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PubMed1987

Human complement C3b/C4b receptor (CR1) mRNA polymorphism that correlates with the CR1 allelic molecular weight polymorphism.

The human C3b/C4b receptor (CR1) is a Mr approximately equal to 200,000 single-chain integral membrane glycoprotein of human erythrocytes and leukocytes. It functions both as a receptor for C3b- and C4b-coated ligands and as a regulator of complement activation. Prior structural studies have defined an unusual molecular weight allelic polymorphism in which the allelic products differ in molecular weight by as much as 90,000. On peripheral blood cells there is codominant expression of CR1 gene products of Mr 190,000 (A), 220,000 (B), 160,000 (C), and 250,000 (D). Results of prior biosynthetic and tryptic peptide mapping experiments have suggested that the most likely basis for the allelic molecular weight differences is at the polypeptide level. In order to define further the molecular basis for these molecular weight differences, human CR1 was purified to homogeneity, tryptic peptide fragments were isolated by HPLC and sequenced, oligonucleotide probes were prepared, and a CR1 cDNA was identified. A subclone of this CR1 cDNA was used as a probe of RNA blots of Epstein-Barr virus-transformed cell lines expressing the allelic variants. Each allelic variant encodes two distinct transcripts. A mRNA size polymorphism was identified that correlated with the gene product molecular weight polymorphism. This finding, in addition to a prior report of several homologous repeats in CR1, is consistent with the hypothesis that the molecular weight polymorphism is determined at the genomic level and may have been generated by unequal crossing-over.

Alleles↗
PubMed2006

Inclusion of sevoflurane in cardioplegia reduces neutrophil activity during cardiopulmonary bypass.

OBJECTIVE: The purpose of this study was to examine the effects of sevoflurane cardioplegia on neutrophil response and complement activation after cardiopulmonary bypass (CPB). DESIGN: A prospective, randomized clinical investigation. SETTING: University-affiliated hospital; single institutional. PARTICIPANTS: Twenty-one male patients undergoing coronary bypass surgery using CPB. INTERVENTIONS: Eleven patients were randomly assigned to receive sevoflurane 2% as a part of the cardioplegic mixture (SEV). The control group (n = 10) received no sevoflurane in their cardioplegia (control). MEASUREMENTS AND MAIN RESULTS: Myeloperoxidase activity (MPO) was assayed in coronary sinus blood as a surrogate for neutrophilic response at the termination of CPB. MPO activity in the coronary sinus blood was lower in the patients who received sevoflurane compared with controls. MPO activity was higher in patients with cardiac events at 4-year follow-up when compared with asymptomatic patients. IL-8, C4b, C3d, C5a, and CH50 were assessed in coronary sinus and peripheral blood at time of CPB initiation (T0) and upon the termination of CPB (T2). Peripheral blood sampling occurred at the sixth hour after T0 (T6). IL-8 levels were significantly inhibited in the SEV group when compared with controls at T2 and T6. CH50 (an index of global activation of complement system) decreased 30% at T2 and 52% at T6. The classic component of the complement pathway (C4b) was effectively inhibited in the SEV group, whereas the common pathway (C3d and C5a) was similar in both groups. CONCLUSIONS: The addition of sevoflurane to cardioplegia is associated with an inhibition of neutrophils after CPB. A major component of the neutrophil response appears to be IL-8 mediated, although the classic complement pathway is also inhibited by sevoflurane.

Cardiopulmonary Bypass↗
PubMed2003

Structural analysis of IgG2A monoclonal antibodies in relation to complement deposition and renal immune complex deposition.

This study explores the structural features of murine monoclonal IgG2a anti-dinitrophenyl (DNP) antibodies that were previously shown to form immune complexes (IC) differing in their capacity to bind complement, their clearance from the circulation and their deposition in the kidney. Interestingly, the sequence of one of these antibodies has a missing stretch of 14 amino acids within FR3. Molecular modeling suggests that this sequence deletion corresponds to the loss of beta-pleated sheet structure for two beta-strands (designated 4-3 and 4-4) on the external surface of the V(H) domain. Despite this sequence and conformational abnormality, the antibody retains affinity for DNP comparable to other IgG2a antibodies. Data presented here identify monoclonal IgG2a antibodies that form IC with varying propensity for both complement binding and renal deposition and yet have similar V(H) domain sequences. In fact, in the case of two IgG2a antibodies that form IC with very different renal tropisms and complement binding capacity, sequence variation within V(H) was observed only at three clustered residues within FR2, a single residue within FR3 and nine clustered residues spanning CDR3 and FR4. Sequence and modeling analysis also yielded the paradoxical finding that an antibody forming IC with a relatively high capacity to serve as a target for complement binding displays a relatively low number of solvent exposed acceptor residues for C4b and C3b. These data underscore the complex relationship between V domain structure, complement activation and renal deposition of model IC.

