Structural homology of complement protein C6 with other channel-forming proteins of complement.
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The amino acid sequence of the amino-terminal half of the complement protein C6 has been found to show overall structural homology with the homologous regions of the channel-forming proteins C7, C8 alpha, C8 beta, and C9. In addition, two specific cysteine-rich segments common to the amino-terminal regions of C7, C8 alpha, C8 beta, and C9 also occur in their expected positions in C6, suggesting functional significance. Two cDNA clones encoding C6 were isolated from a human liver library in the bacteriophage vector lambda gt11. The predicted protein sequence contains an apparent initiation methionine and a putative signal peptide of 21 residues, as well as a site for N-glycosylation at residue 303. The sequence of the C6 protein reported here has 47-52% similarity with C7, C8 alpha, C8 beta, and C9, as well as 31-38% similarity with thrombospondin, thrombomodulin, and low density lipoprotein receptor. The sequence data have been interpreted by using computer algorithms for estimation of average hydrophobicity and secondary structure. (+info)
Relative contributions of chemo-attractant and terminal components of complement to anti-glomerular basement membrane (GBM) glomerulonephritis.
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The relative contributions of chemo-attractant and terminal components of complement to heterologous phase glomerular injury was studied in anti-GBM glomerulonephritis in rabbits. Normal rabbits (complement intact) were given anti-GBM antibody at a dose which resulted in 140 micrograms specific kidney-fixed antibody per gram of renal cortex, and developed significant proteinuria (1910 +/- 327 mg/24 h; control 18.2 +/- 6.1 mg/24 h; P less than 0.01). Leucocyte depletion significantly reduced but did not abolish proteinuria (574 +/- 186 mg/24 h, P less than 0.05). Complement depletion of neutrophil-depleted rabbits resulted in a further significant reduction in proteinuria 50.1 +/- 12.2 mg/24 h, P less than 0.05; versus neutrophil-depleted, complement-intact rabbits), indicating that both neutrophil accumulation and complement activation independent of neutrophils contribute to injury in this model. Rabbits congenitally deficient in the sixth component of complement (C6D) developed similar levels of proteinuria (2099 +/- 796 mg/24 h) to normal rabbits given an identical dose of antibody. However, after leucocyte depletion, C6D rabbits developed significantly less proteinuria (135 +/- 56 mg/24 h) than did leucocyte-depleted, complement-intact rabbits (P less than 0.05). These studies show that terminal complement components are not necessary for the full expression of acute anti-GBM antibody-initiated injury in leucocyte-intact rabbits. However, in the absence of leucocytes, C6 and the terminal complement components are apparently responsible for the majority of the complement-dependent glomerular injury. (+info)
Depletion of C6 prevents development of proteinuria in experimental membranous nephropathy in rats.
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To study the possible role of the complement membrane attack complex, C5b-9, in an experimental rat model that is morphologically indistinguishable from membranous nephropathy in man (passive Heymann nephritis [PHN]), an antibody to rat C6 was used to deplete C6 levels to less than 5% of pretreatment values (C6D) during disease development. C3, C7, C8, and C9 levels were not different in C6D and control rats. After injection of nephritogenic quantities of 125I-anti-Fx1A antibody, the kinetics of disappearance of labeled IgG from the blood were identical in the complement deficient and sufficient groups, and glomerular deposition of 125I-antibody was the same in both groups at 5 days. Glomerular deposits of sheep IgG and C3 were also similar in C6D and controls, but glomerular deposits of C6 and C5b-9 neoantigens were markedly reduced or absent in C6 depleted rats. However, despite equivalent antibody deposits, proteinuria was abolished in C6D rats compared with normocomplementemic controls. Similar results were obtained when F(ab')2 anti-rat C6 IgG was used to deplete C6 during development of PHN. These results demonstrate that C6 is required for the development of the increased glomerular permeability that occurs in PHN, presumably because C6 is required for formation of C5b-9. We conclude that glomerular injury in the PHN model of membranous nephropathy in the rat is mediated by C5b-9. (+info)
A case of hereditary combined deficiency of complement components C6 and C7 in man.
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Immunological investigation of a patient presenting with candidiasis and toxoplasmosis revealed a combined deficiency of C6 and C7. Deficiency of C6 was total, but small amounts (less than 1 microgram/ml) of apparently normal C7 were present in the serum. All family members (three sibs and both parents) were heterozygous for the combined deficiency. This is only the second reported case of combined homozygous deficiency of the closely linked and immunochemically similar proteins C6 and C7, and only the third kindred in which this defect has been demonstrated. (+info)
The molecular architecture of human complement component C6.
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The molecular architecture of human complement component C6 was elucidated at several levels of structural organization. The entire primary structure of C6 was determined by sequencing C6 cDNA that was cloned from a human liver lambda gt11 library. The polypeptide chain of C6 contains 913 amino acids. The protein is homologous with the other terminal components of complement, C7-C9. Specifically, C6 has 29% of its residues identical with C7, and 55 of the 56 cysteines found in C7 match those in C6. The C6 polypeptide chain is cross-linked by 32 disulfide bonds, and most of the cysteines are located in short (34-77 amino acids) discrete segments that exhibit homology with a wide variety of other proteins such as thrombospondin, the low density lipoprotein receptor, epidermal growth factor, and complement factors H and I. C6 is a glycoprotein, and it has two oligosaccharide groups attached to asparagines located near the amino and the carboxyl termini of the molecule. The organization of secondary structural elements in C6 was elucidated using circular dichroism spectroscopy and an empirical method based on sequence analysis. C6 has an estimated 12% alpha-helix, but is comparatively richer in beta-sheet (29%) and beta-turns (21%). Most of the predicted alpha-helical structure resides in a portion of the polypeptide chain that is free of cysteine and which shares homology with C9 and perforin. The tertiary structure of the C6 molecule was visualized by transmission electron microscopy; it has a sickle shape with dimensions of 144 x 66 A. The combined results are discussed and comparisons made with the other late acting components of complement and perforin. (+info)
Complete primary structure and functional characterization of the sixth component of the human complement system. Identification of the C5b-binding domain in complement C6.
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Complement C6 is one of five plasma proteins that are incorporated into the lytic terminal complement complex on lipid membranes (C5b-9m) upon activation of the complement cascade. Oligonucleotide probes derived from partial amino acid sequences of purified C6 were used to isolate cDNA clones from a human liver cDNA library. The complete polypeptide structure of mature C6 deduced from the cDNA sequence consists of 913 amino acid residues preceded by a typical 21-residue signal peptide. C6 is most similar in structure to complement C7, sharing 33.5% identical residues with C7 including all 56 cysteine residues. The low density lipoprotein receptor class A and B modules, the thrombospondin type I module at the carboxyl terminus, and the two short consensus repeat modules are arranged in the same way as in C7. In contrast to C7 and other terminal complement proteins, the thrombospondin type I module at the amino terminus occurs as a tandem repeat in C6. The last tandem repeat at the carboxyl terminus of C6 and C7 has been identified as a new distinct module (factor I module), which is closely related to a segment in the heavy chain of complement control factor I. Binding studies with filter-bound C6 fragments generated by proteolysis showed that the C5b-binding domain of C6 was located in the 34-kDa carboxyl terminal fragment consisting of two short consensus repeats and two factor I modules. (+info)