Tunicamycin resistant glycosylation of coronavirus glycoprotein: demonstration of a novel type of viral glycoprotein

Abstract

Tunicamycin has different effects on the glycosylation of the two envelope glycoproteins of mouse hepatitis virus (MHV), a coronavirus. Unlike envelope glycoproteins of other viruses, the transmembrane glycoprotein El is glycosylated normally in the presence of tunicamycin. This suggests that glycosylation of El does not involve transfer of core oligosaccharides from dolichol pyrophosphate intermediates to asparagine residues, but may occur by 0-linked glycosylation of serine or threonine residues. Synthesis of the peplomeric glycoprotein E2 is not readily detectable in the presence of tunicamycin. Inhibition of N-linked glycosylation of E2 by tunicamycin either prevents synthesis or facilitates degradation of the protein moiety of E2. Radiolabeling with carbohydrate precursors and borate gel electrophoresis of glycopeptides show that different oligcsaccharide side chains are attached to El and E2. The two coronavirus envelope glycoproteins thus appear to be glycosylated by different mechanisms. In tunicamycin-treated cells, noninfectious virions lacking peplomers are formed at intracytoplasmic membranes and released from the cells. These virions contain normal amounts of nucleocapsid protein and glycosylated El, but lack E2. Thus the transmembrane glycoprotein El is the only viral glycoprotein required for the formation of the viral envelope or for virus maturation and release. The peplomeric glycoprotein E2 appears to be required for attachment to virus receptors on the plasma membrane. The coronavirus envelope envelope glycoprotein E1 appears to be a novel type of viral glycoprotein which is post-translationally glycosylated by a tunicamycin-resistant process that yields oligosaccharide side chains different from those of N-linked glycoproteins. These findings suggest that El may be particularly useful as a model for studying the biosynthesis, glycosylation, and intracellular transport of 0-linked glycoproteins.