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PubMed2001

Some patients with antiphospholipid syndrome express hitherto undescribed antibodies to cardiolipin-binding proteins.

Contrary to infective anticardiolipin (aCL) antibodies, autoimmune aCL antibodies react with phospholipids (PL) mainly via binding to the plasma glycoprotein cofactor beta2-Glycoprotein I (beta2GPI). While there is a well-documented link between the risk of thrombosis and the presence of beta2GPI-dependent anticardiolipin antibodies, the pathological impact of other antiphospholipid antibodies is less clear. By means of cardiolipin affinity-chromatography, we isolated and identified 3 CL-binding proteins, complement component C4, complement factor H and a kallikrein-sensitive glycoprotein, and tested for the presence of autoantibodies against these proteins in patients with antiphospholipid syndrome (APS), systemic lupus erythematosus (SLE) and other autoimmune diseases. High titers of autoantibodies to C4 as compared to age- and sex-matched healthy controls were present in 3 of 26 patients with APS, and weak titers were found in 2 of 26 patients with SLE and in none of 26 patients with other autoimmune diseases. Autoantibodies to complement factor H were found in 4 APS, 3 SLE and none of the other autoimmune patients. Autoantibodies to kallikrein-sensitive glycoprotein were detected in 6 APS patients, 1 SLE patient, and 1 patient with another autoimmune disease. A close relationship between these antibodies was found, suggesting their origin from a common macromolecular complex. However, no relationship with anti-beta2GPI antibodies was found, with the three patients with higher levels of autoantibodies having a low titer of anti-beta2GPI antibodies. In conclusion, some patients with APS harbor circulating antibodies to other CL-binding proteins which might be useful to further characterize these patients.

Adolescent↗
PubMed1998

Regulation of transcription of the TATA-less human complement component C4 gene.

The 5'-sequences flanking the human complement component C4 genes (C4A and C4B) have been analyzed for their ability to direct expression of a reporter gene in cell lines that constitutively express or do not express C4. No difference in the level of reporter gene expression was detected in cells transfected with C4A- or C4B-specific constructs. A series of reporter constructs containing progressively truncated C4 promoter fragments transfected into the hepatocyte Hep G2 cell line, identified the sequence contained within the region -178 to -39 as that associated with maximal reporter gene expression. This region contains consensus binding motifs for nuclear factor 1 (-110 to -97), Sp1 (-57 to -49), and three basic helix-loop-helix (-137 to -132, -98 to -93, and -78 to -73)-like transcription factors. Electromobility shift assays and DNase I footprinting analysis showed specific DNA-protein interactions of the C4 promoter at the nuclear factor 1, two E box (-98 to -93 and -78 to -73), and Sp1 binding domains. Site-directed mutagenesis of the Sp1 binding site resulted in total abrogation of reporter gene expression and mutation of the E box (-78 to -73) resulted in a 8-fold reduction in expression. We conclude that the Sp1 binding site at position -57 to -49 is critical for accurately initiated, basal transcription of C4.

Base Sequence↗
PubMed1983

cDNA clone spanning the alpha-gamma subunit junction in the precursor of the murine fourth complement component (C4).

cDNA clones carrying parts of murine fourth complement component (C4, serum substance protein) mRNA sequences have been identified by differential hybridization to mRNA from a high C4-producing strain, B10.WR, and a congeneic low C4 strain, B10.BR, followed by hybrid-selected translation and DNA sequence analysis. One clone, pMLC4/w7-2, encodes an open amino acid reading frame that includes four tandem arginine residues immediately preceding a sequence 85% homologous with the NH2-terminal sequence of the human C4 gamma-chain. The amino acid composition of the predicted sequence upstream of the tandem arginines matches quite closely with the composition of a similar sized peptide at the COOH terminus of the human C4 alpha chain. The latter result raises questions regarding the nature and extent of plasma-mediated postsynthetic processing of the C4 alpha-chain COOH terminus. The results also demonstrate that strain differences in plasma C4 levels (low C4 vs. high C4) reflect differences in steady-state levels of liver C4 mRNA in these strains.

Amino Acid Sequence↗
PubMed1995

Complement component C4 gene intron 9 as a phylogenetic marker for primates: long terminal repeats of the endogenous retrovirus ERV-K(C4) are a molecular clock of evolution.

