exact molecular mechanisms have thus far not been completely described but it has been shown that both the protease activated receptor 1 and EPCR are essential for the cytoprotective activity of APC. Recently it was shown that also other receptors like sphingosine 1 phosphate receptor 1, Cd11b/CD18 and tyrosine kinase with immunoglobulin-like and EGFlike domains 2 are likewise important for APC signalling. Mutagenesis studies are being performed to map the various APC functions and interactions onto its 3D structure and to dissect anticoagulant and cytoprotective properties. The results of these studies have provided a wealth of structure-function information. With this review we describe the state-of-the-art of the intricate structure-function relationships of APC, a protein that harbours several important functions for the maintenance of both humoral and tissue homeostasis.

Lessons from natural and engineered mutations

Protective effects of non-anticoagulant activated protein C variant (D36A/L38D/A39V) in a murine model of ischaemic stroke

Ischaemic stroke is caused by occlusive thrombi in the cerebral vasculature. Although tissue-plasminogen activator (tPA) can be administered as thrombolytic therapy, it has major limitations, which include disruption of the blood-brain barrier and an increased risk of bleeding. Treatments that prevent or limit such deleterious effects could be of major clinical importance. Activated protein C (APC) is a natural anticoagulant that regulates thrombin generation, but also confers endothelial cytoprotective effects and improved endothelial barrier function mediated through its cell signalling properties. In murine models of stroke, although APC can limit the deleterious effects of tPA due to its cell signalling function, its anticoagulant actions can further elevate the risk of bleeding. Thus, APC variants such as APC(5A), APC(Ca-ins) and APC(36-39) with reduced anticoagulant, but normal signalling function may have therapeutic benefit. Human and murine protein C (5A), (Ca-ins) and (36-39) variants were expressed and characterised. All protein C variants were secreted normally, but 5-20% of the protein C (Ca-ins) variants were secreted as disulphide-linked dimers. Thrombin generation assays suggested reductions in anticoagulant function of 50- to 57-fold for APC(36-39), 22- to 27-fold for APC(Ca-ins) and 14- to 17-fold for APC(5A). Interestingly, whereas human wt APC, APC(36-39) and APC(Ca-ins) were inhibited similarly by protein C inhibitor (t_½ - 33 to 39 mins), APC(5A) was inactivated ~9-fold faster (t_½ - 4 mins). Using the murine middle cerebral artery occlusion ischaemia/repurfusion injury model, in combination with tPA, APC(36-39), which cannot be enhanced by its cofactor protein S, significantly improved neurological scores, reduced cerebral infarct area by ~50% and reduced oedema ratio. APC(36-39) also significantly reduced bleeding in the brain induced by administration of tPA, whereas wt APC did not. If our data can be extrapolated to clinical settings, then APC(36-39) could represent a feasible adjunctive therapy for ischaemic stroke.

Activated protein C action in inflammation

Activated protein C (APC) is a natural anticoagulant that plays an important role in coagulation homeostasis by inactivating the procoagulation factor Va and VIIIa. In addition to its anticoagulation functions, APC also has cytoprotective effects such as anti-inflammatory, anti-apoptotic, and endothelial barrier protection. Recently, a recombinant form of human APC (rhAPC or drotrecogin alfa activated; known commercially as 'Xigris') was approved by the US Federal Drug Administration for treatment of severe sepsis associated with a high risk of mortality. Sepsis, also known as systemic inflammatory response syndrome (SIRS) resulting from infection, is a serious medical condition in critical care patients. In sepsis, hyperactive and dysregulated inflammatory responses lead to secretion of pro- and anti-inflammatory cytokines, activation and migration of leucocytes, activation of coagulation, inhibition of fibrinolysis, and increased apoptosis. Although initial hypotheses focused on antithrombotic and profibrinolytic functions of APC in sepsis, other agents with more potent anticoagulation functions were not effective in treating severe sepsis. Furthermore, APC therapy is also associated with the risk of severe bleeding in treated patients. Therefore, the cytoprotective effects, rather than the anticoagulant effect of APC are postulated to be responsible for the therapeutic benefit of APC in the treatment of severe sepsis.

Hyperantithrombotic, noncytoprotective Glu149Ala-activated protein C mutant

Activated protein C (APC) reduces mortality in severe sepsis patients. APC exerts anticoagulant activities via inactivation of factors Va and VIIIa and cytoprotective activities via endothelial protein C receptor and protease-activated receptor-1. APC mutants with selectively altered and opposite activity profiles, that is, greatly reduced anticoagulant activity or greatly reduced cytoprotective activities, are compared here. Glu149Ala-APC exhibited enhanced in vitro anticoagulant and in vivo antithrombotic activity, but greatly diminished in vitro cytoprotective effects and in vivo reduction of endotoxin-induced murine mortality. Thus, residue Glu149 and the C-terminal region of APC's light chain are identified as functionally important for expression of multiple APC activities. In contrast to Glu149Ala-APC, 5A-APC (Lys191-193Ala + Arg229/230Ala) with protease domain mutations lacked in vivo antithrombotic activity, although it was potent in reducing endotoxin-induced mortality, as previously shown. These data imply that APC molecular species with potent antithrombotic activity, but without robust cytoprotective activity, are not sufficient to reduce mortality in endotoxemia, emphasizing the need for APC's cytoprotective actions, but not anticoagulant actions, to reduce endotoxin-induced mortality. Protein engineering can provide APC mutants that permit definitive mechanism of action studies for APC's multiple activities, and may also provide safer and more effective second-generation APC mutants with reduced bleeding risk.

gamma-Glutamate and beta-hydroxyaspartate in proteins

Vitamin K-dependent coagulation plasma proteins possess from 9-12 residues of gamma-carboxyglutamic acid (Gla) distributed over a ca. 45 amino acid peptide sequence, i.e., the Gla domain, which encompasses the NH2-terminal region. In addition, epidermal growth factor (EGF) homology units present in many of these same proteins contain beta-hydroxyaspartate (Hya) residues, which is a modification decoupled from gamma-carboxylation. The function of Gla residues in these proteins, viz., prothrombin, coagulation factors VII, IX, and X, along with anticoagulant protein C and protein S, is to coordinate Ca2+. This results in a large conformational alteration in the proteins or peptides, which allows adsorption to membrane phospholipids (PL), an event that is critical is to their proper functions in the blood coagulation system. Less certain is the role of Hya in EGF domains, but it has been proposed that modification at this residue may negatively regulate fucosylation of these regions. In several proteins, these modules also interact with Ca2+, but it has been shown that although the particular aspartate containing the beta-OH group is critical to that interaction, beta-hydroxylation of that Asp residue is not. Because of their widespread distribution, quantitative detection protocols for both Gla and Hya are of importance. It is the purpose of this communication to detail a reliable method for these analyses that is employed in our laboratories.

