ROLE OF N-LINKED OLIGOSACCHARIDE RECOGNITION, GLUCOSE TRIMMING, AND CALNEXIN IN GLYCOPROTEIN FOLDING AND QUALITY-CONTROL

Using a pulse-chase approach combined with immunoprecipitation, we sho wed that newly synthesized influenza virus hemagglutinin (HA) and vesi cular stomatitis virus G protein associate transiently during their fo lding with calnexin, a membrane-bound endoplasmic reticulum (ER) chape rone. Inhibitors of N-linked glycosylation (tunicamycin) and glucosida ses I and II (castanospermine and 1-deoxynojirimycin) prevented the as sociation, whereas inhibitors of ER alpha-mannosidases did not. Our re sults indicated that binding of these viral glycoproteins to calnexin correlated closely with the composition of their N-linked oligosacchar ide side chains. Proteins with monoglucosylated oligosaccharides were the most likely binding species. On the basis of our data and existing information concerning the role of monoglucosylated oligosaccharides on glycoproteins, we propose that the ER contains a unique folding and quality control machinery in which calnexin acts as a chaperone that binds proteins with partially glucose-trimmed carbohydrate side chains . In this model glucosidases I and II serve as signal modifiers and UD P-glucose:glycoprotein glucosyltransferase, as a folding sensor.