The complement component C4 genes of Old World primates exhibit a long/short dichotomous size variation, except that chimpanzee and gorilla only contain short C4 genes. In human it has been shown that the long C4 gene is attributed to the integration of an endogenous retrovirus, HERV-K(C4), into intron 9. This 6.36 kilobase retroviral element is absent in short C4 genes. Here it is shown that the homologous endogenous retrovirus, ERV-K(C4), is present precisely at the same position in the long C4 gene of orangutan and African green monkey. Determination of the short C4 gene intron 9 sequences from human, three apes, two Old World monkeys, and a New World monkey allowed the establishment of consistent phylogenetic trees for primates, which favors a chimpanzee-gorilla clade. The 5' long terminal repeats (LTR) and 3' LTR of ERV-K(C4) in long C4 genes of human, orangutan, and African green monkey have similar sequence divergence values of 9.1%-10.5%. These values are more than five-fold higher than the sequence divergence of the homologous intron 9 sequences between the long and short C4 genes in higher primates. The latter is probably a result of homogenization or concerted evolution. We suggest that the 5' LTR and 3' LTR of an endogenous retrovirus can serve as a reliable reference point or a molecular clock for studies of gene duplication and gene evolution. This is because the 5'/3' LTR sequences were identical at the time of retroviral integration and evolved independently of each other afterwards. Our data provides strong evidence for the short C4 gene being the ancestral form in primates, trans-species evolution, and the "slow-down" phenomenon of the sequence divergence in great apes.

Amino Acid Sequence↗
PubMed1985

Polymorphism of human complement component C4.

An assessment has been made of the polymorphism of human complement component C4 by comparing derived amino acid sequences of cDNA and genomic DNA with limited amino acid sequences. In all, one complete and six partial sequences have been obtained from material from three individuals and include two C4A and two C4B alleles. Differences were found between the 4 alleles from 2 loci in only 15 of the 1722 amino acid residues, and 12 lie within one section of 230 residues, which in 1 allele also contains a 3-residue deletion. In three variable positions, an allelic difference in one C4 type was common to the other types. Three nucleotide differences were found in four introns. In spite of marked differences in their chemical reactivity, the many allelic forms appear to differ in less than 1% of their amino acid residue positions. This unusual pattern of polymorphism may be due to recent duplication of the C4 gene, or may have arisen by selection as a result of the biological role of C4, which interacts in the complement sequence with nine other proteins necessitating conservation of much of the surface structure.

Amino Acid Sequence↗
PubMed1989

Murine complement component C4 and sex-limited protein: identification of amino acid residues essential for C4 function.

Murine sex-limited protein (Slp) is an isotype of murine complement component C4 that shares 95% sequence identity with C4 as well as the intramolecular thioester necessary for C4 function but has no complement activity. Slp is nonfunctional at least in part because it is not cleaved by the activated form of complement protease C1s (C1s), which proteolytically activates C4 in the classical complement pathway. Slp is also distinct from C4 in that its expression in some mouse strains is under testosterone control. In the present studies, we used site-directed mutagenesis of C4 and expression of the mutant proteins in cultured cells to identify the amino acid substitutions in Slp that are responsible for resistance to C1s cleavage. We focused on sequence changes immediately downstream of the cleavage site in C4 because the arginine at that site is conserved in Slp, but the downstream sequences diverge substantially, with six differences in the first 7 residues followed by a 3-residue deletion in Slp. We found that a C4 mutant carrying only the 3-residue deletion is not cleaved by C1s and has essentially no hemolytic activity, whereas a mutant carrying only the six replacement changes is cleaved by C1s and has normal hemolytic activity. Both mutants have intact thioesters. A third mutant in which two acidic residues in the segment deleted in Slp were replaced by aliphatic residues is also cleaved by C1s, has an intact thioester group, and has normal hemolytic activity. These results indicate that the downstream mutations are responsible for the resistance of Slp to C1s cleavage and suggest that the length rather than the specific sequence of this segment is critical in determining susceptibility to the protease.

Amino Acid Sequence↗
PubMed2026

Complement component C4 and neuroimaging in psychiatry: A systematic review.