Activated protein C mutant with minimal anticoagulant activity, normal cytoprotective activity, and preservation of thrombin activable fibrinolysis inhibitor-dependent cytoprotective functions

Activated protein C (APC) reduces mortality in severe sepsis patients and exhibits beneficial effects in multiple animal injury models. APC anticoagulant activity involves inactivation of factors Va and VIIIa, whereas APC cytoprotective activities involve the endothelial protein C receptor and protease-activated receptor-1 (PAR-1). The relative importance of the anticoagulant activity of APC versus the direct cytoprotective effects of APC on cells for the in vivo benefits is unclear. To distinguish cytoprotective from the anticoagulant activities of APC, a protease domain mutant, 5A-APC (RR229/230AA and KKK191-193AAA), was made and compared with recombinant wild-type (rwt)-APC. This mutant had minimal anticoagulant activity but normal cytoprotective activities that were dependent on endothelial protein C receptor and protease-activated receptor-1. Whereas anticoagulantly active rwt-APC inhibited secondary-extended thrombin generation and concomitant thrombin-dependent activation of thrombin activable fibrinolysis inhibitor (TAFI) in plasma, secondary-extended thrombin generation and the activation of TAFI were essentially unopposed by 5A-APC due to its low anticoagulant activity. Compared with rwt-APC, 5A-APC had minimal profibrinolytic activity and preserved TAFI-mediated anti-inflammatory carboxypeptidase activities toward bradykinin and presumably toward the anaphlatoxins, C3a and C5a, which are well known pathological mediators in sepsis. Thus, genetic engineering can selectively alter the multiple activities of APC and provide APC mutants that retain the beneficial cytoprotective effects of APC while diminishing bleeding risk due to reduction in APC's anticoagulant and APC-dependent profibrinolytic activities.

Selective modulation of protein C affinity for EPCR and phospholipids by Gla domain mutation

Uniquely amongst vitamin K-dependent coagulation proteins, protein C interacts via its Gla domain both with a receptor, the endothelial cell protein C receptor (EPCR), and with phospholipids. We have studied naturally occurring and recombinant protein C Gla domain variants for soluble (s)EPCR binding, cell surface activation to activated protein C (APC) by the thrombin-thrombomodulin complex, and phospholipid dependent factor Va (FVa) inactivation by APC, to establish if these functions are concordant. Wild-type protein C binding to sEPCR was characterized with surface plasmon resonance to have an association rate constant of 5.23 x 10(5) m(-1).s(-1), a dissociation rate constant of 7.61 x 10(-2) s(-1) and equilibrium binding constant (K(D)) of 147 nm. It was activated by thrombin over endothelial cells with a K(m) of 213 nm and once activated to APC, rapidly inactivated FVa. Each of these interactions was dramatically reduced for variants causing gross Gla domain misfolding (R-1L, R-1C, E16D and E26K). Recombinant variants Q32A, V34A and D35A had essentially normal functions. However, R9H and H10Q/S11G/S12N/D23S/Q32E/N33D/H44Y (QGNSEDY) variants had slightly reduced (< twofold) binding to sEPCR, arising from an increased rate of dissociation, and increased K(m) (358 nm for QGNSEDY) for endothelial cell surface activation by thrombin. Interestingly, these variants had greatly reduced (R9H) or greatly enhanced (QGNSEDY) ability to inactivate FVa. Therefore, protein C binding to sEPCR and phospholipids is broadly dependent on correct Gla domain folding, but can be selectively influenced by judicious mutation.

Molecular characterization of an extended binding site for coagulation factor Va in the positive exosite of activated protein C

The anticoagulant human plasma serine protease, activated protein C (APC), inhibits blood coagulation by specific inactivation of the coagulation cofactors factor Va (FVa) and factor VIIIa. Site-directed mutagenesis of residues in three surface loops of a positive exosite located on APC was used to identify residues that play a significant role in binding to FVa. Eighteen different residues were mutated to alanine singly, in pairs, or in triple mutation combinations. Mutant APC proteins were purified and characterized for their inactivation of FVa. Three APC residues were identified that provide major contributions to FVa interactions: Lys(193), Arg(229), and Arg(230). In addition, four residues made significant minor contributions to FVa interactions: Lys(191), Lys(192), Asp(214), and Glu(215). All of these residues primarily contribute to APC cleavage at Arg(506) in FVa and play a small role in the interaction of APC with the Arg(306) cleavage site. In conjunction with previously published work, these results define an extensive FVa binding site in the positive exosite of APC that is primarily involved in binding and cleaving at Arg(506) on FVa.

The autolysis loop of activated protein C interacts with factor Va and differentiates between the Arg506 and Arg306 cleavage sites

The anticoagulant human plasma serine protease, activated protein C (APC), inactivates blood coagulation factors Va (FVa) and VIIIa. The so-called autolysis loop of APC (residues 301-316, equivalent to chymotrypsin [CHT] residues 142-153) has been hypothesized to bind FVa. In this study, site-directed mutagenesis was used to probe the role of the charged residues in this loop in interactions between APC and FVa. Residues Arg306 (147 CHT), Glu307, Lys308, Glu309, Lys311, Arg312, and Arg314 were each individually, or in selected combinations, mutated to Ala. The purified recombinant protein C mutants were characterized using activated partial thromboplastin time (APTT) clotting assays and FVa inactivation assays. Mutants 306A, 308A, 311A, 312A, and 314A had mildly reduced anticoagulant activity. Based on FVa inactivation assays and APTT assays using purified Gln506-FVa and plasma containing Gln506-FV, it appeared that these mutants were primarily impaired for cleavage of FVa at Arg506. Studies of the quadruple APC mutant (306A, 311A, 312A, and 314A) suggested that the autolysis loop provides for up to 15-fold discrimination of the Arg506 cleavage site relative to the Arg306 cleavage site. This study shows that the loop on APC of residues 306 to 314 defines an FVa binding site and accounts for much of the difference in cleavage rates at the 2 major cleavage sites in FVa.