Amino Acid Sequence↗
PubMed1984

Ultrastructure of C4b-binding protein fragments formed by limited proteolysis using chymotrypsin.

C4b-binding protein is a regulator of the classical pathway of the complement system, acting as a cofactor to the serine protease factor I in the degradation of C4b. Its molecular weight is approximately 570,000 and it is composed of multiple, disulfide-linked 70-kDa subunits. Visualized by electron microscopy (Dahlbäck, B., Smith, C. A., and Muller-Eberhard, H. J. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 3641-3645), it has an unusual spider-like structure with multiple thin (30 A), elongated (330 A) tentacles. The number of tentacles was estimated to be seven. Limited proteolysis by chymotrypsin produces fragments of approximately 50- and 160-kDa, the latter composed of multiple, disulfide-linked, 25-kDa polypeptides. We now have isolated the undenatured C4b-binding protein fragments formed by treatment of the protein with chymotrypsin and have visualized them by electron microscopy. The 160-kDa fragment comprises the central portion of the C4b-binding protein, which appears as a ringlike structure with an inner diameter of 13 A and an outer diameter of 60 A and having attached an approximately 40-A long piece of each tentacle. The liberated 50-kDa fragment constitutes the major part (290-A long) of the tentacles. Chymotrypsin digestion of C4b-binding protein was also monitored as a function of time by polyacrylamide gel electrophoresis and the number of subunits cleaved was found to be seven, supporting our previous ultrastructural data which suggested that C4b-binding protein contains seven identical tentacle-like subunits.

Amino Acid Sequence↗
PubMed1995

Erythrocyte complement receptors.

Primate erythrocytes express complement receptors (E-CR), which can extrinsically bind C3b and Cb4. This interaction allows primate erythrocytes to bind complement opsonized particles and immune complexes, a phenomenon historically referred to as immune adherence. The binding of C3b and C4b by E-CR also leads to inhibition of complement activation. The human E-Cr is the complement activation. The human E-CR is the complement receptor type 1, or CR1, which is codominantly expressed as four polymorphic allotypes, ranging in size from 190,000 to 280,000 M(r). Non-human primate E-CR are similar to CR1 in function and antigenicity and are likely homologous to CR1 in structure; however, they are one third to one half the size of CR1. The physiological role of E-CR, determined from studies in monkeys and humans, is to allow erythrocytes to perform as inert shuttles for circulating immune complexes (IC), safely directing IC to the organs of the monocyte phagocytic system, thus preventing indiscriminate IC deposition in vulnerable tissue. In IC-mediated diseases, such as systemic lupus erythematosus (SLE), detectable erythrocyte CR1 levels are reduced, an abnormality that in part is acquired during disease activity. The loss of erythrocyte CR1 may be an important pathogenic factor in the development and severity of SLE.

Animals↗
PubMed1984

Rodgers (Rg) and Chido (Ch) determinants on human C4: characterization of two C4 B5 subtypes, one of which contains Rg and Ch determinants.

The genetically determined polymorphism of the fourth component of human complement was further extended with the aid of a panel of human allo-anti-C4 sera, anti-Rodgers and anti-Chido. These antisera were found previously to react with the alpha-chains of the C4 molecules controlled by the C4A and C4B loci, respectively. We analyzed a number of new and rare C4 allotypes, and found that they generally followed the expected pattern. Some interesting exceptions, however, were found. The alpha-chain of the allotype C4A1 was found to react with anti-Chido, unlike all other C4A allotypes. Also the C4B5 allotype could be subdivided into two subtypes on the basis of their reaction with anti-Rodgers. They were tentatively named B5Rg+ and B5Rg-. Moreover, the B5Rg+ subtype reacted not only with anti-Rodgers but also with some anti-Chido sera, indicating for the first time that Chido and Rodgers determinants are present on the same allotype.