INTRODUCTION: Genomic, transcriptomic, and proteomic studies suggest that the complement system contributes to the pathophysiology of various psychiatric disorders partly through neurodevelopmental effects linked to C4A protein levels variations. We conducted a systematic review to characterize how brain micro- and macrostructure and connectivity vary with proxies of in vivo brain C4A protein levels in both psychiatric and general-population cohorts. METHODS: We used Medline, Web of Science, and Embase, and included all studies published before April 14, 2025. Inclusion criteria were: (1) inclusion of healthy controls and/or individuals with psychiatric disorders assessed according to recognized diagnostic manuals (DSM or ICD); (2) use of MRI-based neuroimaging; and (3) use of genomic, transcriptomic and/or proteomic approaches as proxies of in vivo brain C4A proteins levels. RESULTS: From 317 identified articles, 11 were included. Associations between C4A levels and brain structure were heterogeneous across regions. Only the mOFC, dlPFC, and entorhinal cortex were implicated in more than one study. Findings for the mOFC and dlPFC varied by the type of metrics and clinical status, whereas higher C4A levels were more consistently associated with smaller entorhinal cortex surface area and cortical thickness in pediatric, middle-aged, and older general-population cohorts. In addition, one study found higher genetically predicted C4A expression to be associated with higher TSPO levels. CONCLUSION: The limited number of available studies and their methodological heterogeneity make synthesis challenging. However, biological hypotheses such as excessive synaptic pruning or broader inflammatory effects on the brain may provide plausible explanatory frameworks for the reported associations.

Humans↗
PubMed1996

Production of the third and fourth component of complement (C3, C4) by smooth muscle cells.

Production of the third and fourth components of complement (C3, C4) by smooth muscle cells was investigated by using normal human aortic smooth muscle cells (AoSMC), human smooth muscle cell line (G402) and vascular smooth muscle cells obtained from human umbilical cord vein (UVSMC). AoSMC spontaneously produced both C3 and C4 at 15 ng/10(6) cells/72 hr and 22 ng/10(6) cells/72 hr, respectively, and both were enhanced by interferon-gamma (IFN-gamma). Although phorbol 12-myristate 13-acetate (PMA) and tumour necrosis factor-alpha (TNF-alpha) enhanced C3 production, C4 production was reduced by these agents. On the other hand, G402 produced C4 but not C3 in a dose-dependent manner when cultured with IFN-gamma. UVSMC produced only a small amount of C3 and C4 compared with AoSMC or G402. C3 and C4 produced by AoSMC were confirmed to be identical with their human serum counterparts as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and measurement of haemolytic activity. Northern blotting analysis showed that the expression of mRNA of C3 and C4 was enhanced by TNF-alpha and IFN-gamma, respectively, in AoSMC. Our findings suggest the importance of smooth muscle cells as a source of components of complement in vascular diseases including vasculitis.

Aorta↗
PubMed1986

Phenotyping of human complement component C4, a class-III HLA antigen.

The plasma complement protein C4 is encoded at two highly polymorphic loci, A and B, within the class-III region of the major histocompatibility complex. At least 34 different polymorphic variants of human C4 have been identified, including non-expressed or 'null' alleles. The main method of identification of C4 polymorphic allotypes is separation on the basis of charge by agarose-gel electrophoresis of plasma. On staining by immunofixation with anti-C4 antibodies, each C4 type gives three major bands, but, since individuals can have up to five allotypes, the overlapping banding pattern is difficult to interpret. We show that digestion of plasma samples with carboxypeptidase B, which removes C-terminal basic amino acids, before electrophoresis, produces a single, sharp, distinct band for each allotype and allows identification of the biochemical basis of the multiple banding pattern previously observed in C4 phenotype determination.

Carboxypeptidase B↗
PubMed1984

The structural basis of the multiple forms of human complement component C4.

cDNA clones of human complement components C4A and C4B alleles were prepared from mRNA obtained from the liver of a donor heterozygous at both loci. cDNA from one C4A allele was sequenced to give the derived complete amino acid sequence of 1722 amino acid residues of the C4 single chain precursor molecule and the estimated sequences of the three peptide chains of secreted C4. Comparison with partial sequences of a second C4A allele and a C4B allele has led to the tentative identification of some class differences in nucleotide sequences between C4A and C4B and of allelic differences between C4A alleles in this highly polymorphic system.

