Complement receptor type 1 (CR1) also known as C3b/C4b receptor or CD35 (cluster of differentiation 35) is a protein that in humans is encoded by the CR1 gene.

This gene is a member of the regulators of complement activation (RCA) family and is located in the 'cluster RCA' region of chromosome 1. The gene encodes a monomeric single-pass type I membrane glycoprotein found on erythrocytes, leukocytes, glomerular podocytes, hyalocytes, and splenic follicular dendritic cells. The Knops blood group system is a system of antigens located on this protein. The protein mediates cellular binding to particles and immune complexes that have activated complement. Decreases in expression of this protein and/or mutations in its gene have been associated with gallbladder carcinomas, mesangiocapillary glomerulonephritis, systemic lupus erythematosus and sarcoidosis. Mutations in this gene have also been associated with a reduction in Plasmodium falciparum rosetting, conferring protection against severe malaria. Alternate allele-specific splice variants, encoding different isoforms, have been characterized. Additional allele specific isoforms, including a secreted form, have been described but have not been fully characterized. In mice, CR1 is an alternatively spliced variant of the complement receptor 2 (CR2) gene.

Certain alleles of this gene have been statistically associated with an increased risk of developing late-onset Alzheimer's disease.

Gene region

In humans, the CR1 gene is located on the long arm of chromosome 1 at band 32 (1q32) and lies within a complex of immunoregulatory genes. In 5'-3' order the genes in this region are: membrane cofactor protein – CR1 – complement receptor type 2 – decay-accelerating factor – C4-binding protein.

  • Membrane cofactor protein is a widely distributed C3b/C4b binding regulatory glycoprotein of the complement system;
  • decay-accelerating factor (DAF: CD55: Cromer antigen) protects host cells from complement-mediated damage by regulating the activation of C3 convertases on host cell surfaces;
  • complement receptor 2 is the C3d receptor.

Factor H, another immunoregulatory protein, also maps to this location.

Gene structure and isoforms

The canonical Cr2/CD21 gene of subprimate mammals produces two types of complement receptor (CR1, ca. 200 kDa; CR2, ca. 145 kDa) via alternative mRNA splicing. The murine Cr2 gene contains 25 exons; a common first exon is spliced to exon 2 and to exon 9 in transcripts encoding CR1 and CR2, respectively. A transcript with an open reading frame of 4,224 nucleotides encodes the long isoform, CR1; this is predicted to be a protein of 1,408 amino acids that includes 21 short consensus repeats (SCR) of ca. 60 amino acids each, plus transmembrane and cytoplasmic regions. Isoform CR2 (1,032 amino acids) is encoded by a shorter transcript (3,096 coding nucleotides) that lacks exons 2–8 encoding SCR1-6. CR1 and CR2 on murine B cells form complexes with a co-accessory activation complex containing CD19, CD81, and the fragilis/Ifitm (murine equivalents of LEU13) proteins.

The complement receptor 2 (CR2) gene of primates produces only the smaller isoform, CR2; primate CR1, which recapitulates many of the structural domains and presumed functions of Cr2-derived CR1 in subprimates, is encoded by a distinct CR1 gene (apparently derived from the gene Crry of subprimates).

Isoforms CR1 and CR2 derived from the Cr2 gene possess the same C-terminal sequence, such that association with and activation through CD19 should be equivalent. CR1 can bind to C4b and C3b complexes, whereas CR2 (murine and human) binds to C3dg-bound complexes. CR1, a surface protein produced primarily by follicular dendritic cells, appears to be critical for generation of appropriately activated B cells of the germinal centre and for mature antibody responses to bacterial infection.

The most common allelic variant of the human CR1 gene (CR1*1) is composed of 38 exons spanning 133kb encoding a protein of 2,039 amino acids with a predicted molecular weight of 220 kDa. Large insertions and deletions have given rise to four structurally variant genes and some alleles may extend up to 160 kb and 9 additional exons. The transcription start site has been mapped to 111 bp upstream of the translation initiation codon ATG and there is another possible start site 29 bp further upstream. The promoter region lacks a distinct TATA box sequence. The gene is expressed principally on erythrocytes, monocytes, neutrophils and B cells but is also present on some T lymphocytes, mast cells and glomerular podocytes.

Structure

The encoded protein has a 47 amino acid signal peptide, an extracellular domain of 1930 residues, a 25 residue transmembrane domain and a 43 amino acid C terminal cytoplasmic region. The leader sequence and 5'-untranslated region are contained in one exon. The large extracellular domain of CR1, which has 25 potential N-glycosylation sites, can be divided into 30 short consensus repeats (SCRs) (also known as complement control protein repeats (CCPs) or sushi domains), each having 60 to 70 amino acids. The sequence homology between SCRs ranges between 60 and 99 percent. The transmembrane region is encoded by 2 exons and the cytoplasmic domain and the 3'-untranslated regions are coded for by two separate exons.

The 30 or so SCRs are further grouped into four longer regions termed long homologous repeats (LHRs) each encoding approximately 45 kDa of protein and designated LHR-A, -B, -C, and -D. The first three have seven SCRs while LHR-D has 9 or more. Each LHR is composed of 8 exons and within an LHR, SCR 1, 5, and 7 are each encoded by a single exon, SCR 2 and 6 are each encoded by 2 exons, and a single exon codes for SCR 3 and 4. The LHR seem to have arisen as a result of unequal crossing over and the event that gave rise to LHR-B seems to have occurred within the fourth exon of either LHR-A or –C. To date the atomic structure have been solved for SCRs 15–16, 16 & 16–17.

Alleles

Four known human alleles encode proteins with predicted molecular weights of 190 kDa, 220 kDa, 250 kDa and 280 kDa. similar to red cell genotyping for any of the other recognized blood group systems. Molecular analysis can identify gene variants (alleles) that may affect Knops antigen expression on the red cell membrane.

References

Further reading

  • Knops blood group system at BGMUT Blood Group Antigen Gene Mutation Database at NCBI, NIH