Calnexin: Proteins and Enzymes

Calnexin, an abundant approx. 90kDa integral protein of the endoplasmic reticulum, is also referred to as IP90, p88 and p90 (1). It consists of a large 50kDa N-terminal calcium-binding luminal domain, a single transmembrane helix and a short acidic cytoplasmic tail (2, 3). Unlike its ER counterparts which have a KDEL sequence on their Cterminus to ensure ER retention (4), calnexin has positively charged cytosolic residues that do the same thing (3). Most ER proteins act as molecular chaperones and participate in the proper folding of polypeptides and their assembly into mulit-subunit proteins. Calnexin together with calreticulin, plays a key role in glycoprotein folding and its control within the ER, by interacting with folding intermediates via their monoglycosylated glycans (5, 6). Calnexin has also been shown to associate with the major histocompatability complex class I heavy chains, partial complexes of the T cell receptor and B cell membrane immunoglobulin (7).