Complement receptor 1

Last updated
CR1
Protein CR1 PDB 1gkg.png
Available structures
PDB Human UniProt search: PDBe RCSB
Identifiers
Aliases CR1 , C3BR, C4BR, CD35, KN, complement component 3b/4b receptor 1 (Knops blood group), complement C3b/C4b receptor 1 (Knops blood group)
External IDs OMIM: 120620 HomoloGene: 55474 GeneCards: CR1
Orthologs
SpeciesHumanMouse
Entrez

1378

n/a

Ensembl

ENSG00000203710

n/a

UniProt

P17927

n/a

RefSeq (mRNA)

NM_000573
NM_000651
NM_001381851

n/a

RefSeq (protein)

NP_000564
NP_000642
NP_001368780

n/a

Location (UCSC) Chr 1: 207.5 – 207.64 Mb n/a
PubMed search [2] n/a
Wikidata
View/Edit Human

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. [3] [4]

Contents

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. [3]

In primates, CR1 serves as the main system for processing and clearance of complement opsonized immune complexes. It has been shown that CR1 can act as a negative regulator of the complement cascade, mediate immune adherence and phagocytosis and inhibit both the classic and alternative pathways. The number of CR1 molecules decreases with aging of erythrocytes in normal individuals and is also decreased in pathological conditions such as systemic lupus erythematosus (SLE), HIV infection, some haemolytic anaemias and other conditions featuring immune complexes. [5] 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. [6] [7]

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.

Factor H, another immunoregulatory protein, also maps to this location. [8]

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. [9]

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. [10]

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. [5] Multiple size variants (55–220 kDa) are also found among non-human primates and a partial amino-terminal duplication (CR1-like gene) that encodes the short (55–70 kDa) forms expressed on non human erythrocytes. These short CR1 forms, some of which are glycosylphosphatidylinositol (GPI) anchored, are expressed on erythrocytes and the 220-kDa CR1 form is expressed on monocytes. The gene including the repeats is highly conserved in primates possibly because of the ability of the repeats to bind complement. LHR-A binds preferentially to the complement component C4b: LHR-B and LHR-C bind to C3b and also, albeit with a lower affinity, to C4b. Curiously the human CR1 gene appears to have an unusual protein conformation but the significance of this finding is not clear.

The mean number of complement receptor 1 (CR1) molecules on erythrocytes in normal individuals lies within the range of 100–1000 molecules per cell. Two codominant alleles exist – one controlling high and the other low expression. Homozygotes differ by a factor of 10–20: heterozygotes typically have 500–600 copies per erythrocyte. These two alleles appear to have originated before the divergence of the European and African populations.

Rosetting

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) interacts with uninfected erythrocytes. This 'stickiness', known as rosetting, is believed to be a strategy used by the parasite to remain sequestered in the microvasculature to avoid destruction in the spleen and liver. Erythrocyte rosetting causes obstruction of the blood flow in microcapillaries. There is a direct interaction between PfEMP1 and a functional site of complement receptor type 1 on uninfected erythrocytes. [5]

Role in blood groups

The Knops antigen was the 25th blood group system recognized and consists of the single antigen York (Yk) a with the following allelic pairs:

The antigen is known to lie within the CR1 protein repeats and was first described in 1970 in a 37-year-old Caucasian woman. Racial differences exist in the frequency of these antigens: 98.5% and 96.7% of American Caucasians and Africans respectively are positive for McC(a). 36% of a Mali population were Kn(a) and 14% of exhibited the null (or Helgeson) phenotype compared with only 1% in the American population. The frequencies of McC (b) and Sl (2) are higher in Africans compared with Europeans and while the frequency of McC (b) was similar between Africans from the United States or Mali, the Sl (b) phenotype is significantly more common in Mali – 39% and 65% respectively. In Gambia the Sl (2)/McC(b) phenotype appears to have been positively selected – presumably due to malaria. 80% of Papua New Guineans have the Helgeson phenotype and case–control studies suggest this phenotype has a protective effect against severe malaria.

Related Research Articles

Glycophorin C plays a functionally important role in maintaining erythrocyte shape and regulating membrane material properties, possibly through its interaction with protein 4.1. Moreover, it has previously been shown that membranes deficient in protein 4.1 exhibit decreased content of glycophorin C. It is also an integral membrane protein of the erythrocyte and acts as the receptor for the Plasmodium falciparum protein PfEBP-2.

<span class="mw-page-title-main">Classical complement pathway</span> Aspect of the immune system

The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM.

<span class="mw-page-title-main">Alternative complement pathway</span> Type of cascade reaction of the complement system

The alternative pathway is a type of cascade reaction of the complement system and is a component of the innate immune system, a natural defense against infections.

<span class="mw-page-title-main">C3-convertase</span>

C3 convertase belongs to family of serine proteases and is necessary in innate immunity as a part of the complement system which eventuate in opsonisation of particles, release of inflammatory peptides, C5 convertase formation and cell lysis.

Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed by phagocytes. Different types of things ("targets") can be tagged by opsonins for phagocytosis, including: pathogens, cancer cells, aged cells, dead or dying cells, excess synapses, or protein aggregates. Opsonins help clear pathogens, as well as dead, dying and diseased cells.

<span class="mw-page-title-main">HLA-DRA</span> Protein-coding gene in the species Homo sapiens

HLA class II histocompatibility antigen, DR alpha chain is a protein that in humans is encoded by the HLA-DRA gene. HLA-DRA encodes the alpha subunit of HLA-DR. Unlike the alpha chains of other Human MHC class II molecules, the alpha subunit is practically invariable. However it can pair with, in any individual, the beta chain from 3 different DR beta loci, DRB1, and two of any DRB3, DRB4, or DRB5 alleles. Thus there is the potential that any given individual can form 4 different HLA-DR isoforms.

<span class="mw-page-title-main">Integrin alpha X</span> Mammalian protein found in Homo sapiens

CD11c, also known as Integrin, alpha X (ITGAX), is a gene that encodes for CD11c.

<span class="mw-page-title-main">Decay-accelerating factor</span> Mammalian protein found in Homo sapiens

Complement decay-accelerating factor, also known as CD55 or DAF, is a protein that, in humans, is encoded by the CD55 gene.

<span class="mw-page-title-main">Complement receptor 2</span> Mammalian protein found in Homo sapiens

Complement receptor type 2 (CR2), also known as complement C3d receptor, Epstein-Barr virus receptor, and CD21, is a protein that in humans is encoded by the CR2 gene.

<span class="mw-page-title-main">CD19</span> Biomarker for B cell lineage

B-lymphocyte antigen CD19, also known as CD19 molecule, B-Lymphocyte Surface Antigen B4, T-Cell Surface Antigen Leu-12 and CVID3 is a transmembrane protein that in humans is encoded by the gene CD19. In humans, CD19 is expressed in all B lineage cells. Contrary to some early doubts, human plasma cells do express CD19, as confirmed by others. CD19 plays two major roles in human B cells: on the one hand, it acts as an adaptor protein to recruit cytoplasmic signaling proteins to the membrane; on the other, it works within the CD19/CD21 complex to decrease the threshold for B cell receptor signaling pathways. Due to its presence on all B cells, it is a biomarker for B lymphocyte development, lymphoma diagnosis and can be utilized as a target for leukemia immunotherapies.

Complement control protein are proteins that interact with components of the complement system.

Complement component 4 (C4), in humans, is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with the other numerous components. Furthermore, it is a crucial factor in connecting the recognition pathways of the overall system instigated by antibody-antigen (Ab-Ag) complexes to the other effector proteins of the innate immune response. For example, the severity of a dysfunctional complement system can lead to fatal diseases and infections. Complex variations of it can also lead to schizophrenia. The C4 protein was thought to derive from a simple two-locus allelic model, which however has been replaced by a much more sophisticated multimodular RCCX gene complex model which contain long and short forms of the C4A or C4B genes usually in tandem RCCX cassettes with copy number variation, that somewhat parallels variation in the levels of their respective proteins within a population along with CYP21 in some cases depending on the number of cassettes and whether it contains the functional gene instead of pseudogenes or fragments. Originally defined in the context of the Chido/Rodgers blood group system, the C4A-C4B genetic model is under investigation for its possible role in schizophrenia risk and development.

<span class="mw-page-title-main">CD46</span> Mammalian protein found in Homo sapiens

CD46 complement regulatory protein also known as CD46 and Membrane Cofactor Protein is a protein which in humans is encoded by the CD46 gene. CD46 is an inhibitory complement receptor.

<span class="mw-page-title-main">Complement factor I</span> Protein

Complement factor I, also known as C3b/C4b inactivator, is a protein that in humans is encoded by the CFI gene. Complement factor I is a protein of the complement system, first isolated in 1966 in guinea pig serum, that regulates complement activation by cleaving cell-bound or fluid phase C3b and C4b. It is a soluble glycoprotein that circulates in human blood at an average concentration of 35 μg/mL.

<span class="mw-page-title-main">C3b</span>

C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apoptotic cells for phagocytosis. Additionally, C3b plays a role in forming a C3 convertase when bound to Factor B, or a C5 convertase when bound to C4b and C2b or when an additional C3b molecule binds to the C3bBb complex.

<span class="mw-page-title-main">LILRA3</span> Protein-coding gene in the species Homo sapiens

Leukocyte immunoglobulin-like receptor subfamily A member 3 (LILR-A3) also known as CD85 antigen-like family member E (CD85e), immunoglobulin-like transcript 6 (ILT-6), and leukocyte immunoglobulin-like receptor 4 (LIR-4) is a protein that in humans is encoded by the LILRA3 gene located within the leukocyte receptor complex on chromosome 19q13.4. Unlike many of its family, LILRA3 lacks a transmembrane domain. The function of LILRA3 is currently unknown; however, it is highly homologous to other LILR genes, and can bind human leukocyte antigen (HLA) class I. Therefore, if secreted, the LILRA3 might impair interactions of membrane-bound LILRs with their HLA ligands, thus modulating immune reactions and influencing susceptibility to disease.

