CHAPERONE FUNCTION OF CALRETICULIN WHEN EXPRESSED IN THE ENDOPLASMIC-RETICULUM AS THE MEMBRANE-ANCHORED AND SOLUBLE FORMS

A unique type of chaperone that requires glucose trimming of the targe t proteins has been shown to be important for their maturation in the endoplasmic reticulum (ER). Calnexin, an ER membrane chaperone, is the first example of such a class. Here, we focus on calreticulin, a majo r ER luminal protein, which shares with calnexin two sets of character istic sequence repeat. We evaluated the chaperone function of calretic ulin by expressing it on the ER luminal membrane surface. We construct ed a membrane-anchored calreticulin chimera by fusing truncated calret iculin to the membrane-anchoring tagged segment of calnexin. When expr essed in HepG2 cells, the calreticulin chimera transiently interacted with a set of nascent secretory proteins in a castanospermine-sensitiv e manner. The spectrum of proteins recognized by the membrane-anchored calreticulin was remarkably similar to that observed with calnexin. N ext, we tested if such a chaperone function of calreticulin is express ed at its physiological location. Luminally expressed calreticulin pre ferentially bound to nascent transferrin and released it upon chase. A ssociation with other calnexin ligands was observed, however, at low e fficiencies. Interactions were abrogated by castanospermine treatment. We conclude that calreticulin per se is another chaperone with appare ntly the same characteristics as calnexin and selectively interacts wi th nascent transferrin in the lumen, suggesting that calreticulin may cover the diversity of maturations.