The autolysis loop of activated protein C interacts with factor Va and differentiates between the Arg506 and Arg306 cleavage sites

The anticoagulant human plasma serine protease, activated protein C (APC), inactivates blood coagulation factors Va (FVa) and VIIIa. The so-called autolysis loop of APC (residues 301-316, equivalent to chymotrypsin [CHT] residues 142-153) has been hypothesized to bind FVa. In this study, site-directed mutagenesis was used to probe the role of the charged residues in this loop in interactions between APC and FVa. Residues Arg306 (147 CHT), Glu307, Lys308, Glu309, Lys311, Arg312, and Arg314 were each individually, or in selected combinations, mutated to Ala. The purified recombinant protein C mutants were characterized using activated partial thromboplastin time (APTT) clotting assays and FVa inactivation assays. Mutants 306A, 308A, 311A, 312A, and 314A had mildly reduced anticoagulant activity. Based on FVa inactivation assays and APTT assays using purified Gln506-FVa and plasma containing Gln506-FV, it appeared that these mutants were primarily impaired for cleavage of FVa at Arg506. Studies of the quadruple APC mutant (306A, 311A, 312A, and 314A) suggested that the autolysis loop provides for up to 15-fold discrimination of the Arg506 cleavage site relative to the Arg306 cleavage site. This study shows that the loop on APC of residues 306 to 314 defines an FVa binding site and accounts for much of the difference in cleavage rates at the 2 major cleavage sites in FVa.

Interaction of the Fungal Pathogen Candida albicans with Integrin CD11b/CD18: Recognition by the I Domain Is Modulated by the Lectin-Like Domain and the CD18 Subunit

Interactions of microorganisms with integrins are central to the host defense mechanisms. The leukocyte integrin CD11b/CD18 is the principal adhesion receptor on leukocytes for Candida albicans, a major opportunistic pathogen. In this study we have investigated the roles of three regions within the receptor, the inserted (I) and lectin-like domains within the CD11b subunit, and the CD18 subunit, in CD11b/CD18-C. albicans interactions. We report four major findings. 1) A mutation in CD18 exerts a dominant negative effect on the function of the CD11b/CD18 complex. This interpretation is based on the observation that in the absence of CD18, the CD11b subunit alone binds C. albicans well, but a single point mutation at Ser138 of CD18 abolishes CD11b/CD18 binding of the fungus. 2) The lectin-like domain is not sufficient for CD11b/CD18-C. albicans interactions. Rather, the lectin-like domain appears to influence CD11b/CD18 binding activity by modulating the function of the I domain. 3) The I domain is the primary binding site for C. albicans in the receptor and is sufficient to support an efficient interaction. 4) We have identified specific amino acid sequences within the I domain that engage the microorganism. Compared with other ligands of CD11b/CD18, C. albicans has some unique as well as common contact sites within the I domain of the receptor. Such unique contact sites may underlie the ability of C. albicans to modulate CD11b/CD18 function and raise the possibility for selective interference of the microorganism-host leukocyte interactions.

Inactivation of the gene for anticoagulant protein C causes lethal perinatal consumptive coagulopathy in mice

Matings of mice heterozygous for a protein C (PC) deficient allele, produced by targeted PC gene inactivation, yielded the expected Mendelian distribution of PC genotypes. Pups with a total deficiency of PC (PC-/-), obtained at embryonic day (E) 17.5 and at birth, appeared to develop normally macroscopically, but possessed obvious signs of bleeding and thrombosis and did not survive beyond 24 h after delivery. Microscopic examination of tissues and blood vessels of E17.5 PC-/- mice revealed their normal development, but scattered microvascular thrombosis in the brain combined with focal necrosis in the liver was observed. In addition, bleeding was noted in the brain near sites of fibrin deposition. The severity of these pathologies was exaggerated in PC-/- neonates. Plasma clottable fibrinogen was not detectable in coagulation assays in PC-/- neonatal mice, suggestive of fibrinogen depletion and secondary consumptive coagulopathy. Thus, while total PC deficiency did not affect the anatomic development of the embryo, severe perinatal consumptive coagulopathy occurred in the brain and liver of PC-/- mice, suggesting that a total PC deficiency is inconsistent with short-term survival.

The functions of the first epidermal growth factor homology region of human protein C as revealed by a charge-to-alanine scanning mutagenesis investigation

Variant proteins containing charge-to-alanine mutations of single amino acid residues and clusters of such groups contained in the epidermal growth factor 1 (EGF1) homology unit of human protein C (PC) have been accomplished, resulting in the following recombinant (r) mutant proteins: r-[E56A/H57A]PC; r-[H66A]PC; r-[D71A]PC; r-[D79A/R81A]PC; r-[E85A/R87A]PC; and r-[R91A/E92A]PC. Studies of the mutant proteins with a variety of Ca2+-dependent and Ca2+-independent monoclonal antibodies not only led to identification of the epitopes of these antibodies, but also confirmed the importance of D/beta-hydroxyaspartic acid (Hya)71 as one probable coordination site for Ca2+. Employing these antibodies, it was also revealed that Ca2+ binding to its site in the EGF1 region of PC did not influence Ca2+ binding or adoption of the Ca2+-dependent conformation of the gamma-carboxyglutamic acid domain of this same protein. In addition, the Ca2+-induced inhibition of PC activation by thrombin, and the kinetic constants for activation of PC by the thrombin/thrombomodulin complex, were only modestly affected by any of the mutations. The mutants r-[E56A/H57A]APC and r-[H66A]APC displayed at least 70% of wild type r-APC activity in a fVIII inactivation assay, while r-[D79A/R81A]APC, r-[E85A/R87A]APC and r-[R91A/E92A]APC possessed only approximately 40% activity in that same assay. The special role of D/Hya71 in this process was confirmed by showing that r-[D71A]APC was inactive in the fVIII-inactivation assay. These findings demonstrate that some of the charged residues of EGF1, most notably those in the carboxy-terminal region of this domain, participate as partial determinants of the anticoagulant activity of APC. Overall, with the exceptions noted, the data generally suggest that the charged residues of the EGF1 domain of PC, and the Ca2+ binding site contained within this module, are likely more involved with maintenance of the overall structural integrity of this module rather than with its direct functional interactions with effectors, activators, or substrates of PC and APC. Lastly, functional Ca2+ binding to the Gla domain of PC is not significantly influenced by the binding of Ca2+ to the EGF1 module.

