CD35 CR1(complement receptor type 1), C3b/C4bR, Immune adherence receptor
Molecule TypeAntigen ExpressionMolecular Weight
Min / Max
Non-lineage Restricted Molecule
Type 1 glycoprotein
Erythrocyte
Blood Cell
Platelet
Monocyte
Neutrophil
Eosinophil
Follicular Cell
Dendritic Cell
Podocyte
B Lymphocyte
T Lymphocyte
Astrocyte
Granulocyte
160 / 160
285 / 285

Expression
CD35 is expressed on erythrocytes, neutrophils, monocytes, eosinophils, B lymphocytes, and 10%-15% of T lymphocytes.  It has been identified on glomerular podocytes, follicular dendritic cells, and some astrocytes.  Blood group antigens are located on CD35.  For most non-primates, including rabbit and quinea pig, CD35 is expressed on platelets rather than erythrocytes.  The immune adherence receptor on mouse and rat platelets remains to be identified.  It does not cross react with antibodies to mouse CR1/CR2.


Structure
MOLECULAR FAMILY NAME:  Belongs to the regulators of complement activation gene family.

CD35 is a single-pass type-1polymorphic 998 glycoprotein.  It contains a 930 aa extracellular domain which contains 24 potential N-linked glycosylation sites, a 25 aa transmembrane domain and a 43 aa cytoplasmic tail.  The extracellular region is comprised entirely of tandemly repeated complement control protein (CCP) repeats and is also termed short consensus repeats (SCRs) of approximately 65 aa.  Each repeat contains 4 conserved cysteines, linked 1-3 and 2-4, and a tryptophan between cysteines 3 and 4.  High sequence homology of 60%-100% between every 7th repeat permits grouping every 7 repeats into a unit termed a long homologous repeat (LHR). LHRs are termed A, B, C, and D, corresponding to repeats 1-7, 8-15, 16-21, and 22-28.  SCRs 29 and 30 show a reduced homology with the repeats grouped into LHRs.  Molecular mass polymorphism is due to the differences in the number of SCRs encoded by different alleles, 190 kDa in 23 SCRs, 220 kDa in 30 SCRs, 250 kDa in 37 SCRs, and 280 kDa in 44 SCRs.  Functional domains localize to the 1st 3 SCRs of LHRs A, B, and C.  The frequency of each form is A 82%, B 18%, C and D <0.01%.

MOLECULAR MASS
CELL TYPE UNREDUCED REDUCED COMMENT
Erythrocyte 160,190,220,250 kDa 190,220,250,280 kDa Molecular mass polymorphism due to codominant expression of alleles with gene frequencies of : 0.01, 190 kDa; 0.83, 220 kDa; 0.16, 250 kDa; and <0.01, 280 kDa (reduced)
Leukocytes 165,195,225,255 kDa 195,225,255,285 kDa There is approximately 5 kDa increase in molecular mass on leukocytes due to glycosylation

POST-TRANSCRIPTIONAL MODIFICATION

A cDNA clone has been identified which encodes a secreted form comprising the amino-terminal 8.5 SCRs. A protein product of this transcript has not been detected in vivo.  A CD35-like genomic clone has been identified with approximately 90% homology to CD35.  It possesses SCRs 1-6 joined to SCR9.  There is no stop codon in these exons.  Smaller molecular mass forms of CD35 are expressed, predominantly on erythrocytes, by non-human primates.  Some of these smaller forms are bound to the cell surface membrane by a GPI-anchor. Mouse CD35 and CD21 are generated by alternative splicing of a common message onto the CD21 message of 15 SCRs, which appends 6 SCRs from the amino terminus of CD35 producing a hybrid CD21/CD35 protein.

POST-TRANSLATIONAL MODIFICATION

N-linked glycosylation accounts for molecular mass differences of CD35 between erythrocytes and leukocytes. There is no O-linked carbohydrate.

Ligands
CD35 binds the complement components C3b and C4b.

LIGANDS AND MOLECULES ASSOCIATED WITH CD35
Molecule Comment
C3b C3b is a primary natural ligand. CD35's affinity is greater for C3b than for C4b, weaker affinity is seen with iC3b and C3dg. In vitro, affinities are enhanced by reducing the ionic strength
C4b C4b is a primary natural ligand. CD35's affinity is greater for C3b than for C4b, weaker affinity is seen with iC3b and C3dg. In vitro, affinities are enhanced by reducing the ionic strength
iC3b CD35's affinity is greater for C3b than for C4b, weaker affinity is seen with iC3b and C3dg. In vitro, affinities are enhanced by reducing the ionic strength
C3dg CD35's affinity is greater for C3b than for C4b, weaker affinity is seen with iC3b and C3dg. In vitro, affinities are enhanced by reducing the ionic strength
iC3 (hemolytically inactive C3, C3(H20), C3u) Binds CD35 with affinity similar to C3b and C4b.
In vitro, affinities are enhanced by reducing the ionic strength
iC4 (hemolytically inactive C4, C4(H20), C4u) Binds CD35 with affinity similar to C3b and C4b.
In vitro, affinities are enhanced by reducing the ionic strength





