Coronavirus glycoprotein E1, a new type of viral glycoprotein

The carbohydrate contents of coronavirus glycoproteins E1 and E2 have been analyzed. E2 has complex and mannose-rich-type oligosaccharide side-chains, which are attached by N-glycosidic linkages to the polypeptide. Glycosylation of E2 is initiated at the co-translational level, and it is inhibited by tunicamycin, 2-deoxy-glucose, and 2-deoxy-2-fluoro-glucose. Thus, E2 belongs to a glycoprotein type found in many other enveloped viruses. E1, in contrast, represents a different class of glycoprotein. The following observations indicate that its carbohydrate side-chains have 0-glycosidic linkage. (1) The constituent sugars of E1 are N-acetylglucosamine, N-acetylgalactosamine, galactose, and neuraminic acid; mannose and fucose are absent. (2) The side-chains can be removed by β-elimination. (3) Glycosylation of E1 is not sensitive to the compounds interfering with N-glycosylation. E1 is the first viral glycoprotein analyzed that contains only 0-glycosidic linkages. Coronaviruses are therefore a suitable model system to study biosynthesis and processing of this type of glycoprotein.