The gamma-carboxylation recognition site is sufficient to direct vitamin K-dependent carboxylation on an adjacent glutamate-rich region of thrombin in a propeptide-thrombin chimera

The propeptides of the vitamin K-dependent proteins contain a gamma-carboxylation recognition site that is required for gamma-glutamyl carboxylation. To determine whether the propeptide is sufficient to direct carboxylation, two mutant prothrombin species were expressed and characterized with regard to posttranslational gamma-carboxylation. A double point mutant, in which serine substituted for cysteines 17 and 22 disrupted a conserved loop formed by a disulfide bond, was fully carboxylated when expressed in Chinese hamster ovary cells. A propeptide/thrombin chimeric protein, constructed by deleting the Gla, aromatic amino acid stack, and kringle domains of prothrombin, has the signal peptide and propeptide juxtaposed to a glutamate-rich COOH-terminal region of prothrombin, residues 249-530. Of the 8 glutamic acid residues contained within the first 40 residues of the NH2 terminus adjacent to the propeptide, at least seven were fully carboxylated as demonstrated by direct gamma-carboxyglutamic acid analysis of the alkaline hydrolysate and by NH2-terminal sequence analysis. These results indicate that the gamma-carboxylation recognition site within the prothrombin propeptide in a prothrombin propeptide-thrombin chimeric protein is sufficient to direct gamma-carboxylase-catalyzed carboxylation of adjacent glutamic acid residues in a glutamate-rich region of thrombin that is not normally gamma-carboxylated. Furthermore, the disulfide loop in the Gla domain of prothrombin is not required for complete carboxylation.

Identification and reconstruction of the binding site within alphaMbeta2 for a specific and high affinity ligand, NIF

Engagement of the alphaMbeta2 (CD11b/CD18, Mac-1) integrin on neutrophils supports adhesion and induces various cellular responses. These responses can be blocked by a specific ligand of alphaMbeta2, neutrophil inhibitory factor (NIF). The molecular basis of alphaMbeta2-NIF interactions was studied. The single chain alphaM subunit, expressed on the surface of human 293 cells, bound NIF with an affinity equivalent to that of alphaMbeta2 heterodimer. This observation, coupled with previous data showing that the alphaMI domain alone supported high affinity NIF binding, indicated that the binding site for NIF is restricted to the I domain. Guided by the crystal structure of the alphaMI domain, 16 segments corresponding to the entire outer hydrated surface of alphaMI domain were switched to their counterparts sequences in alphaL, which does not bind NIF. Surface expression and heterodimer formation were achieved for all mutants, and correct folding was confirmed. Of the 16 switches, only 5 affected NIF binding substantially, reducing affinity by 8-300-fold. These data confined the NIF-binding site to a narrow region composed of Pro147-Arg152, Pro201-Lys217, and Asp248-Arg261 of alphaM. Verifying this localization, when these segments were introduced into the alphaXI-domain, the resulting chimeric receptor was converted into a high affinity NIF-binding protein.

Highly conserved residue arginine-15 is required for the Ca2+-dependent properties of the gamma-carboxyglutamic acid domain of human anticoagulation protein C and activated protein C

The function of the rigidly conserved amino acid residue R15 in the Ca2+/phospholipid-dependent properties of the gamma-carboxyglutamic acid (Gla)-containing domain (GD) of human Protein C (PC) were investigated through site-directed mutagenesis strategies. A series of recombinant (r) mutants, namely r-[R15K]PC, r-[R15H]PC, r-[R15L]PC, and r-[R15W]PC, were constructed, expressed and purified, and their relevant properties investigated. As revealed by intrinsic fluorescence analysis, all of the variant proteins underwent Ca2+-dependent structural transitions. Nonetheless, they displayed altered binding properties to acidic phospholipid vesicles, and also did not interact with a monoclonal antibody specific for the type of Ca2+-dependent conformation of the GD that characterizes the wild-type protein. On conversion into their activated forms, these variant enzymes possessed less than 10% of the ex vivo plasma anticoagulant activity of wild-type r-PC. Similar activities were found when the r-active PC mutants were assayed directly for inactivation of factor Va and factor VIII, in the complete prothrombinase and tenase complexes respectively. We conclude that R15 is a critical residue in allowing the GD of PC, and probably of other proteins of this class, to adopt a Ca2+-dependent conformation that allows functional phospholipid binding, thus explaining the strict conservation of this amino acid residue in GD modules of various proteins. As a result of an analysis of structural models of the Ca2+-GD complex of PC, it is postulated that hydrogen bonds between the side chain of R15 and the functionally important Gla16 residue, as well as between the side chain of R15 and the carbonyl oxygen in the peptide bond of H10, are critical for adoption of a Ca2+-dependent conformation of the GD that allows functional phospholipid binding.

Nonenzymatic anticoagulant activity of the mutant serine protease Ser360Ala-activated protein C mediated by factor Va

The human plasma serine protease, activated protein C (APC), primarily exerts its anticoagulant function by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. A recombinant active site Ser 360 to Ala mutation of protein C was prepared, and the mutant protein was expressed in human 293 kidney cells and purified. The activation peptide of the mutant protein C zymogen was cleaved by a snake venom activator, Protac C, but the "activated" S360A APC did not have amidolytic activity. However, it did exhibit significant anticoagulant activity both in clotting assays and in a purified protein assay system that measured prothrombinase activity. The S360A APC was compared to plasma-derived and wild-type recombinant APC. The anticoagulant activity of the mutant, but not native APC, was resistant to diisopropyl fluorophosphate, whereas all APCs were inhibited by monoclonal antibodies against APC. In contrast to native APC, S360A APC was not inactivated by serine protease inhibitors in plasma and did not bind to the highly reactive mutant protease inhibitor M358R alpha 1 antitrypsin. Since plasma serpins provide the major mechanism for inactivating APC in vivo, this suggests that S360A APC would have a long half-life in vivo, with potential therapeutic advantages. S360A APC rapidly inhibited factor Va in a nonenzymatic manner since it apparently did not proteolyze factor Va. These data suggest that native APC may exhibit rapid nonenzymatic anticoagulant activity followed by enzymatic irreversible proteolysis of factor Va. The results of clotting assays and prothrombinase assays showed that S360A APC could not inhibit the variant Gln 506-FVa compared with normal Arg 506-FVa, suggesting that the active site of S360A APC binds to FVa at or near Arg 506.

