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Calnexin

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Identifiers
SymbolsCANX ; CNX; IP90; P90
External IDsOMIM114217 MGI88261 HomoloGene1324 ChEMBL: 2719 GeneCards: CANX Gene
RNA expression pattern
File:PBB GE CANX 208852 s at tn.png
File:PBB GE CANX 208853 s at tn.png
More reference expression data
Orthologs
SpeciesHumanMouse
Entrez82112330
EnsemblENSG00000127022ENSMUSG00000020368
UniProtP27824P35564
RefSeq (mRNA)NM_001024649NM_001110499
RefSeq (protein)NP_001019820NP_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

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. It specifically 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 α-chain in the membrane of the ER. As newly synthesized MHC class I α-chains enter the endoplasmic reticulum, calnexin binds on to them retaining them in a partly folded state.[1] After the β2-microglobulin binds to the MHC class I peptide-loading complex (PLC), calreticulin and ERp57 take over the job of chaperoning the MHC class I protein while the tapasin links the complex to the Transporter associated with antigen processing (TAP) complex. This association prepares the MHC class I for binding an antigen for presentation on the cell surface.

Cofactors

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

External links

References

  1. ^ Murphy K (2011). Janeway's Immunobiology (8th ed.). Oxford: Taylor & Francis. ISBN 978-0815342434. 

Further reading

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