Coronavirus proteins: structure and function of the oligosaccharides of the avian infectious bronchitis virus glycoproteins

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Abstract
The recent finding that the E1 glycoproteins of murine coronaviruses contain only O-linked oligosaccharides suggested that this unusual modification might be a distinguishing feature of coronaviruses and might play an essential role in the life cycle of this family of viruses. To examine these possibilities, the oligosaccharide moieties of the membrane proteins of the avian coronavirus infectious bronchitis virus were analyzed. The effect of inhibiting the glycosylation of these proteins on viral maturation and infectivity was determined. Infectious bronchitis virus virions contain 9 proteins. Four of these proteins, GP36, GP31, GP28 and P23, are closely related structurally and appear to be homologous to the E1 proteins of murine coronaviruses. The oligosaccharides of GP31 and GP28 could be removed with endoglycosidase H and neither of these glycoproteins was detectable in tunicamycin-treated [chicken embryo kidney] cells. These two results indicated that GP31 and GP28 contain N-linked oligosaccharides. Thus, O-linked oligosaccharides are not a universal feature of the small coronavirus membrane glycoproteins. Tunicamycin inhibited glycosylation of all the viral glycoproteins but did not inhibit production of virions by infectious bronchitis virus-infected cells. The virions released by these cells contained only the 3 non-glycosylated viral proteins P51, P23 and P14. These particles were not infectious. Thus, it appears that glycosylated infectious bronchitis virus polypeptides are not required for particle formation. The viral glycoproteins are apparently indispensible for viral infectivity.