Inhibition of complement activation by recombinant Sh-CRIT-ed1 analogues.
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Sh-CRIT-ed1 is a potent anti-complement peptide that inhibits the classical complement-activation pathway by interfering with the formation of the C3-convertase complex, C4b2a. C2 is an essential serum glycoprotein that provides the catalytic subunit of the C3 and C5 convertases of the classical pathways of complement activation. Because only in its C4-bound state is C2a capable of cleaving its physiological protein substrates C3 and C5, the interaction of Sh-CRIT-ed1 with C2 plays a decisive role of inhibition in the classical complement-activation process. However, the role of individual Sh-CRIT-ed1 amino acid residues in C2 binding is not fully understood. We constructed nine recombinant Sh-CRIT-ed1 (rSh1) analogues, substituted at conserved residues, and evaluated their anti-complement and C2-binding activities. Results from glutathione S-transferase (GST) pull-down and haemolytic assays suggested that residues 10K, 17E, 19K and 26Y are critical for the interaction of rSh1 with C2. We then constructed an improved anti-complement peptide by duplicating Sh-CRIT-ed1 C-terminal motifs (17H-26Y). This linear homodimer (rH17d) was more potent than rSh1 with respect to binding to C2 and anti-complement activity (the 50% inhibitory concentration value was approximately equal 1.2 micro m versus approximately equal 6.02 micro m for rSh1). Furthermore, rH17d showed higher anti-complement activity in vivo, providing additional evidence that this duplication is a more effective inhibitor of complement activation than rSh1. Taken together, these results identify four key residues in rSh1 and strongly suggest that rH17d is a potent inhibitor of complement activation that may have therapeutic applications. (+info)
A variable number of tandem repeats locus within the human complement C2 gene is associated with a retroposon derived from a human endogenous retrovirus.
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We have previously described multiallelic restriction fragment length polymorphisms of the C2 gene, suggesting the presence of a variable number of tandem repeats (VNTR) locus. We report here the cloning and sequencing of the polymorphic fragments from the two most common alleles of the gene, a and b. The results confirm the presence of a VNTR locus consisting of a nucleotide sequence, 41 bp in average length, repeated tandemly 23 and 17 times in alleles a and b, respectively. The difference in the number of repeats between the two alleles is due to the deletion/insertion of two noncontiguous segments, 143 and 118 bp long, of allele a, and of a 40-bp segment of allele b. The VNTR region is associated with a SINE (short interspersed sequence)-type retroposon, SINE-R.C2, located within the third intron of the C2 gene. SINE-R.C2 is a member of a previously described large retroposon family of the human genome, apparently derived from the human endogenous retrovirus, (HERV) K10, which is homologous to the mouse mammary tumor virus. (+info)
Complement factor 2 deficiency: a clinical and serological family study.
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Inherited complement deficiencies are associated with a variety of connective tissue diseases. A family with inherited deficiency of complement factor 2 (C2) is described in which two family members with homozygous C2 deficiency developed cutaneous vasculitis and sicca syndrome. The other family members had heterozygous C2 deficiency and each member had the HLA-A25, B18, DR2 (w15) haplotype. The mother had seropositive rheumatoid arthritis. Further studies showed the presence of cryoglobulins, antibodies against endothelial cells, and anticardiolipin antibodies. (+info)
Nomenclature for human complement component C2. WHO-IUIS Nomenclature Sub-Committee.
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This note describes the designations for variants of the human complement component C2, which were approved by the Nomenclature Committee of the International Union of Immunological Societies (IUIS). (+info)
Production and interferon-gamma-mediated regulation of complement component C2 and factors B and D by the astroglioma cell line U105-MG.
