CD227 MUC 1 (mucin 1), DF3 antigen, H23 antigen, episialin, PUM (peanut reactive urinary mucin), PEM (polymorphic epithelial mucin) EMA (epithelial membrane antigen)
Molecule TypeAntigen ExpressionMolecular Weight
Min / Max
Non-lineage Restricted Molecule
Type 1 glycoprotein
Stem Cell
Epithelial Cell
Lymphocyte
Monocyte
B Cell
Dendritic Cell
Myeloma
Leukocyte
Breast
Ovarian cancer
25 / 25
700 / 700

Expression
CD227 is expressed by epithelia of glands and ducts as goblet and columnar cells epithelial tissues.  Wide expression is at high levels in most human adenocarcinomas and many myelomas.  Expression is on  follicular dendritic cells, monocytes, leukocytes, subsets of lymphocytes, B and stem cells.  CD227 is a large cell surface mucin glycoprotein that is strongly expressed on the apical surface of almost all grandular and ductal epithelial cells and some hematopoietic cell lineages.   CD227 is expressed on in vitro activated CD4+ and CD8+ T lymphocytes and monocyte-derived and blood derived HLA-DR+ Lin-dendritic cells.  Low to moderate expression occurs on all circulating monocytes.  CD227 has been reported to be expressed on some circulating B cells.  It is present at high levels on many myelomas.  Glycosylated forms are expressed in human epithelial tumors, such as breast or ovarian cancers and also in non-epithelial tumor cells.  There is immunohistochemical evidence for expression on follicular dendritic cells.  CD227 is expressed on 2% to 10% of bone marrow hematopoietic mononuclear cells.  CD227 is localized on the plasma membrane, vesicle vacuole and is secreted.  The CD227 protein is polymorphic depending on the number of tandem repeats that are present in the gene.



Structure
MOLECULAR FAMILY NAME: Belongs to the mucin family.

CD227 is a single-pass type-1 1255 aa glycoprotein.  It contains a 58 aa extracellular domain which contains C-terminal peptide and a membrane-bound subunit in the endoplasmic reticulum and several potential N-linked glycosylation sites, a 28 aa transmembrane domain and a 72 aa cytoplasmic domain which contains 7 tyrosine residues and a potential clathrin-mediated endocytic signal sequence.  CD227 is cleaved into a C-terminal extracellular peptide and a membrane-bound subunit in the endoplasmic reticulum.  These two subunits reassemble to form a heterodimer by can dissociate or be proteolytically cleaved, leading to shedding of the extracellular peptide.  The non-membrane bound subunit has a large tandem repeat domain that is highly polymorphic, varying from 21 to 125 repeats between individuals.  Each repeat consists of 20 aa rich in serine, threonine and proline residues and is potentially O-glycosylated.  The density of O-glycosylation varies between tissues and with cell differentiation.  The protein is anchored to the apical surface of many epithelia by a transmembrane domain that includes a 20 aa variable number tandem repeats and with the degree of glycosylation varying with cell type.  Alterations in glycosylation have been found in epithelial cancer cells.

MOLECULAR MASS
Cell Type Unreduced Reduced Comment
various 25, 300-700 kDa In SDS-PAGE gels the sub-units disassociate into a high Mr (300-700 kDas or larger, depending upon the level of glycosylation) band often seen as a smear and often with distinguishable alleles of different Mr, and an approximately 25 kDa band representing the membrane tethered sub-unit
various 220-350 kDa Unglycosylated precursor proteins are resolved as distinct 220-350 kDa bands by SDS-PAGE

Mr is variable due to expressed VNTR polymorphisms and also due to variation in glycosylation.

POST-TRANSCRIPTIONAL MODIFICATION

Alternative splicing yields 9 different isoforms.  Isoform 5 and 9 are secreted and isoform 1 and 7 undergo transphorylation.  The significance of these variant forms is not well established nor is the existence of the protein products MUC1/X/Y/Z/SEC.  Most analyses have used RT-PCR.   MUC1/Y is a transmembrane form that lacks the entire mucin repeat domain.  MUC1/X and MUC1/Z is a similar transmembrane isoform also lacking the mucin domain.  MUC1/SEC is a secreted form lacking the transmembrane domain and CT , and MUC1 with a variant amino termini.

