High-level expression of the Toxoplasma gondii STT3 gene is required for suppression of the yeast STT3 gene mutation.

N-linked glycosylation is the most frequent modification of secretory proteins. The central reaction of this process in eukaryotic cells is catalyzed by the hetero-oligomeric protein complex oligosaccharyltransferase (OST). The gene STT3 gene encodes a protein, which is the most conserved among the components of the OST. In this report, we describe the isolation and functional characterization of a STT3 homologue from Toxoplasma gondii. The topology of the TgStt3p is similar to that of the yeast Stt3p with 47% identity. We demonstrate that high level expression of the homologues gene is required to completely suppress the defect caused by a stt3 mutation in yeast, suggesting that homologous Stt3 proteins can serve analogous functions in distantly related eukaryotic cells regardless of their degree of conservation.