Synthesis and processing of Semliki forest virus polyprotein in Saccharomyces cerevisiae: a yeast type glycosylation of El envelope protein

A cDNA coding for the structural proteins of Semliki Forest virus (SFV) was ligated between the ADC1 promoter and terminator in a yeast expression vector, pAAH5. Synthesis of the SFV-specific proteins in Saccharomyces cerevisiae transformed with this vector was shown by immunoblotting and immunoprecipitation. Detection of the N-terminal and the C-terminal components of the viral polyprotein, capsid protein and E1 envelope protein, respectively, suggested that the entire polyprotein was translated in yeast. The capsid protein was effectively released from the polyprotein as a normal size polypeptide, but the following protein, p62 (E3, E2 precursor) was not detected, suggesting that it was rapidly degraded. Electrophoretic analyses indicated that the final protein, El, entered the secretory pathway, the signal sequence was cleaved off and the protein became extensively and heterogeneously glycosylated. These data suggest that El was transported to the Golgi complex and that yeast-characteristic outer-chain glycans were added to the protein.