Amino Acid Sequence↗
PubMed1991

Complement C4-derived monocyte-directed chemotaxis-inhibitory factor. A molecular mechanism to cause polymorphonuclear leukocyte-predominant infiltration in rheumatoid arthritis synovial cavities.

To reveal the mechanism of the lesser infiltration of monocytes in synovial cavities with rheumatoid arthritis despite the presence of chronic inflammation, the synovial fluid from 15 rheumatoid arthritis patients was analyzed with respect to leukocyte chemotaxis. The synovial fluid possessed strong chemotactic activity to polymorphonuclear leukocytes but rather suppressed one to monocytes. The synovial fluid contained two different inhibitory activities in monocyte chemotaxis. One, which also suppressed polymorphonuclear leukocyte chemotaxis, was identified as alpha 1 protease inhibitor. The other, with molecular weight of 8 kd, possessed the specificity to monocytes and shared the antigenicity with complement C4 but not with C3 or C5. A similar inhibitor was generated in normal human plasma when the classical pathway of the complement system was initiated with aggregated human IgG, while it was not when alternative pathway was initiated with zymosan. The small size factor in the synovial fluid, apparently derived from C4, seemed to be a cyto-directed factor that might block an early part of signal transduction system of monocytes in the chemotaxis. After removal of the small-size inhibitor, the synovial fluid exhibited chemotactic ability to monocytes. Therefore the apparent C4-derived factor might play a key role in the polymorphonuclear leukocyte-predominant infiltration in the synovial fluid of rheumatoid arthritis.

Adult↗
PubMed1993

Effect of thiol compounds on human complement component C4.

Thiol compounds have been investigated as inhibitors of the covalent binding reaction of human complement protein C4 using Sepharose-C1s as a combined activating and binding surface. o- and p-substituted aminothiophenols are equally effective inhibitors, whereas the m-substituted compound is a less potent inhibitor. The anti-hypertensive drug captopril is also shown to inhibit the covalent binding reaction. A comparison of the effects of these compounds on the covalent binding reaction of isolated C4A and C4B has been made. Results suggest that a Pro-to-Leu substitution in C4B is likely to account for the differences in inhibitory potency of C4B compared with C4A observed with the aromatic inhibitors.

Aminophenols↗
PubMed1980

The binding of human complement component C4 to antibody-antigen aggregates.

The binding of human complement component C4 to antibody-antigen aggregates and the nature of the interaction have been investigated. When antibody-antigen aggregates with optimal C1 bound are incubated with C4, the C4 is rapidly cleaved to C4b, but only a small fraction (1-2%) is bound to the aggregates, the rest remaining in the fluid phase as inactive C4b. It has been found that C4b and th antibody form a very stable complex, due probably to the formation of a covalent bond. On reduction of the C4b-immunoglobulin G (IgG) complex, the beta and gamma chains, but not the alpha' chain, of C4b are released together with all the light chain, but only about half of the heavy chain of IgG. The reduced aggregates contain two main higher-molecular-weight complexes, one shown by the use of radioactive components to contain both IgG and C4b and probably therefore the alpha' chain of C4b and the heavy chain of IgG, and the other only C4b and probably an alpha' chain dimer. The aggregates with bound C1 and C4b show maximal C3 convertase activity, in the presence of excess C2, when the alpha'-H chain component is in relatively highest amounts. When C4 is incubated with C1s in the absence of aggregates, up to 15% of a C4b dimer is formed, which on reduction gives an alpha' chain complex, probably a dimer. The apparent covalent interaction between C4b and IgG and between C4b and other C4b molecules cannot be inhibited by iodoacetamide and hence cannot be catalysed by transglutaminase (factor XIII). The reaction is, however, inhibited by cadaverine and putrescine and 14C-labelled putrescine is incorporated into C4, again by a strong, probably covalent, bond. It is suggested that a reactive group, possibly an acyl group, is generated when C4 is activated by C1 and that this reactive group can react with IgG, with another C4 molecule, or with water.

Antigen-Antibody Complex↗
PubMed1984

A comparison of the properties of two classes, C4A and C4B, of the human complement component C4.

