Human, Mouse, and Rat Calnexin cDNA Cloning: Identification of Potential Calcium Binding Motifs and Gene Localization to Human Chromosome 5

Calnexin is a 90-kDa integral membrane protein of the endoplasmic reticulum (ER). Calnexin binds Ca²⁺ and may function as a chaperone in the transition of proteins from the ER to the outer cellular membrane. We have purified human calnexin in association with the human interferon-γ receptor and cloned calnexin cDNA from placenta. Fragments of calnexin have been prepared as glutathione S-transferase fusion proteins and analyzed for their abilities to bind ⁴⁵Ca²⁺ and ruthenium red. A subdomain containing four internal repeats binds Ca²⁺ with the highest affinity. This sequence is highly conserved when compared to calreticulin (a luminal ER protein), an Onchocerca surface antigen, and yeast and plant calnexin homologues. Consequently, this sequence represents a conserved motif for the high-affnity binding of Ca²⁺, which is clearly distinct from the “E-F hand” motif. An adjacent subdomain, also highly conserved and containing four internal repeats, fails to bind Ca²⁺. The carboxyl-terminal, cytosolic domain is highly charged and binds Ca²⁺ with moderate affinity, presumably by electrostatic interactions. The calnexin amino-terminal domain (residues 1–253) also binds Ca²⁺, in contrast to the amino-terminal domain of calreticulin, which is relatively less acidic. We have also determined the cDNA sequences of mouse and rat calnexins. Comparison of the known mammalian calnexin sequences reveals very high conservation of sequence identity (93–98%), suggesting that calnexin performs important cellular functions. The gene for human calnexin is located on the distal end of the long arm of human chromosome 5, at 5q35.