Antigen-Antibody Reactions↗
PubMed1998

Membrane cofactor protein: importance of N- and O-glycosylation for complement regulatory function.

Membrane cofactor protein (MCP; CD46) is a type 1 membrane glycoprotein that inhibits complement activation on host cells. It also is a measles virus (MV) receptor, an adherence factor for group A Streptococcus pyogenes, and a cellular pilus receptor for pathogenic Neisseria. The amino terminus of MCP consists of four complement control protein (CCP) repeats, three of which (CCP-1, -2, and -4) possess N-glycans. Immediately following the CCP modules is an alternatively spliced region for extensive O-glycosylation (termed the STP domain). Previous studies established that the N-glycan of CCP-2 is essential for MV binding and infection and that the splicing variants of the STP domain not only affect MV binding and fusion, but also differentially protect against complement-mediated cytolysis. In this report, we dissect the role of these carbohydrates on complement regulatory function. We constructed, expressed, and characterized proteins deleting these carbohydrates. For MCP-mediated protection against cytolysis, the N-glycans of CCP-2 and -4 were necessary, the STP segment influenced but was not essential, and the N-glycan of CCP-1 was not required. In addition, the rate and magnitude of cell surface cleavage of C4b to C4c and C4d by MCP and factor I correlated with cytoprotection. These studies expand the structure-function understanding of the active sites of MCP and elucidate an important role for carbohydrates in its function, a finding consistent with their conservation in the MCP of other species.

Animals↗
PubMed1988

C1q acts synergistically with phorbol dibutyrate to activate CR1-mediated phagocytosis by human mononuclear phagocytes.

The adherence of human monocytes and culture-derived macrophages to surfaces coated with complement subcomponent C1q has been previously shown to enhance Fc receptor (FcR)-mediated phagocytosis by these cells. We examined the effects of C1q on C3b/C4b receptor (CR1)-mediated phagocytosis by mononuclear phagocytes. A small percentage of human monocytes cultured in the presence of serum became competent to ingest sheep erythrocytes bearing IgM and C4b (EAC4b). This phagocytic activity was enhanced when these cultured-derived macrophages were adhered to C1q-coated surfaces. However, when cultured in a defined serum-free medium, these cells did not ingest EAC4b, even in the presence of C1q. To investigate this differential responsiveness, we studied the effects of C1q in conjunction with cell-activating agents on CR1 activation. Treatment of serum-free cultured monocytes with phorbol dibutyrate (PDBu), prior to addition of the targets, induced these cells to ingest EAC4b. In addition, when exposed to C1q, both the percentage of these PDBu mononuclear phagocytes ingesting EAC4b and the number of targets ingested increased threefold over the level achieved by macrophages treated with PDBu alone. The chemoattractant N-formyl-methionyl-leucyl-phenylalanine did not activate CR1-mediated phagocytosis and did not substitute for PDBu in causing synergy with C1q. Freshly isolated monocytes adhered to human serum albumin-coated glass slides in the absence or presence of PDBu did not phagocytose EAC4b. Also C1q did not stimulate monocyte CR1-mediated phagocytosis. However, addition of PDBu to cells adherent to the C1q surface triggered phagocytosis of EAC4b. The concentration of PDBu and the time of addition of PDBu relative to addition of the EAC4b targets were found to be important parameters for the achievement of maximal synergy in both the freshly isolated and cultured cell systems. This enhanced phagocytic activity was also seen with cells adhered to the purified collagen-like, pepsin-resistant, fragment of C1q. Since this region was previously shown to interact with C1q surface receptors, it appears that occupancy of this receptor is triggering events contributing to the enhanced cellular function. These experiments suggest that C1q and PDBu promote ingestion via CR1 by different but synergistic mechanisms. These data also demonstrate that the CR1-mediated enhancement of phagocytosis is not specific for FcR-mediated ingestion, but also applies to phagocytosis via CR1.

Antigen-Antibody Complex↗
PubMed1996

Engineering of recombinant soluble CD46: an inhibitor of complement activation.

