The significance of carbohydrate trimming for the antigenicity of the Semliki Forest virus glycoprotein E2.

Six groups, designated a-f, of noncompeting murine monoclonal antibodies to the envelope glycoprotein E2 of Semliki Forest virus (SFV) have been used to analyze antigenic changes caused by differences in the carbohydrate chain composition of the envelope glycoprotein E2 in the virion. Deletion of terminal sialic acids as observed in virus progeny from mosquito cells did not affect antigenic properties. Inhibition of the trimming pathway in infected chicken cells by the mannosidase I inhibitor dMM led to infectious virus particles containing mannose-rich oligosaccharides of the composition Man9(GlcNAc)2 in the envelope glycoproteins. This alteration had no effect on antigenicity. If inhibition was, however, performed with MdN which acts on alpha-glucosidase giving rise to virions with glycoproteins containing three additional glucose residues in the carbohydrate chains [Glc3Man7,8,9(GlcNAc)2], significant antigenic changes were observed. The six epitopes were differently affected by the underlying structural change and the pattern of exposition of epitopes was not identical with that observed after cleavage of intramolecular disulfide bonds. Concomitantly, the cleavage rate of gp62, the intracellular precursor molecule of the glycoproteins E2 and E3 of the virus particle, was reduced causing a reduction of virus yield. It is concluded that the existence of untrimmed carbohydrate chains is sufficient to allow SFV maturation. The trimming reactions improve this process in a matter suggesting that the carbohydrate chains influence intracellular traffic (addressing) of the respective glycoprotein.