Immune adherence was described by Nelson (1953) for an in vitro immunological reaction between normal erythrocytes and a wide variety of microorganisms sensitized with their individually specific antibody and complement; erythrocytes were observed to adhere to microorganisms. It was later recognized to occur in vivo.

Sushi domain is an evolutionarily conserved protein domain. It is also known as Complement control protein (CCP) modules or short consensus repeats (SCR). The name derives from the visual similarity of the domain to nigiri sushi when the primary structure is drawn showing the loops created by the disulfide bonds.

<span class="mw-page-title-main">Complement component 4B</span> Protein-coding gene in the species Homo sapiens

Complement component 4B (Chido blood group) is a kind of the Complement component 4 protein that in humans is encoded by the C4B gene.

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a family of proteins present on the membrane surface of red blood cells that are infected by the malarial parasite Plasmodium falciparum. PfEMP1 is synthesized during the parasite's blood stage inside the RBC, during which the clinical symptoms of falciparum malaria are manifested. Acting as both an antigen and adhesion protein, it is thought to play a key role in the high level of virulence associated with P. falciparum. It was discovered in 1984 when it was reported that infected RBCs had unusually large-sized cell membrane proteins, and these proteins had antibody-binding (antigenic) properties. An elusive protein, its chemical structure and molecular properties were revealed only after a decade, in 1995. It is now established that there is not one but a large family of PfEMP1 proteins, genetically regulated (encoded) by a group of about 60 genes called var. Each P. falciparum is able to switch on and off specific var genes to produce a functionally different protein, thereby evading the host's immune system. RBCs carrying PfEMP1 on their surface stick to endothelial cells, which facilitates further binding with uninfected RBCs, ultimately helping the parasite to both spread to other RBCs as well as bringing about the fatal symptoms of P. falciparum malaria.

References

  1. 1 2 3 GRCh38: Ensembl release 89: ENSG00000203710 - Ensembl, May 2017
  2. ↑ "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
  3. 1 2 "Entrez Gene: CR1 complement component (3b/4b) receptor 1 (Knops blood group)".
  4. ↑ Moulds JM, Nickells MW, Moulds JJ, Brown MC, Atkinson JP (May 1991). "The C3b/C4b receptor is recognized by the Knops, McCoy, Swain-langley, and York blood group antisera". The Journal of Experimental Medicine. 173 (5): 1159–63. doi:10.1084/jem.173.5.1159. PMC   2118866 . PMID   1708809.
  5. 1 2 3 Khera R, Das N (February 2009). "Complement Receptor 1: disease associations and therapeutic implications". Molecular Immunology. 46 (5): 761–72. doi:10.1016/j.molimm.2008.09.026. PMC   7125513 . PMID   19004497.
  6. ↑ Lambert JC, Heath S, Even G, Campion D, Sleegers K, Hiltunen M, et al. (October 2009). "Genome-wide association study identifies variants at CLU and CR1 associated with Alzheimer's disease". Nature Genetics. 41 (10): 1094–9. doi:10.1038/ng.439. hdl: 10281/9031 . PMID   19734903. S2CID   24530130.
  7. ↑ Fonseca MI, Chu S, Pierce AL, Brubaker WD, Hauhart RE, Mastroeni D, et al. (2016). "Analysis of the Putative Role of CR1 in Alzheimer's Disease: Genetic Association, Expression and Function". PLOS ONE. 11 (2): e0149792. Bibcode:2016PLoSO..1149792F. doi: 10.1371/journal.pone.0149792 . PMC   4767815 . PMID   26914463.
  8. ↑ Das, N; Biswas, B; Khera, R (2013). "Membrane-Bound Complement Regulatory Proteins as Biomarkers and Potential Therapeutic Targets for SLE". Complement Therapeutics. Advances in Experimental Medicine and Biology. Vol. 735. pp. 55–81. doi:10.1007/978-1-4614-4118-2_4. ISBN   978-1-4614-4117-5. PMID   23402019.
  9. ↑ Jacobson AC, Weis JH (September 2008). "Comparative functional evolution of human and mouse CR1 and CR2". Journal of Immunology. 181 (5): 2953–9. doi:10.4049/jimmunol.181.5.2953. PMC   3366432 . PMID   18713965.
  10. ↑ Donius LR, Handy JM, Weis JJ, Weis JH (July 2013). "Optimal germinal center B cell activation and T-dependent antibody responses require expression of the mouse complement receptor Cr1". Journal of Immunology. 191 (1): 434–47. doi:10.4049/jimmunol.1203176. PMC   3707406 . PMID   23733878.

Further reading

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