Human CD46 (membrane cofactor protein) is a type 1 glycoprotein that functions to protect autologous cells from complement-mediated damage by binding C3b and C4b for their factor I-mediated cleavage. We now describe the production and function of recombinant soluble CD46 (rsCD46), which was produced as a truncated form by mutagenesis using the splice overlap extension polymerase chain reaction, by inserting a translational stop codon into the CD46 cDNA at the junction of the transmembrane and extracellular domains. After transfection of an expression construct into 293-EBNA (Epstein-Barr nuclear antigen)-transformed cells, secretion of rsCD46 protein was detected by immunoradiometric assay using monoclonal antibodies. Following a single-step immunoaffinity purification, the protein resolved as a single band of approximately 56,000 MW on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The purified rsCD46 (51 micrograms/ml) protected Chinese hamster ovary (CHO) cells from lysis initiated by a high titre rabbit anti-CHO antibody and complement from rabbit or human. The protection was specifically mediated by rsCD46 because the monoclonal antibody M177, which blocks interaction between CD46 and C3b/C4b, abrogated the protection. The results demonstrate that rsCD46 is effective as a fluid-phase regulator of complement activation on cell surfaces, even when initiated by the classical complement pathway. The in vivo efficacy of rsCD46 was investigated using a mouse heart to rat xenograft model. Administration of a bolus injection of rsCD46 was effective at delaying hyperacute graft rejection. These data suggest that rsCD46 may have a role as a therapeutic agent.

Animals↗
PubMed1996

CRP-mediated activation of complement in vivo: assessment by measuring circulating complement-C-reactive protein complexes.

The in vivo function of C-reactive protein (CRP) is unknown. Among the in vitro functions assigned to CRP is the ability to activate complement via the classical pathway. To date, there is no evidence supporting that CRP exerts this function in vivo. We here show a novel approach to assess CRP-mediated complement activation in vivo, which is based on the property that activated complement factors C3 and C4 fix to CRP during complement activation induced by this acute phase protein. We developed specific ELISAs for complexes between CRP and C4b, C4d, C3b, or C3d. We established that in vitro complement-CRP complexes were formed only during CRP-dependent activation, and not during activation by other activators, even in the presence of high CRP levels. Circulating levels of complement-CRP complexes were undetectable in normal donors, but significantly increased in nine patients following implantation of a renal allograft. Importantly, levels of complement-CRP complexes did not change in these patients upon a bolus infusion of mAb OKT3, which induces activation of the classical complement pathway, demonstrating in vivo that complement-CRP complexes are not formed during CRP-independent activation of complement, even when CRP is elevated. We conclude that measurement of complement-CRP complexes provides a suitable tool to study CRP-mediated activation of complement in vivo. Furthermore, increased levels of these complexes occur in clinical samples, indicating that CRP may induce activation of complement in vivo.

Adult↗
PubMed1997

A dysfunctional allele of the mannose binding protein gene associates with systemic lupus erythematosus in a Spanish population.

OBJECTIVE: To determine dysfunctional mannose binding protein (MBP) status of Spanish patients with systemic lupus erythematosus (SLE) and to determine whether MBP and complement C4 null alleles contribute in an additive way to SLE susceptibility. METHODS: The frequencies of MBP alleles (characterized by polymorphisms at codon 54 and codon 57 of exon 1) were determined by the amplification refractory mutation system-polymerase chain reaction in 50 Spanish patients with SLE and 49 matched controls. Mutant genotypes for the codon 54 mutation were confirmed using a Ban I restriction enzyme digest method. Complement C4 allotyping was achieved by agarose gel electrophoresis of neuraminidase/carboxypeptidase B digested plasma samples followed by immunofixation and staining. RESULTS: At least one dysfunctional MBP allele, unable to activate complement, was present in 52% of patients with SLE and in 31% of controls (OR = 2.4, 95% CI 1.1-5.6). Complement C4 null alleles (either C4A or C4B) were present in 61% of patients and 43% of controls (OR = 2.1, 95% CI 0.9-4.9). A dysfunctional MBP allele and C4 null allele were present in 41% of patients and 16% of controls (OR = 3.2, 95% CI 1.2-8.1). CONCLUSION: The presence of a dysfunctional MBP allele is a risk factor for developing SLE in this Spanish population and may affect susceptibility in an additive way with C4 null alleles.

Acute-Phase Proteins↗
PubMed2001

Structure-function relationships of complement receptor type 1.