Function
The major role of CD35 is the removal and processing of immune complexes and facilitating transport to lymphoid follicles.  On leukocytes and tissue sources, CD35 mediates adherence of C4b/C3b coated particles in preparation for phagocytosis.  Phagocytosis most often requires the cooperation of CD35 with complement receptor type 3, CR3, CD11b/CD18, and the Fcg receptors.  Primary function of erythrocyte CD35 is mediating the adherence of targets coated with C4b/C3b and their transport to the fixed phagocyte systems of the spleen and liver.  CD35 limits complement activation and creates ligands for complement receptors by facilitating C3b and C4b cleavage by factor 1 and accelerating the decay of the C3 and C5 convertases and acting as a cofactor to plasma serine protease factor 1.   Invasion by a number of bacterial and protozoan pathogens is facilitated by CD35.  Soluble CD35 is currently undergoing clinical trials as a complement inhibitor.   

BIOCHEMICAL ACTIVITY

Receptors for C3b and C4b are bound to immune complexes.  The co-factor for specific proteolytic cleavage of C3b and C4b is by the plasma serine protease, factor I.  CD35 accelerates the decay of both the classical and the alternative pathway C3 and C5 convertases on cells to which it is bound by intrinsic and extrinsic activity.

DISEASE RELEVANCE AND FUNCTION OF CD35 IN INTACT ANIMAL

CD35 plays a major role in the removal and processing of immune complexes and facilitating of their localization to lymphoid follicles.  A recombinant soluble form of CD35, consisting of SCRs 1-30 but lacking the transmembrane and cytoplasmic domains, is in clinical trials as a complement inhibitor.  This reagent has provided the means to establish a major role for complement in ischemia-reperfusion injury.  A CD35 knockout model in the mouse demonstrates impaired B-cell responses to T-cell dependent antigens.  The use of monoclonal antibodies to CD35 crosslinked to either autoantigens or antibodies to pathogens is currently being investigated as a means of promoting clearance.  A transient, acquired and disease activity-related reduction in CR1/number erythrocyte occurs in SLE and other syndromes with excessive amounts of complement fixing immune complexes.  CD35 contains a system of antigens that carry Knops blood group polymorphism with one allele more common in individuals of African origin reported to confer some protection against severe malaria.  Mutations cause a reduction in P. falciparum rosetting.  Mutations of CD35 have been associated with gallbladder carcinomas, mesangiocapillary glomerulonephritis, systemic lupus erythematosus and sarcoidosis 

Comments
The CD35 gene is located within the regulators of complement activation (RCA) gene complex which contains the genes for CD21, CD35, CD46, CD55, factor H, and the 2 subunits of the C4 binding protein, all of which contain CCP domains.  This region appears to have evolved rapidly in mammals because in many cases orthologues of these genes have not been identified in other species.  Furthermore, identified  orthologues often differ in structure, expression, and function.  For example, the most abundant form of CD35 on chimpanzee erythrocytes has only 1 LHR, equivalent to LHR-A, and binds both C3b and C4b.  Mice lack a structural equivalent of human CD35 but instead have 1 gene that codes for both CD35 (CR1) and CD21 (CR2).  Mouse CD21 has 15 CCP domains and CD35 contains an additional 6 CCP at its N-terminus.  These additional CCP domains confer a C3b binding activity.  A related mouse gene Crry encodes a membrane protein with 5 CCP domains.  The sequence of the first 4 CCP domains of Crry are most similar to the first 4 CCP domains of human CD35.  However, Crry can mediate both decay accelerating activity, like CD55, and cofactor activity, like CD46, for both the classical and alternative complement activation pathways. The CD35/Crry regulatory proteins of the rat are different.

MOLECULAR INTERACTIONS -
PROTEINS AND DNA ELEMENTS WHICH REGULATE TRANSCRIPTION OF CD35: No information.

SUBSTRATES

By binding C3b or C4b, CD35's primary ligands, CD35 is thought to induce a conformational change in these ligands making them more susceptible to site-specific cleavage by the plasma serine protease, factor I.  CD35 is a co-factor for the 1st cleavage reaction, which leads to production of C3bi.  It is a necessary co-factor for the cleavage of C3bi to C3c and C3dg.

ENZYMES WHICH MODIFY CD35: No information.


Database accession numbers
AnimalPIRSWISSPROTEMGBL/GENBANK
 
HumanEntrezgene 1378P17927
MouseA43519M34164-74
MouseM36470
Antibodies
Ber-Mac-DRC   View Reactivity
E11   View Reactivity
To5   View Reactivity

Revised June 25, 2008


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