C. Hammond et al., ROLE OF N-LINKED OLIGOSACCHARIDE RECOGNITION, GLUCOSE TRIMMING, AND CALNEXIN IN GLYCOPROTEIN FOLDING AND QUALITY-CONTROL, Proceedings of the National Academy of Sciences of the United Statesof America, 91(3), 1994, pp. 913-917
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.