Human complement receptor type 1 (CR1) is a large, multifunctional glycoprotein which is a member of the regulators of complement activation family. Like other members of this family, it is composed mainly of tandemly arranged modules, each about 60-70 amino acids long, known as complement control protein repeats (CCPs). Each domain folds independently and contains a hydrophobic core wrapped in beta sheets. These domains mediate interactions with C3/C4-derived fragments. CR1 is the most versatile inhibitor of both classical and alternative pathway C3 and C5 convertases due to its decay-accelerating activity and co-factor activity for C3b/C4b cleavage. Moreover, CR1 plays a major role in immune complex clearance due to its high affinity for C3b and C4b. CR1 is an excellent model to study structure-function relationships because its functions are mediated by two distinct but highly homologous sites, each composed of three CCPs. CR1 derivatives carrying just one active site were used to define critical sequences/amino acids. This was achieved by testing functional profiles of the proteins carrying a mutated active site produced by substituting peptides/amino acids with their counterparts from the other site. These mutated proteins, of which we analyzed over 100, permitted the fine mapping of the functional sites. CR1 on primate erythrocytes varies in size. In most cases it is smaller and has fewer active sites than does human CR1. This variation was used to determine that increased copy number (3,000 to 20,000 versus 300 for human CR1) compensates for a smaller size. Moreover, studies of primate CR1 led to the finding that subtle differences in the critical areas, as compared to human sites, produce active sites with a broader functional repertoire. These alterations ensure that short CR1 forms possess similar biologic activities to the large CR1 forms. There is much interest in producing therapeutic agents to inhibit unwanted complement activation. Based on these structure-function analyses, smaller and more potent complement inhibitors derived from CR1 can be produced.

Amino Acid Sequence↗
PubMed1987

High resolution electrophoresis for the allotyping of human C4. Proposal of a relational nomenclature.

Among the major histocompatibility complex (MHC)-linked complement genes, the loci for C4A and C4B exhibit the most extensive structural polymorphisms. Therefore the differentiation of variant and complex C4 phenotypes often proves difficult in conventional immunofixation electrophoresis. To improve the available technique of C4 typing a closed horizontal electrophoresis system was combined with poly- and monoclonal alkaline phosphatase immunoprobing on contact diffusion blots. The high resolution and sensitivity of this method not only facilitated C4 allotyping but also revealed additional polymorphic variation. Relative electrophoretic mobilities specific for each C4 allotype were established by computerized remission densitometry and provided the basis for a quantitative denomination of C4 variants. Typing by high resolution electrophoresis and the proposed relational C4 nomenclature could be valuable for further immunogenetical studies of the C4 protein polymorphism.

Alkaline Phosphatase↗
PubMed1989

Bf and C4 phenotypes in patients with psoriasis.

Phenotype frequencies for the complement proteins Bf (factor B), C4A and C4B were performed in a sample of 49 Caucasian patients with psoriasis followed in Memphis, Tennessee. The genes for these proteins are located in the major histocompatibility complex between the HLA-B and HLA-DR loci. Bf phenotype frequency did not differ significantly for the patients as compared to regional controls. The C4*A6 allele was present in 26.6% of the patients as compared to 5.4% of the controls, p less than 0.001, relative risk = 4.93. The C4*A6 allele is known to be in linkage disequilibrium with the HLA B17 allele and produces a functionally defective gene product. The role, if any, of C4*A6 in the pathogenesis of psoriasis is uncertain.

Complement C4↗
PubMed2006

Measuring human complement activation by HLA antibodies.

CONTEXT: The clinical significance of HLA antibodies in transplantation depends on their ability to activate complement, which is measured by the standard complement-dependent cytotoxicity test. Recent reports indicate that C3b measurement on target cells may be a better indicator of human complement activation than standard cytotoxicity. OBJECTIVE: To determine the characteristics of the test, the role of other complement components, and the potential influence of the patient's own complement activity. DESIGN: The T-cell deposition of multiple complement components triggered by HLA alloantibodies was evaluated by flow cytometry using normal human serum as the source of complement. Complement activity in patients' sera was measured after activation with a standard antibody. RESULTS: When HLA antibodies activate complement, C3b deposition is usually highest, followed by that of C4b and C5b. Deposition of C1q, C3d, iC3b, or C5b-9 was low or undetectable in this study. The C5 was activated only when relatively high levels of C3b were present. There was a critical concentration of antibody, unique for each patient, below which activation of C3 declined abruptly. The complement activity in the serum of candidates for liver, heart, and lung transplantation that was tested with a standard antibody was similar to normal serum. In contrast, kidney transplant candidates exhibited higher complement activity. Unexpectedly, serum C3 activity was retained for at least 72 hours after collection. CONCLUSIONS: Measurement of C3b, and in some instances C4b or C5b, offers a better definition of the ability of HLA antibodies to activate human complement than tests that are in current use. The results provide a necessary baseline to conduct clinical correlation studies.

