| Gene Ontology | |
|---|---|
| Molecular function | • calcium ion binding • protein binding • carbohydrate binding • unfolded protein binding |
| Cellular component | • endoplasmic reticulum • endoplasmic reticulum lumen • endoplasmic reticulum membrane • melanosome • integral to lumenal side of endoplasmic reticulum membrane |
| Biological process | • antigen processing and presentation of peptide antigen via MHC class I • protein folding • protein secretion • protein N-linked glycosylation via asparagine • antigen processing and presentation of exogenous peptide antigen via MHC class II • post-translational protein modification • cellular protein metabolic process • synaptic vesicle endocytosis • clathrin-mediated endocytosis |
| Sources: Amigo / QuickGO | |
179.11 – 179.16 Mb
50.29 – 50.33 Mb
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.
Calnexin is one of the chaperone molecules, which are characterized by their main function of assisting protein folding and quality control, ensuring that only properly folded and assembled proteins proceed further along the secretory pathway.
The function of calnexin is to retain unfolded or unassembled N-linked glycoproteins in the ER. Antibodies against calnexin can be used as markers for the ER in immmunofluorescence experiments.
Calnexin binds only those N-glycoproteins that have GlcNAc2Man9Glc1 oligosaccharides.
Oligosaccharides with three sequential glucose residues are added to asparagine residues of the nascent proteins in the ER.
The monoglucosylated oligosaccharides that are recognized by calnexin 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.
ATP and calcium ions are two of the cofactors involved in substrate binding for calnexin.
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.