The Yeast Oligosaccharyltransferase Complex can be replaced by STT3 from Leishmania major

The key step of protein N-glycosylation is catalyzed by the multimeric oligosaccharyltransferase complex (OST). Biochemical and genetic studies have revealed that OST from Saccharomyces cerevisiae consists of nine subunits: Wbp1, Swp1, Stt3, Ost1, Ost2, Ost3, Ost4, Ost5, and Ost6. With the exception of Stt3, assumed to contain the catalytic site, little is known about the function of other OST subunits. The existence of the OST complex is suggested to allow substrate specificity and efficient transfer, a close interaction with the translocon and the prevention of protein folding to ensure the efficient co-translational modification of proteins. However, in the recently completed genome of the trypanosomatid parasite Leishmania major STT3 (of which four paralogs exist, STT3-1, STT3-2, STT3-3, and STT3-4) is the only OST subunit that can be identified. Here we report that L.m.STT3 proteins, except STT3-3, are able to complement stt3 deficiency in yeast during vegetative growth, but only poorly during sporulation. By blue native electrophoresis we demonstrate that the L.mSTT3 is active mainly as a free, monomeric enzyme. In cell-free assays and also in vivo we find that L.mSTT3, expressed in yeast, has a broad specificity for nonglucosylated lipid-linked mannose-oligosaccharides, typical for several protists. But when incorporated into the OST complex, L.mSTT3 transfers also the common eukaryotic Glc3Man9GlcNAc2-PP-Dol donor. Finally, three L.m.STT3 paralogs were shown to complement not only stt3 but also ost1, ost2, wbp1, or swp1 mutants. Thus, STT3 from Leishmania can substitute for the whole OST complex.