Complement Activation↗
PubMed1990

Complotypes in individuals of African origin: frequencies and possible extended MHC haplotypes.

We analyzed the frequency distribution of 106 complotypes [four allele sets of the major histocompatibility complex (MHC) genes for the complement proteins factor B, C2, C4A, and C4B] from 32 Black families residing in Boston and Washington, DC. Twenty-five different complotypes were identified, among which there were four complotypes that had not been previously observed in our large database of complotypes compiled from family studies of Boston Caucasians and that are, presumably, unique to individuals of African origin. These four African-derived complotypes are FC(1,90)0, FC63, S1C2, 17, and SC(3,2,90)0. The frequencies of two of these four unique Black complotypes, FC(1,90)0 and FC63, were increased significantly when compared to Caucasians (pcorr less than 0.00042, pcorr = 0.00294, respectively). The complotype FC(1,90)0 was in positive linkage disequilibrium with HLA-DR3 haplotypes containing the B locus antigens Bw42, Bw52, Bw53, and Bw58, while FC63 was associated with HLA-Bw70, -DR5. These findings demonstrate the extensive polymorphism of complotypes in Blacks, and also suggest that it may be possible to define unique extended haplotypes of African origin.

Alleles↗
PubMed1999

An initiator element and a proximal cis-acting sequence are essential for transcriptional activation of the complement factor I (CFI) gene.

Human complement factor I (CFI) is a serine protease which regulates the complement system by inactivation of C3b and C4b in the presence of appropriate cofactors. In this study, we have analyzed the mechanism controlling the constitutive transcriptional activation of the CFI gene. Using deletion analysis and transient CAT expression assays, we have mapped the minimal promoter to the region located between -46 and +160 bp relative to the major transcription start point (tsp), and also shown that cis-acting elements both upstream and downstream of the tsp are important for promoter activity. A silencer element was also found between -71 and -46 bp. The sequence surrounding the tsp was related to the mouse terminal deoxynucleotidyltransferase initiator element (Inr) and point mutations in this sequence, from -3 to +4, drastically reduced CFI promoter activity. Mutations in a -9 to -5 bp CTGAA sequence immediately upstream of the tsp also reduced promoter activity. Gel mobility shift analysis demonstrated the binding of nuclear factors to a CTGAA repeat located at -9 to -5 and +101 to +105. Our results suggest that CFI promoter contains a functional Inr element that is essential for promoter activity, and the interactions of the CTGAA element located between -9 and +5 with nuclear factor(s) may be part of the machinery required for CFI Inr-dependent transcription.

Animals↗
PubMed1987

Characterization of the interaction between human protein S and C4b-binding protein (C4bp).

C4b-binding protein, C4bp, is a regulatory factor of the complement system and is also known to be a binding protein of vitamin K-dependent coagulation factor, protein S. Whereas the C4b-binding site is known to be located in the middle part of the subunit chains of C4bp, the location and properties of protein S-binding site are uncertain. Therefore, we have examined the characteristics of the interaction between human protein S and C4bp. Proteolysis of C4bp-protein S complex with chymotrypsin yielded N-terminal-derived 48-kDa fragments of C4bp subunit chains and a C-terminal-derived 160-kDa core fragment of C4bp, to which protein S was still bound. This result suggested that the protein S-binding site is located in the core domain of C4bp. Gel filtration of guanidine-treated C4bp-protein S complex in the absence of guanidine resulted in the separation of C4bp and protein S. Binding assay with 125I-labeled protein S showed that the guanidine-treated C4bp lacked the protein S-binding activity. This result suggests that the protein S-binding site in C4bp is denatured irreversibly by guanidine treatment and therefore seems to be dependent on a specific conformation of C4bp. The C4bp-binding site of protein S was lost upon thrombin treatment, suggesting that the N-terminal thrombin-sensitive region of protein S may be related to the C4bp-binding site. Although free protein S was susceptible to chymotrypsin, leukocyte elastase, and cathepsin G, C4bp-bound protein S was found to be resistant to these proteases.(ABSTRACT TRUNCATED AT 250 WORDS)

Carrier Proteins↗