A discrete site modulates activation of I domains. Application to integrin alphaMbeta2

A central characteristic of integrin adhesion receptors is their capacity to become activated, thereby enhancing their affinity for ligands. Here, we report the identification of a discrete site within the I domain of integrin alphaMbeta2, which modulates the adhesive activity of this receptor. Based upon the crystal structure, this region is composed of two short and spatially proximal loops, E162QLKKSKTL and Q190NNPNPRS. Mutations in these loops yield receptors which support spontaneous cell adhesion to fibrinogen, whereas mutation of an adjacent region and wild-type receptors require activation to adhere to this substrate. An activating monoclonal antibody enhanced the adhesive activity of one but not the other loop mutants, suggesting that the activation states of these two mutant receptors were not identical. Given that similar I domains exist in several other integrin alpha subunits and non-integrin proteins, and possibly in all integrin beta subunits, these two loop segments may represent a universal target for controlling integrin activation and the function of other I domain-containing proteins. In support of this hypothesis, several naturally occurring mutations that activate von Willebrand factor map to the same loops of its I(A) domain.

Affinity purification of biologically active and inactive forms of recombinant human protein C produced in porcine mammary gland

Recombinant human protein C (rhPC) secreted in the milk of transgenic pigs was studied. Transgenes having different regulatory elements of the murine milk protein, whey acidic protein, were used with cDNA and genomic human protein C (hPC) DNA sequences to obtain lower and higher expressing animals. The cDNA pigs had a range of expression of about 0.1-0.5 g/l milk. Two different genomic hPC pig lines have expressed 0.3 and 1-2 g/l, respectively. The rhPC was first purified at yields greater than 60 per cent using a monoclonal antibody (mAb) to the activation site on the heavy chain of hPC. Subsequent immunopurification with a calcium-dependent mAb directed to the gamma-carboxyglutamic acid domain of the light chain of hPC was used to fractionate a population having a higher specific anticoagulant activity in vitro. The higher percentages of Ca(2+)-dependent conformers isolated from the total rhPC by immunopurification correlated well with higher specific activity and lower expression. A rate limitation in gamma-carboxylation of rhPC was clearly identified for the higher expressing animals. Thus, transgenic animals with high expression levels of complex recombinant proteins produced a lower percentage of biologically active protein.

The entire gamma-carboxyglutamic acid- and helical stack-domains of human coagulation factor IX are required for optimal binding to its endothelial cell receptor

The minimal region of the gamma-carboxyglutamic acid (Gla) domain of human factor (f) IX that interacted with its putative bovine aortic endothelial cell (BAEC) receptor was examined by chemical synthesis of peptides with sequence counterparts in this region of the protein, and assessment of their relative abilities to compete with fIX for receptor binding. We found that IC50 values (total peptide concentrations needed to achieve 50% inhibition of binding of [125I]-fIX to BAEC) were ca. 18 nM for unlabeled fIX and 23 nM for the peptide consisting of the entire Gla domain/helical stack (HS) region (residues 1-47) of fIX. The peptide containing only the Gla domain of fIX (residues 1-38) displayed an IC50 value of > 500 nM for this same competitive binding, whereas peptides containing sequences present in positions 1-14 and 1-24 of the Gla domain of human fIX did not significantly compete with [125I]-fIX for BAEC binding. We conclude that whereas a specific receptor recognition element is present within residues 1-14 of fIX, as has previously been concluded by others and by us, full expression of this epitope requires its presence within the entire Gla domain and HS for proper folding. All determinants for proper folding of fIX that lead to BAEC receptor binding appear to be present within these two domains.

Overlapping, but not identical, sites are involved in the recognition of C3bi, neutrophil inhibitory factor, and adhesive ligands by the alphaMbeta2 integrin

The alphaMbeta2 (CD11b/CD18, Mac-1) integrin receptor binds numerous ligands, including neutrophil inhibitory factor (NIF), C3bi, and certain immobilized protein substrates, represented by denatured ovalbumin. These ligands share no obvious structural similarities, yet their interactions with receptor are inhibited by NIF and involve the I domain, a stretch of approximately 200 amino acids in the alphaM subunit. Recombinant wild-type and mutant forms of alphaMbeta2 have been used to compare the recognition requirements of these ligands. The various constructs were expressed efficiently on the surface of human embryonic kidney 293 cells and formed alpha.beta heterodimeric complexes. The wild-type transfectants bound the three ligands in a similar fashion to naturally occurring alphaMbeta2. NIF inhibited these interactions, and deletion of the D248PLGY from within the I domain abolished binding of all three ligands, suggesting an overlapping recognition specificity. A single point mutation of Ser138 to Ala in the beta2 subunit abolished C3bi binding and cell adhesion but did not affect NIF binding. A switch of the R281QELNTI sequence in helix 6 of the alphaM I domain to the corresponding sequence in the I domain of the alphaL (QETLHKF) subunit completely abrogated adhesion while not affecting C3bi and NIF binding. The two mutant receptors also did not support activation-dependent adhesion to fibrinogen. Thus, the contact sites for NIF, C3bi, and adhesive proteins, represented by denatured ovalbumin and fibrinogen, in alphaMbeta2 are overlapping but not identical.

Review of conformation-specific affinity purification methods for plasma vitamin K-dependent proteins

There are seven known vitamin K-dependent proteins in blood. These proteins require calcium ion for expressing their full biological activities. Calcium ion also induces conformational changes in this class of proteins. Taking advantage of the ligand induced conformational changes, a number of unique approaches of affinity chromatography have been developed. These methodologies have been very useful tools for both the purification and for understanding the structure-function relationships of this class of proteins. One method is the use of metal ion dependent immunoaffinity chromatography. The antigen can be dissociated from the antibodies with either the removal or addition of calcium ion under physiological conditions. The other method is pseudoaffinity chromatography. This method uses conventional ion-exchange or hydrophobic resin and manipulates the mobilities of the proteins on these resins by the presence or absence of calcium ions. Researchers working with other calcium binding proteins or other proteins that are known to undergo ligand induced conformational changes may benefit from the experience of these unique conformation-specific affinity chromatography approaches.

