The lectin chaperone calnexin utilizes polypeptide-based interactions to associate with many of its substrates in vivo

Calnexin and calreticulin are molecular chaperones of the endoplasmic retic ulum that bind to newly synthesized glycoproteins in part through a lectin site specific for monoglucosylated (Glc(1)Man(7-9)GlcNAc(2)) oligosaccharid es. In addition to this lectin-oligosaccharide interaction, in vitro studie s have demonstrated that calnexin and calreticulin can bind to polypeptide segments of both glycosylated and nonglycosylated proteins. However, the in vivo relevance of this latter interaction has been questioned. We examined whether polypeptide-based interactions occur between calnexin and its subs trates in vivo using the glucosidase inhibitor castanospermine or glucosida se-deficient cells to prevent the formation of monoglucosylated oligosaccha rides. We show that if care is taken to preserve weak interactions, the blo ck in lectin-oligosaccharide binding leads to the loss of some calnexin-sub strate complexes, but many others remain readily detectable. Furthermore, w e demonstrate that calnexin is capable of associating in vivo with a substr ate that completely lacks Asn-linked oligosaccharides. The binding of calne xin to proteins that lack monoglucosylated oligosaccharides could not be at tributed to nonspecific adsorption nor to its inclusion in protein aggregat es. We conclude that both lectin-oligosaccharide and polypeptide-based inte ractions occur between calnexin and diverse proteins in vivo and that the s trength of the latter interaction varies substantially between protein subs trates.