Calnexin

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Calnexin
Calnexin 1JHN.png
Lumenal domain of calnexin from PDB 1JHN.
Available structures
PDB Ortholog search: PDBe, RCSB
Identifiers
Symbols CANX ; CNX; IP90; P90
External IDs OMIM114217 MGI88261 HomoloGene1324 ChEMBL: 2719 GeneCards: CANX Gene
RNA expression pattern
PBB GE CANX 208852 s at tn.png
PBB GE CANX 208853 s at tn.png
More reference expression data
Orthologs
Species Human Mouse
Entrez 821 12330
Ensembl ENSG00000127022 ENSMUSG00000020368
UniProt P27824 P35564
RefSeq (mRNA) NM_001024649 NM_001110499
RefSeq (protein) NP_001019820 NP_001103969
Location (UCSC) Chr 5:
179.11 – 179.16 Mb
Chr 11:
50.29 – 50.33 Mb
PubMed search [1] [2]

Calnexin (CNX) is a 67kDa integral protein (that appears variously as a 90kDa, 80kDa or 75kDa band on western blotting depending on the source of the antibody) of the endoplasmic reticulum (ER). It consists of a large (50 kDa) N-terminal calcium-binding lumenal domain, a single transmembrane helix and a short (90 residues), acidic cytoplasmic tail.

Function[edit]

Calnexin is a chaperone, characterized by assisting protein folding and quality control, ensuring that only properly folded and assembled proteins proceed further along the secretory pathway.

Calnexin acts to retain unfolded or unassembled N-linked glycoproteins in the ER.

Calnexin binds only those N-glycoproteins that have GlcNAc2Man9Glc1 oligosaccharides. These monoglucosylated oligosaccharides result from the trimming of two glucose residues by the sequential action of two glucosidases, I and II. Glucosidase II can also remove the third and last glucose residue.

If the glycoprotein is not properly folded, an enzyme called UGGT (for UDP-glucose:glycoprotein glucosyltransferase) will add the glucose residue back onto the oligosaccharide thus regenerating the glycoprotein's ability to bind to calnexin.

The improperly-folded glycoprotein chain thus loiters in the ER, risking the encounter with MNS1 (alpha-mannosidase), which eventually sentences the underperforming glycoprotein to degradation by removing its mannose residue.

If the protein is correctly translated, the chance of it being correctly folded before it encounters MNS1 is high.

Calnexin also functions as a chaperone for the folding of MHC class I alpha chain in the membrane of the ER. After folding is completed Calnexin is replaced by Calreticulin, which assists in further assembly of MHC class I.

Uses in molecular biology[edit]

Antibodies against calnexin can be used as markers for the ER in immmunofluorescence experiments. [1]

Cofactors[edit]

ATP and calcium ions are cofactors involved in substrate binding for calnexin.

External links[edit]

References[edit]

Further reading[edit]