Hydrophobic amino acid residues of human anticoagulation protein C that contribute to its functional binding to phospholipid Vesicles

The contributions to functional phospholipid (PL) binding of the cluster of amino acid side chains of human protein C (PC) comprising F4, L5, and L8 have been assessed by construction of mutants of PC and activated protein C (APC) designed wherein a hydrophilic side chain replaced the wild-type hydrophobic groups at these positions. The PL-dependent plasma-based anticoagulant activities of [F4Q]-r-APC and [L8Q]r-APC were severely reduced to 5% and < 2%, respectively, of wild-type r-APC. Activity losses of the mutants toward inactivation of coagulation factor VIII, measured in the complete in vitro tenase system, have also been observed. As evidenced through Ca(2+)-induced intrinsic fluorescence changes, both [F4Q]r-PC and [L8Q]r-PC were able to adopt Ca(2+)-dependent conformations that appeared similar to that of wtr-PC, ruling out shortcomings associated with such Ca(2+)-induced transitions as the basis for their anticoagulant activity losses. However, despite this, [L8Q]r-PC showed greatly defective macroscopic binding properties to PL vesicles, as did to a lesser extent [F4Q]r-PC. These findings were similar to those reported previously for [L5Q]r-PC/APC [Zhang, L., & Castellino, F. J. (1994) J. Biol. Chem. 269, 3590-3595]. We thus propose that the PL-dependent activity losses of these mutants are related to their suboptimal binding to PL or to their misorientation on the PL surface leading to poor alignment of the active sites of the r-APC mutants with the complementary cleavage sites on fVIII/fVIIIa and fV/fVa.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfer of specific endothelial cell-binding properties from the procoagulant protein human factor IX into the anticoagulant protein human protein C

A series of recombinant (r) chimeric mutants of human coagulation protein C (PC) and activated protein C (APC) containing replacements of homologous PC domains by those of human coagulation factor IX (fIX) were generated, with the intention of determining whether the specific bovine aortic endothelial cell (BAEC) receptor-binding characteristics of fIX could be incorporated into the chimeric r-PC while maintaining the essential properties of PC and APC. Using a competitive BAEC displacement assay with [125I]fIX, we found that a chimeric r-PC (r[delta PC1-46/delta fIX1-47]PC), consisting of the entire gamma-carboxyglutamic domain ([GDIX], residues 1-38) and helical stack ([HSIX], residues 38-47) of fIX as replacements for these same domains of PC, provided an IC50 for fIX-related BAEC binding of 13 nM, as compared to 10 nM for that of unlabeled fIX. This showed that all of the BAEC tight binding determinants for fIX existed within the [GDIX/HSIX]. Additionally, this chimera reacted to the same extent as fIX with the Ca(2+)-dependent, [GDIX]-specific monoclonal antibody H5B7 and lost its reactivity to a similar antibody specific for the [GDPC], JTC1. A synthetic peptide containing residues 1-47 of fIX also competed effectively (IC50 = 16 nM) with intact fIX for BAEC binding. Displacement of [125I]fIX from BAEC did not occur with a chimera containing the [HSIX] alone or with another mutant protein possessing a replacement of the two epidermal growth factor (EGF) homology regions of r-PC (residues 47-137) with those same domains of fIX.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino acids 225-235** of the protein C serine-protease domain are important for the interaction with the thrombin-thrombomodulin complex

Protein C (PC) is a vitamin K-dependent zymogen that inactivates factors Va and VIIIa after its activation by thrombin complexed to thrombomodulin. We characterized a monoclonal antibody (mAb) against PC, whose only influence on PC functions was to inhibit PC activation by the thrombin-thrombomodulin complex. It recognized an epitope in the PC heavy chain, the conformation of which is calcium-dependent. The mAb did not recognize a natural PC variant that was not activated by the thrombin-thrombomodulin complex (mutation R229Q) and did recognize a synthetic peptide corresponding to PC amino acids 225-235 in an Elisa assay. The peptide inhibited PC activation by the thrombin-thrombomodulin complex. These data confirm that the calcium-binding loop of the serine-protease domain is involved in the interaction of PC with the thrombin-thrombomodulin complex.

Two Drosophila genes that encode the alph and beta subunits of the brain soluble guanylyl cyclase

We identified two Drosophila genes (dgc alpha 1 and dgc beta 1) that encode the soluble guanylyl cyclase alpha and beta subunits, respectively. The putative Dgc alpha 1 protein is 76 kDa, has 35% amino acid identity with previously isolated alpha subunits, and was immunolocalized to the adult retina, to the optic lobes, and throughout the brain neuropil. The Dgc beta 1 protein is 86 kDa and exhibits 59% amino acid identity with the rat beta 1 protein. However, the Dgc beta 1 protein has an additional 118 amino acids inserted near the amino terminus, which makes it significantly larger than the rat beta 1. The Dgc beta 1 protein was immunolocalized to the optic lobes and throughout the brain neuropil, with no detectable expression in the retina. The Dgc alpha 1 and Dgc beta 1 cDNAs were stably transfected into human kidney 293 cells. Expression of the individual subunits and mixing of the individually expressed subunits failed to generate significant guanylyl cyclase activity. Only coexpression of the subunits resulted in significant guanylyl cyclase activity. Our results indicate that Dgc alpha 1 and Dgc beta 1 are soluble guanylyl cyclase alpha and beta subunits that are capable of forming a functional guanylyl cyclase heterodimer.

Structure of the metal-free gamma-carboxyglutamic acid-rich membrane binding region of factor IX by two-dimensional NMR spectroscopy

The gamma-carboxyglutamic acid-rich domain of blood coagulation Factor IX is required for the binding of the protein to phospholipid membranes. To investigate the three-dimensional structure of this domain, a synthetic peptide corresponding to residues 1-47 of Factor IX was studied by 1H NMR spectroscopy. In the absence of metal ions, the proton chemical shift dispersion in the one-dimensional NMR spectrum indicated that the peptide contains regular structural elements. Upon the addition of Ca(II) or Mg(II), large chemical shift changes were observed in the amide proton and methyl proton regions of the spectrum, consistent with the conformational transitions that metal ions are known to induce in native Factor IX. The apopeptide was studied by two-dimensional NMR spectroscopy at 500 MHz to determine its solution structure. Protons were assigned using total correlation spectroscopy, nuclear Overhauser effect spectroscopy, and double quantum-filtered correlation spectroscopy experiments. Intensities of cross-peaks in the nuclear Overhauser effect spectrum were used to generate a set of interproton distance restraints. The structure of the apopeptide was then calculated using distance geometry methods. There are three structural elements in the apopeptide that are linked by a flexible polypeptide backbone. These elements include a short amino-terminal tetrapeptide loop (amino acids 6-9), the disulfide-containing hexapeptide loop (amino acids 18-23), and a carboxyl-terminal alpha helix (amino acids 37-46). Amide hydrogen exchange kinetics indicate that the majority of the peptide is solvent accessible, except in the carboxyl-terminal element. The structured regions in the apopeptide are insufficient to support phospholipid binding, indicating the importance of additional structural features in the Ca(II)-stabilized conformer.

