Calnexin

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Lumenal domain of calnexin from PDB 1JHN.
calnexin
Identifiers
Symbol	CANX
HUGO	1473
Entrez	821
OMIM	114217
RefSeq	NM_001024649
UniProt	P27824
Other data
Locus	Chr. 5 q35

Calnexin (CNX) is a 90kDa integral protein 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 belongs among chaperones, 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 endoplasmic reticulum.

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 ability to bind to calnexin.

The glycoprotein chain which for some reason has difficulty folding up properly 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.

ATP and Ca++ are two of the cofactors involved in substrate binding for calnexin.