C1qRP C1qRP (C1q receptor enhances phagocytosis)
Molecule TypeAntigen ExpressionMolecular Weight
Min / Max
Non-lineage Restricted Molecule
Type 1 glycoprotein
Endothelial Cell
Myeloid Cell
Monocyte
Neutrophil
Progenitor Cell
95 / 130

Expression
C1qRP is expressed strongly on all endothelial cells, hematopoietic progenitor cells and on cells of myeloid origin such as monocytes and neutrophils, though not on many macrophages.  While high expression is detected in U937 cells, variable expression is seen in lines of THP-1, weak expression in K562 cells and no detectable expression in HL-60.  There is no expression in the human CEM, RAJI, HELA or in murine F9 cells.  C1qRP is present in platelets as demonstrated by Western blot on platelet extracts.  In humans, expression is on lin-, CD34+ and -cells, all of which have stem cell reconstituting ability.  It is expressed on murine fetal liver hematopoietic progenitor cells, vascular endothelial cells and in adult bone marrow CD34-/loLin-Sca-I+Kit+ and CD34 hiLin-Sca-I+Kit+ stem and progenitor cell subsets.  mRNA is present on the mouse brain endothelial cell line,b.End3, as demonstrated by Northern blot.  C1qRP is also seen in rat primary microglia, mouse microgial cell line, BV-2, and other murine myeloid cell lines BM2.3 and IC-21.  In rat, the C1qRP orthologue is expressed on cells of myeloid origin, but also on subsets of lymphoid cells such as NK cells.  Additional differences between rat and human tissue expression of C1qRP have been reported.

Structure
MOLECULAR FAMILY NAME: No information.

C1qRP is a single chain type 1 glycoprotein.  It is comprised of 652 aa.  Human, murine and rat protein sequences have been deduced from cDNA clones and are known to be similar in sequence and organization.  Each contains a leader sequence of 21 aa, 20 aa and 22 aa respectively that is absent in the mature protein.  C1qRP contains a domain which is structurally similar to a C-type lectin carbohydrate recognition domain.  There are 5 EGF-domains, a mucin-like domain enriched in serine and threonine potential glycosylation sites and a single transmembrane.  There is a 47 aa intracellular domain with a tyrosine residue that is a consensus motif for phosphorylation.  It also has a highly charged jutamembrane motif, resembling other molecules that have been shown to engage ERM proteins.  The deduced murine sequence has a similar domain structure with over 68% identity with the human aa sequence and an 89% identity to the human receptor in the intracellular domain.  The tyrosine phosphorylation consensus motif is not present in the murine intracellular domain.  The rat has a similar domain with 87% identity to the mouse and 66% identity with the human.  The rat, like the human and unlike the mouse, has a tyrosine residue in the cytoplasmic tail that is in a consensus motif for phosphorylation.

MOLECULAR MASS
Cell Type Unreduced Reduced Comment
97 kDa 126 kDa Aberrant migration on SDS-PAGE of the nascent chain due to both the aa composition and O-glycosylation

POST-TRANSCRIPTIONAL MODIFICATION: No information.

POST-TRANSLATIONAL MODIFICATION

Migration of SDS-PAGE suggests extensive post-translational modification in all species.  Data suggests that O-glycosylation is N-glycosylation in the human, rat and murine molecule, as well as an additional putative N-glycosylation site in the murine C1qRP, (position 102-105) but there is as of yet no indication that the site(s) is/are glycosylated.

 



Ligands
LIGANDS AND MOLECULE ASSOCIATED WITH C1qRP
Molecule Comment
Adiponectin Adiponectin suppresses phagocytosis and this is blocked by an anti-C1qRP antibody (R3)
C1q Triggers enhancement of phagocytosis through C1qRP.  A critical interaction sequence has been mapped to GE(K/Q/R)GEP found within the collagen-like N-terminal third MBL, C1q and SPA
Ficolin Serum lectin
MBL (mannose binding lectin) Serum lectin.  A critical interaction sequence has been mapped to GE(K/Q/R)GEP found within the collagen-like N-terminal third MBL, C1q and SPA
SPA (pulmonary surfactant protein A) A critical interaction sequence has been mapped to GE(K/Q/R)GEP found within the collagen-like N-terminal third MBL, C1q and SPA
SPD (pulmonary surfactant protein D)


Function
Multivalent interaction of cells expressing this receptor with C1qRP, MBL and SPA or cross-linking, induced by immobilization of an IgM anti-C1qRP (R3), induces enhancement of phagocytosis of subotimally opsonized particles and/or cellular debris.  The ligands can be associated with the target to be ingested or can be independently interacting with the cell to induce enhanced function.  The enhancement of cell function is induced within less than 1 hour after ligand engagement.  In vitro, this molecule may mediate the enhancement of phagocytosis upon interaction with soluble defense collagens, although additional functions are possible.

BIOCHEMICAL ACTIVITY: No information.

DISEASE RELEVANCE AND FUNCTION OF C1qRP IN INTACT ANIMAL: No information.


Comments
MOLECULAR INTERACTIONS -
PROTEINS AND DNA ELEMENTS WHICH REGULATE TRANSCRIPTION OF C1qRP: No information.

SUBSTRATES: No information.

ENZYMES WHICH MODIFY C1qRP: No information.

ADDITIONAL INSIGHTS

In all cases C1qRP should be distinquished from a protein previously called cC1qR.  These proteins are not the same molecules.  Rather the cC1qR is identical with the chaperone protein calreticulin.  Calreticulin is promiscuous in that it interacts with a wide variety of molecule, and thus there is a large controversy as to whether this should continue to be referred to as cC1qR.  Another molecule that also has no predicted transmembrane domain has been designated as gC1qR as it binds the globular domain of C1q.  However, this intracellular protein binds many different proteins and its function is unknown.  It is sometimes referred to as a collectin receptor.  However, since C1q is not collectin (ie. has not been demonstrated to have lectin binding activity) this is an incorrectly attributed name.  In contrast, it can be classified as a receptor, or part of a receptor complex, for soluble defense collagens.

Database accession numbers
AnimalPIRSWISSPROTEMGBL/GENBANK
 
Antibodies

Revised June 25, 2008


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