Functions of individual gamma-carboxyglutamic acid (Gla) residues of human protein c. Determination of functionally nonessential Gla residues and correlations with their mode of binding to calcium

Previous studies from this laboratory have been directed toward elucidation of the roles of individual gamma-carboxyglutamic acid (Gla) residues in Gla domain-related Ca(2+)-directed properties of human protein C (PC) and activated protein C (APC). On the basis of results using recombinant variants of PC containing highly conservative (Asp) mutations of individual Gla residues, it was previously proposed that Gla6, Gla14, and Gla19 may not be essential for properties associated with the Ca(2+)-dependent conformation of the Gla domain of these proteins. In this study, we have demonstrated that radical mutations to Val of Gla residues 14 and 19 resulted in 94% and 82%, respectively, of the Gla domain-related, Ca(2+)- and phospholipid- (PL-) dependent anticoagulant (APTT) activity of wild-type recombinant (wtr) APC, while [Gla6-->Val]r-APC showed a complete loss of this same activity. The more conservative mutant [Gla6-->Gln]r-APC possessed 4% of the APTT activity of wtr-APC, whereas [Gla6-->Asp]r-APC was nearly fully active. As with wtr-PC, both [Gla6-->Val]r-PC and [Gla6-->Gln]r-PC displayed Ca(2+)-dependent intrinsic fluorescence quenching, suggesting that they adopted a Ca(2+)-induced conformation. However, Ca2+ titration data suggested that these conformations were not identical to that undergone by wtr-PC. In addition, the Ca(2+)-mediated binding parameters of [Gla6-->Val]r-PC and [Gla6-->Gln]r-PC to acidic PL vesicles were found to be defective. These data were interpreted at the molecular level using a model for the Gla domain of PC based on the X-ray crystal structure of the Ca2+/bovine prothrombin fragment 1 complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of recombinant chimeric human protein C and activated protein C containing the gamma-carboxyglutamic acid and trailing helical stack domains of protein C replaced by those of human coagulation factor IX

The properties of a recombinant (r) chimeric human protein C (PC) containing replacement of its gamma-carboxyglutamic acid (Gla) and helical stack (HS) domains by those of human coagulation factor IX (fIX) have been examined. Titration with Ca2+ of the divalent cation-induced intrinsic fluorescence quenching of this chimera (r-GDIX/PC) allowed determination of the [Ca2+], of 1.8 mM, required to produce this alteration in 50% of the protein molecules. These values were 0.41 and 0.61 mM for wtr-PC and fIX, respectively. The chimera did not react with a Ca(2+)-dependent, Gla domain-directed conformational monoclonal antibody (MAb) to r-PC but did interact with a similar MAb (H5B7) to fIX. The [Ca2+] required to induced H5B7 binding to 50% of the r-GDIX/PC molecules was 6.6 mM, while this same value for fIX was a nearly identical 7.2 mM. The [Ca2+] needed for binding of 50% of r-GDIX/PC to acidic phospholipid (PL) vesicles was 0.58 mM, while that for wtr-PC and fIX were 1.2 and 0.55 mM, respectively. The [protein] required for 50% binding of r-GDIX/PC to PL at 20 mM Ca2+ was 0.29 microM. These same values for r-PC and fIX were 0.38 and 1.8 microM, respectively. The Ca(2+)-mediated inhibition of the thrombin-catalyzed activation of r-GDIX/PC was characterized by a Ki of 118 microM, a value similar to that of 125 microM obtained for this same inhibition of wtr-PC activation. The thrombin-catalyzed activation of both r-GDIX/PC and wtr-PC was stimulated by soluble r-thrombomodulin. Similar to the case of wtr-PC, Ca2+ initially enhanced and, at higher concentrations, inhibited the activation of r-GDIX/PC. The Km and kcat values for this latter activation at optimal [Ca2+] (100 microM) were 4.1 microM and 2.5 s-1, respectively. These same kinetic constants for activation of wtr-PC were 4.3 microM and 2.9 s-1, respectively. These results show that many of the features needed for functional integrity of the Ca2+-bound Gla/HS domains of PC are also present in those same modules of fIX, a finding that points to a generalized functional role for the Ca2+-induced conformation of the structural unit consisting of the Gla and HS domains. The data also suggest that the Ca2+-bound form of the Gla/HS region is an independently folded unit in PC and perhaps in fIX.(ABSTRACT TRUNCATED AT 400 WORDS)

The porcine mammary gland as a bioreactor for complex proteins

The similar biological activity of rhPC and hPC indicates that porcine mammary gland can perform many of the processing reactions necessary for recombinant synthesis of complex human proteins and produce them at levels suitable for industrial bioreactor applications. The health of the transgenic pigs appeared unaffected by the expression of high levels of the heterologous protein. We suggest that one of the advantages of using the mammary gland as a bioreactor appears to be the high cell density relative to that of cell culture.

The activities of recombinant gamma-carboxyglutamic-acid-deficient mutants of activated human protein C toward human coagulation factor Va and factor VIII in purified systems and in plasma

The dependence of the activity of recombinant activated human protein C (r-APC) on each of its nine gamma-carboxyglutamic (Gla) residues (sequence positions 6, 7, 14, 16, 19, 20, 25, 26, and 29) has been assessed in purified systems and in plasma using r-mutants in which each Gla residue of r-APC was individually altered to an Asp (D) residue. The assays employed included a factor Va inactivation assay in the prothrombinase system with purified components and in plasma. In addition, a factor VIII inactivation assay in the tenase system, also with purified components, was utilized. Compared to wild-type protein (wtr-APC), the r-mutants that possessed nearly full activity in all assays were the Gla6-->D variant ([Gla6D]r-APC]) as well as [Gla14D]r-APC and [Gla19D]r-APC. In addition, another mutant (Q32-->Gla) in which a Gla was substituted for Gln (Q) at position 32, a situation that exists with other vitamin-K-dependent clotting proteins (e.g., factor IX and prothrombin), displayed full activity in all assays. Those mutants that possessed very-low-to-no activity in all assays included [Gla16D]r-APC and [Gla26D]r-APC. The other mutants showed partial and, in some cases, differential activity in these assay systems, with [Gla25D]r-APC being the most remarkable example. In this case, the factor V/Va plasma assay and the plasma-based activated partial thromboplastin time assay yielded < 25% activity, whereas nearly full activity was observed for this variant in the prothrombinase and tenase assays with purified components.(ABSTRACT TRUNCATED AT 250 WORDS)