POST-TRANSLATIONAL MODIFICATION

Extensive O-glycosylation is localized mainly on the tandem repeat VNTR domain consisting of repeating 20 aa sequences of which all 5 serines/threonines can be glycosylated.  CD227 contains several possible N-glycosylation sites.  Different tissues/cells produce differing glycoforms of CD227.  Glycosylation of CD227 in malignant cells is often altered compared to normal tissue.  CD227 is cleaved into 2 subunits in the endoplasmic reticulum during biosynthesis.  These 2 subunits re-associate but they can disassociate resulting in the shedding of the extracellular sub-unit containing the mucin variable number tandem repeat (VNTR) domain from the cell surface.


Ligands
LIGANDS AND MOLECULES ASSOCIATED WITH CD227
Molecule Comment
CD54 CD227 has been shown to bind CD54 (ICAM1) via the CD227 VNTR domain
selectins CD227 molecules bearing sialyl-Lex carbohydrate structures are capable of binding endothelial selectins
CD169 CD227 has been shown to bind to CD169 (sialoadhesin) a sialic acid binding lectin found on macrophages
Grb2 CD227 has been shown to bind Grb2
b-catenin CD227 has been shown to bind b-catenin
GSK-3b CD227 has been shown to bind GSK-3b


Function
Originally, it was assumed that CD227 had a purely protective role via its mucin repeat domain which forms an elongated rigid structure extending well above other molecules in the glycocalyx.  It has a protective role by lubricating and hydrate the cell surface, protecting against degradative enzymes and sterically inhibiting microbial access to the cell wall.  Transfection studies have shown that the high cell surface expression of CD227 can interfere with both cell-cell and cell-substrate adhesion.  A number of potential ligands for CD227 have been identified.  However, the CD227 CT is highly conserved across species and research over the last 5 years has firmly implicated the CD227 CT in multiple signal transduction pathways.  Based on observations in epithelial cells, the most likely roles for CD227 in hematopoietic cells include cell surface protection by binding to pathogens, modulation of adhesion and functions in cell signaling  capacity in response to specific ligands and cell migration.  Very high cell surface expression can sterically hinder both cell-cell and cell-extracellular matrix interactions and it may play a role in cell-cell interactions and promote metastasis of tumor cells, providing a protective layer on epithelial surfaces, and have a direct or indirect interaction with actin cytoskeleton.  Epithelial tissues can be highly variable in response to steroid hormones and cytokines.

BIOCHEMICAL ACTIVITY

The CD227 cytoplasmic domain does not have kinase activity but undergoes tyrosine phosphorylation in epithelial and hematopoietic cells, and has been implicated in signal transduction.

DISEASE RELEVANCE AND FUNCTION OF CD227 IN INTACT ANIMAL

CD227 is a prognostic marker and is highly expressed by the majority of human adenocarcinomas and associated with poor prognosis.  Malignant transformation often leads to CD227 over expression of altered RNA spliced variants with aberrant glycosylation and loss of apical restriction.  In some cases, a tumor-specific epitope may arise that may trigger an immune response.  Overexpression, aberrant intracellular localization, and changes in glycosylation of CD227 have been associated with carcinomas.  CD227 is shed into the bloodstream of adenocarcinoma patients and is the antigen in commercial serum tumor marker assays, CA15-3, Truquant-Br, CASA, CA549, MCA.  CD227 is the target of current cancer immunotherapy trials and may play a role in non-malignant inflammatory epithelial diseases.


Comments
CD227 is a prognostic marker in adenocarcinoma.

MOLECULAR INTERACTIONS -
PROTEINS AND DNA ELEMENTS WHICH REGULATE TRANSCRIPTION OF CD227: No information.

SUBSTRATES: No information.

ENZYMES WHICH MODIFY CD227
Enzyme Comment
polypeptidyl N-acetyl-galactosaminyl-transferases (GalNAc Transferases T1, T2, T3, T4)


Database accession numbers
AnimalPIRSWISSPROTEMGBL/GENBANK
 
HumanEntrezgene 4582P15941
MouseM76179
MouseM77226
Antibodies

Revised June 25, 2008


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