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In this paper, we demonstrate the synthesis of the complement component C2 and factors B and D by the human astroglioma cell line U105-MG. All three components were structurally and antigenically similar to their serum counterparts, as determined by biosynthetic labelling studies or Western blot analysis. Northern blot analysis demonstrated that the mRNAs of all three components had the same apparent sizes as the equivalent mRNAs from hepatocyte and monocyte cell lines. Interestingly, U105-MG cells produce two C2 transcripts with sizes of approximately 2.8 and 2.3 kb. Interferon-gamma (IFN-gamma) enhanced the expression of C2 and factor B mRNA and protein in a dose- and time-dependent fashion, while factor D expression was refractory to IFN-gamma. IFN-gamma appeared to predominantly enhance the expression of the large (2.8 kb) C2 transcript. Kinetic studies demonstrated peak C2 and factor B expression in 48 h in response to IFN-gamma, similar to the acute-phase response of factor B in serum. These data are the first to demonstrate the synthesis of C2 and factor D by astroglioma cells. Combined with previous reports documenting the synthesis of C3 by astrocytes, our data suggest that endogenous synthesis of complement proteins, and particularly of alternative pathway activation components (C3, factors B and D), may play an important role in host defence in the central nervous system. (+info)
Activation of the classical complement pathway by mannose-binding protein in association with a novel C1s-like serine protease.
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Serum mannose-binding protein (MBP) is a C-type lectin that binds to terminal mannose and N-acetylglucosamine moieties present on surfaces of certain pathogens and activates the classical complement pathway. In the present study, we describe the mechanism underlying the activation triggered by MBP. The human serum MBP fraction was obtained by sequential affinity chromatography on mannan-Sepharose, anti-IgM-Sepharose and anti-MBP-Sepharose in the presence of calcium ions. This fraction contained a C1s-like serine protease as assessed by C4 consumption. The C1s-like serine protease, designated MBP-associated serine protease (MASP), was separated from MBP by rechromatography on anti-MBP-Sepharose in the presence of ethylenediaminetetraacetic acid. MASP exhibited both C4- and C2-consuming activities. The molecular mass of MASP was estimated to be 83 kD with two polypeptides of heavy (66 kD) and light (L) (31 kD) chains linked by disulfide bonds. The serine residue responsible for protease activity is located on the L chain. Reconstitution experiments using MASP and MBP revealed that combination of the two components restores C4- and C2-activating capacity on mannan. Based on analyses of molecular size, antigenicity, and 11 NH2-terminal amino acid sequences of the L chain, we conclude that MASP is a novel protein different from C1r or C1s. Our findings are not in accord with a proposed mechanism by which MBP utilizes the C1r2-C1s2 complex to initiate the classical complement pathway. (+info)
Crystal structure of the A domain from complement factor B reveals an integrin-like open conformation.
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Complement factor B is a 90 kDa protein consisting of three domains: a three-module complement control protein, a von Willebrand factor A domain, and a C-terminal serine protease (SP) domain that adopts a default inactive (zymogen) conformation. The interaction between factor B and pathogen-bound C3b is mediated by its A domain, triggering a conformational change in factor B that ultimately creates the "C3 convertase" of the alternative complement pathway. We report the crystal structure of the A domain from factor B and show that it contains an integrin-like MIDAS motif that adopts the "open" conformation typical of integrin-ligand complexes, with an acidic residue (provided by a fortuitous crystal contact) completing the coordination of the metal ion. Modeling studies indicate that the factor B A domain can also adopt the closed conformation, supporting the hypothesis that an "integrin-like switch" is conserved in complement proteins and perhaps in 60 other A domains found within the human proteome. (+info)
Two mechanisms for mannose-binding protein modulation of the activity of its associated serine proteases.
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Serum mannose-binding protein (MBP) neutralizes invading microorganisms by binding to cell surface carbohydrates and activating MBP-associated serine proteases-1, -2, and -3 (MASPs). MASP-2 subsequently cleaves complement components C2 and C4 to activate the complement cascade. To analyze the mechanisms of activation and substrate recognition by MASP-2, zymogen and activated forms have been produced, and MBP.MASP-2 complexes have been created. These preparations have been used to show that MBP modulates MASP-2 activity in two ways. First, MBP stimulates MASP-2 autoactivation by increasing the rate of autocatalysis when MBP.MASP-2 complexes bind to a glycan-coated surface. Second, MBP occludes accessory C4-binding sites on MASP-2 until activation occurs. Once these sites become exposed, MASP-2 binds to C4 while separate structural changes create a functional catalytic site able to cleave C4. Only activated MASP-2 binds to C2, suggesting that this substrate interacts only near the catalytic site and not at accessory sites. MASP-1 cleaves C2 almost as efficiently as MASP-2 does, but it does not cleave C4. Thus MASP-1 probably enhances complement activation triggered by MBP.MASP-2 complexes, but it cannot initiate activation itself. (+info)