Calnexin is a 90 kDa integral membrane protein located primarily in the endoplasmic reticulum (ER). The structure of calnexin includes a long N-terminal calcium-binding domain that extends into the lumen of the ER and a short, acidic cytosolic domain. Calnexin associates with several cell surface proteins as they pass through the ER, and may be involved in the Ca2+-dependent retention of proteins in the ER. The amino acid sequence of calnexin is highly conserved among various species and is similar in sequence to calreticulin, another Ca2+-binding protein found in the ER. Phosphorylation may regulate the activity of the C-terminal region of Calnexin. Both proline-dependent kinase and casein kinase sites have been identified, and the phosphorylation of these sites may regulate calnexin functions involved with detection of ER protein quality control and transport.