A remarkable difference has been observed between the reactivity of the two forms of human complement component C4. C4B binds twice as effectively as C4A to antibody-coated red cells, but the reverse occurs with protein-antigen complexes. C4B reacts much more effectively with hydroxyl groups than C4A and this is reversed for reaction with amino groups in spite of the very small difference in amino acid sequence between the two forms of C4. No other differences in stability, activation or inactivation were observed. These findings emphasise the biological advantage of the duplication of the C4 gene in its reaction with a wide range of antigenic structures. The correlation of the presence of different forms of C4 with susceptibility to autoimmune diseases may be explicable by these big differences in binding reactivity.

Amino Acid Sequence↗
PubMed1976

Persistence of immunoglobulin and complement components C4 and C3 bound to guinea pig tumor cells.

Guinea pig hepatoma cells (line-10) growing as ascites were studied for the presence of immunoglobulin, C4, and C3 (components of complement) on their surfaces. Immunoglobulin, C4, and C3 content increased with length of time spent in the peritoneal cavity. The persistence of these factors bound in vivo or in vitro was also determined. Complement-fixing (CF) activity of cellbound antibody disappeared from the surface more rapidly at 37 degrees C than at 4 degrees C; the continued presence of cellbound immunoglobulin in non-complement-fixing form could be demonostrated at either temperature. No CF activity was released into the medium. C4 and C3 were released into the medium and could be demonstrated in the medium by immunochemical methods.

Animals↗
PubMed1999

Deficiency of human complement protein C4 due to identical frameshift mutations in the C4A and C4B genes.

The complement protein C4, encoded by two genes (C4A and C4B) on chromosome 6p, is the most polymorphic among the MHC III gene products. We investigated the molecular basis of C4 deficiency in a Finnish woman with systemic lupus erythematosus. C4-specific mRNA was present at low concentrations in C4-deficient (C4D) patient fibroblasts, but no pro-C4 protein was detected. This defect in C4 expression was specific in that synthesis of two other complement proteins was normal. Analysis of genomic DNA showed that the proposita had both deleted and nonexpressed C4 genes. Each of her nonexpressed genes, a C4A null gene inherited from the mother, a C4A null gene, and a C4B null gene inherited from the father, all contained an identical 2-bp insertion (TC) after nucleotide 5880 in exon 29, providing the first confirmatory proof of the C4B pseudogene. This mutation has been previously found only in C4A null genes. Although the exon 29/30 junction is spliced accurately, this frameshift mutation generates a premature stop at codon 3 in exon 30. These truncated C4A and C4B gene products were confirmed through RT-PCR and sequence analysis. Among the possible genetic mechanisms that produce identical mutations is both genes, the most likely is a mutation in C4A followed by a gene conversion to generate the mutated C4B allele.

Adult↗
PubMed1987

The chemical structure of the C4d fragment of the human complement component C4.

The complete amino acid sequence of the C4d fragment (380 residues long) of the human complement component C4 is presented. Most of the sequence was determined by analysis of CNBr peptides and tryptic peptides obtained from S-carboxymethylated protein. The sequence of the amino terminal 88 residues [Campbell R. D., Gagnon J. and Porter R. R. (1981) Biochem. J. 199, 359-370] and a 106 residue polymorphic segment of C4d [Chakravarti D. N., Campbell R. D. and Gagon J. (1983) FEBS Lett. 154, 387-390] was extended. Some overlaps not provided by the protein sequence analysis were obtained from the amino acid sequence predicted by the nucleotide sequence [Belt K. T., Carroll M. C. and Porter R. R. (1984) Cell 36, 907-914]. The present protein sequence data provide information for the isolation of all the CNBr and succinylated tryptic peptides of C4d. In addition to the polymorphism previously described, two other sets of polymorphic amino acid residues at positions 153 (Ile/Ser) and 154 (Gln/Ala) have been identified. The major site of glycosylation has been shown to be an asparagine residue located in the sequence -Asn-Val-Thr- in the carboxy terminal end of C4d. A remarkable difference in the predicted secondary structure of C4d arising from one set of four polymorphic residues in a stretch of six residues and another single polymorphic residue suggests a structural basis for the origin of the different chemical reactivities of the C4 isotypes (C4A and C4B) and their serological difference in the expression of Rodgers or Chido blood group antigens. Possible non-covalent membrane attachment sites have been suggested from the hydropathy profile. Comparison of the C4d sequence with human C3, C5 and alpha 2-macroglobulin revealed extended stretches of sequence similarity (between 19 and 38% homology) with the corresponding regions of these proteins.

Amino Acid Sequence↗