Construction, expression, and properties of a recombinant chimeric human protein C with replacement of its growth factor-like domains by those of human coagulation factor IX

The cDNA encoding a chimeric human protein C (PC), in which its epidermal growth factor-(EGF) like regions have been replaced with equivalent structures from human factor IX (fIX), was constructed and the gene product was expressed in human 293 cells. A molecular subpopulation of the recombinant chimeric protein (r-[PC/delta EGF-1,2/delta fIXEGF-1,2]) was purified that contained the full complement (9 residues/mol) of gamma-carboxyglutamic acid (Gla). After conversion by thrombin to its activated form (r-[APC/delta EGF-1,2/delta fIXEGF-1,2]), this latter enzyme was found to possess approximately 10% of the activity of wild-type recombinant APC (wtr-APC) in an APTT assay. In assay systems employing purified components, the activity of the mutant enzyme toward prothrombinase cofactor Va (fVa) and tenase cofactor VIII (fVIII) was approximately 30% and < 10%, respectively, of that of wtr-APC. The chimeric protein displayed full reactivity with a Ca(2+)-dependent monoclonal antibody to the Gla domain of PC, yielding a C50 for Ca2+ that was very similar to that obtained with wtr-PC (ca. 3.7 mM). Titrations of the dependency on Ca2+ of the intrinsic fluorescence of r-[PC/delta EGF-1,2/delta fIXEGF-1,2] allowed calculation of a C50 value of 0.34 mM, again very similar to that of wtr-PC. As with wtr-PC, Ca2+ inhibited the thrombin-catalyzed activation of r-[PC/delta EGF-1,2/delta fIXEGF-1,2] with aKi of 148 microM, as compared to a Ki of 125 microM for wtr-PC. At a saturating level of Ca2+, activation of r-[PC/delta EGF-1,2/delta fIXEGF-1,2/] by the thrombin/thrombomodulin (thrombin/TM) complex occurred at approximately 70% of the rate of that of wtr-PC.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions and inhibition of blood coagulation factor Va involving residues 311-325 of activated protein C

Activated protein C (APC) exerts its physiologic anticoagulant role by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. The synthetic peptide-(311-325) (KRNRTFVLNFIKIPV), derived from the heavy chain sequence of APC, potently inhibited APC anticoagulant activity in activated partial thromboplastin time (APTT) and Xa-1-stage coagulation assays in normal and in protein S-depleted plasma with 50% inhibition at 13 microM peptide. In a system using purified clotting factors, peptide-(311-325) inhibited APC-catalyzed inactivation of factor Va in the presence or absence of phospholipids with 50% inhibition at 6 microM peptide. However, peptide-(311-325) had no effect on APC amidolytic activity or on the reaction of APC with the serpin, recombinant [Arg358]alpha 1-antitrypsin. Peptide-(311-325) surprisingly inhibited factor Xa clotting activity in normal plasma, and in a purified system it inhibited prothrombinase activity in the presence but not in the absence of factor Va with 50% inhibition at 8 microM peptide. The peptide had no significant effect on factor Xa or thrombin amidolytic activity and no effect on the clotting of purified fibrinogen by thrombin, suggesting it does not directly inhibit these enzymes. Factor Va bound in a dose-dependent manner to immobilized peptide-(311-325). Peptide-(311-315) inhibited the binding of factor Va to immobilized APC or factor Xa.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of calcium to synthetic peptides containing gamma-carboxyglutamic acid

The Ca2+ binding properties of various gamma-carboxyglutamic acid (gla)-containing synthetic peptides with counterpart sequences in human protein C were investigated employing potentiometry with a Ca(2+)-selective electrode and titration calorimetric techniques. The shortest peptides, FL(gla)(gla)LR, DF(gla)(gla)AK, and the oxidized form of the cyclic hexapeptide CI(gla)(gla)IC, each of which contains one pair of gla residues, have a weak affinity for Ca2+, with some peptides probably involved in intermolecular bridging of the Ca2+. The best example of this is the oxidized form of the peptide, CI(gla)(gla)IC, where one g-atom of Ca2+ interacts with 2 mol of peptide (n = 0.5) with a Kd value of 1.6 mM. A second g-atom of Ca2+ interacts with 2 mol of this same peptide (n = 0.5) and is characterized by a Kd of 8.8 mM. A longer peptide containing this same sequence, viz. L(gla)R(gla)CI(gla)(gla)IC, possesses two binding sites (n = 2.0) for Ca2+ of Kd = 16.1 mM, as well as a tighter site (n = 1), of Kd = 0.4 mM. An increase in stoichiometry of tight binding sites as the peptide is elongated is observed from binding data obtained on a 38-residue peptide that possesses all nine of the gla-residues of protein C in their proper sequence positions. The strongest Ca2+ binding sites (n = 3-4) possess an average Kd of 0.4 mM, followed by another class of sites (n = 5-10, average Kd = 1.5-3.0 mM). The affinity and stoichiometry of these stronger sites mimic those observed for binding of Ca2+ to the gla region of prothrombin fragment 1.(ABSTRACT TRUNCATED AT 250 WORDS)

High-level expression of a heterologous protein in the milk of transgenic swine using the cDNA encoding human protein C

Transgenic pigs were generated that produced human protein C in their milk at up to 1 g/liter. The gene construct was a fusion gene consisting of the cDNA for human protein C inserted into the first exon of the mouse whey acidic protein gene. These results demonstrate that the mouse whey acidic protein gene contains regulatory elements that can direct cDNA expression at high levels in the pig mammary gland. Recombinant human protein C that was produced at about 380 micrograms/ml per hr in transgenic pig milk possessed anticoagulant activity that was equivalent to that of protein C derived from human plasma. These studies provide evidence that gamma-carboxylation can occur at high levels in the mammary gland of a pig.