Complement component 4

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Complement component 4 (C4), in humans, is a protein involved in the intricate

One of the earlier genetic studies on the C4 protein identified two different groups, found within a human serum, called the Chido/Rogers (Ch/Rg) blood groups. O’Neill et al. have demonstrated that two different C4 loci express the different Ch/Rg antigens on the membranes of erythrocytes.[3] More specifically, the two proteins, Ch and Rg, function together as a medium for interaction between the Ab-Ag complex and other complement components.^[4] Moreover, the two loci are linked to the HLA, or the human analog of the major histocompatibility complex (MHC) on the short arm of chromosome 6, whereas previously they were believed to have been expressed by two codominant alleles at a single locus.^[3][5] In gel electrophoresis studies, O’Neill et al. have identified two genetic variants: F, signifying the presence (F+) or absence (f0/ f0) of four fast moving bands, and S, signifying the presence (S+) or absence (s0/ s0) of four slow moving bands.^[3] The homogeneity or heterogeneity of the two loci, with the addition of these null (f0, s0) genes, allow for duplication/non-duplication of the C4 loci.^[6] Therefore, having separate loci for C4, C4F and C4S (later identified as C4A or C4B, respectively), possibly account for producing multiple allelic forms, leading to the great size and copy number variation.^{[citation needed]}

Two important contributors, Carroll and Porter, in their study of cloning the human C4 gene showed that all six of their clones contained the same C4 gene.^[7] The C4 protein consists of 3 subunits (α, β, and γ) having molecular weights (MWs) of ~95,000, 78,000, and 31,000, respectively and they are all joined by interchain disulfide bridges.^{[7][8][9][10]} In a study by Roos et al., the α-chains between the C4A and C4B were found to be slightly different (MW of ~96,000 and 94,000, respectively), proving that there is actually a structural difference between the two variants.^[9] Moreover, they implicated that a lack of C4 activity could be attributed to the structural differences between the α-chains.^[9] Nevertheless, Carroll and Porter demonstrated that there is a 1,500-bp region that acts as an intron in the genomic sequence, which they believed to be the known C4d region, a byproduct of C4 activity.^[7] Carroll et al. later published work that characterized the structure and organization of the C4 genes, which are situated in the HLA class III region and linked with C2 and factor B on the chromosome.^[11] Through experiments involving restriction mapping, nucleotide sequence analysis, and hybridization with C4A and C4B, they found that the genes are actually fairly similar though they have their differences.^[11] For example, single nucleotide polymorphisms were detected, which allowed them to be class differences between C4A and C4B.^[11] Furthermore, class and allelic differences would affect the performance of the C4 proteins with the immune complex.^[11] Finally, by overlapping cDNA cloned fragments, they were able to determine that the C4 loci, an estimated 16 kilobase (kb) long, are spaced by 10 kb and aligned 30 kb from the factor B locus.^[10][11]

In the same year, studies relatedly identified a 98 kb region of the chromosome the four class III genes (that express C4A, C4B, C2, and factor B) are closely linked, which does not allow for cross-overs to occur.

The early studies vastly expanded the knowledge of the C4 complex, laying down the foundations that paved the way to discovering the gene and protein structures. C. Yu successfully determined the complete sequence of the human complement component C4A gene.[4] In the findings, the whole genome was found to have of 41 exons, with a total of 1744 residues (despite avoiding the sequence of a large Intron 9).^[4] The C4 protein is synthesized into a single chain precursor, which then undergoes proteolytic cleavage into three chains (in order of how they are chained, β-α-γ).^[4]

The β-chain consists of 656 residues, coded by exons 1-16.^[4] The most prominent aspect of the β-chain is the presence of a large intron, ranging from six to seven kilobases in size.^[4] It is present in the first locus (coding for C4A) for all C4 genes and in the second locus (coding for C4B) only in a few C4 genes.^[4] The α-chain consists of residues 661-1428, encoding exons 16-33.^[4] Within this chain, two cleavage sites marked by exons 23 and 30 produces the C4d fragment (where the thioester, Ch/Rg antigens, and isotypic residues are located); moreover, most of the polymorphic sites cluster in this region.^[4] The γ-chain consists of 291 residues, encoding exons 33-41.^[4] Unfortunately, no specific function has been attributed to the γ-chain.^[4]

The study completed by Vaishnaw et al. sought to identify the key region and factors related to the efforts of gene expression of the C4 gene.^[13] Their research concluded with the fact that the Sp1 binding site (positioned at -59 to -49) plays an important role in accurately starting basal transcription of C4.^[13] Utilization of electromobility shift assays and DNase I footprint analyses demonstrated specific DNA-protein correlations of the C4 promoter at the nuclear factor 1, two E box (-98 to -93 and -78 to -73), and Sp1 binding domains.^[13] These findings were later added to in another extensive study, that found a third E box site.^[14] In addition, the same findings postulated that two physical entities within the gene sequence could have a role in the expression levels of human C4A and C4B, which include the both presence of the endogenous retrovirus that can have positive or negative regulatory influences affecting C4 transcription and the varying genetic environment (dependent on which genetic modular component is present) past position -1524.^[14]

To provide more context, in the latter study, the previously noted bimodular structure (C4A-C4B) has been updated to a quadrimodular structure of one to four discrete segments, containing one or more RP-C4-CYP21-TNX (

Additionally, the same study identified the expression of human complement C4 transcripts in multiple tissues. The results of a Northern blot analysis, using a C4d probe and RD probe as positive control, showed that the liver contains the majority of transcripts throughout the body.[14] Even so, moderate quantities were expressed in adrenal cortices/medulla, thyroid, and kidney.^[14]

Function and mechanism

As noted, C4 (mixture of C4A and C4B) participates in all three of the complement pathways (classical, alternative, and lectin); the alternative pathway is "triggered spontaneously," while the classical and lectin pathways are elicited in response to the recognition of particular microbes.^[16] All three pathways converge at a step in which complement protein C3 is cleaved into proteins C3a and C3b, which results in a lytic pathway and formation of a macromolecular assembly of multiple proteins, termed the membrane-attack complex (MAC), which serves as a pore in the membrane of the targeted pathogen, leading to invading cell disruption and eventual lysis.^[16]

In the classical pathway, the complement component—hereafter abbreviated by the "C" preceding the protein number— termed C1s, a

C4b has further functions. It interacts with protein C2; the same protease invoked earlier, C1s, then cleaves C2 into two parts, termed C2a and C2b, with C2b being released, and C2a remaining in association with C4b; the C4b-C2a complex of the two proteins then exhibits a further system-associated protease activity toward protein C3 (cleaving it), with subsequent release of both proteins, C4b and C2a, from their complex (whereupon C4b can bind another protein C2, and conduct these steps again).[16] Because C4b is regenerated, and a cycle is created, the C4b-C2a complex with protease activity has been termed the C3 convertase.^[16] Protein 4b can be further cleaved into 4c and 4d.^[19]

Clinical significance

Although other diseases (i.e.

Substantial data from all over the world has been collected and analyzed to determine that schizophrenia, indeed, has a strong genetic relationship with a region in the MHC locus on chromosome arm 6.[21]^[1]

Data and information collected internationally can shed light onto the mysteries of

• Complement component 4A
• Complement component 4B
• HLA A1-B8-DR3-DQ2 haplotype
• Complement system
• Complement deficiency

References