Hydrolytic activity of yeast oligosaccharyltransferase is enhanced when misfolded proteins accumulate in the endoplasmic reticulum

It is known that oligosaccharyltransferase (OST) has hydrolytic activity toward dolichol-linked oligosaccharides (DLO), which results in the formation of free N-glycans (FNGs), i.e. unconjugated oligosaccharides with structural features similar to N-glycans. The functional importance of this hydrolytic reaction, however, remains unknown. In this study, the hydrolytic activity of OST was characterized in yeast. It was shown that the hydrolytic activity of OST is enhanced in ubiquitin ligase mutants that are involved in endoplasmic reticulum-associated degradation. Interestingly, this enhanced hydrolysis activity is completely suppressed in asparagine-linked glycosylation (alg) mutants, bearing mutations related to the biosynthesis of DLO, indicating that the effect of ubiquitin ligase on OST-mediated hydrolysis is context-dependent. The enhanced hydrolysis activity in ubiquitin ligase mutants was also found to be canceled upon treatment of the cells with dithiothreitol, a reagent that potently induces protein unfolding in the endoplasmic reticulum (ER). Our results clearly suggest that the hydrolytic activity of OST is enhanced under conditions in which the formation of unfolded proteins is promoted in the ER in yeast. The possible role of FNGs on protein folding is discussed. Oligosaccharyltransferase (OST) has hydrolytic activity toward dolichol-linked oligosaccharides, which results in the formation of free N-glycans. The functional importance of this hydrolytic reaction, however, remains unknown. Our results suggest that the hydrolytic activity of OST is enhanced under conditions in which the formation of unfolded proteins is promoted in the endoplasmic reticulum in yeast. image

Automated summary

It is known that oligosaccharyltransferase (OST) has hydrolytic activity toward dolichol-linked oligosaccharides, but the functional importance of this activity remains unknown. In this study, we found that the hydrolytic activity of OST is enhanced in yeast under conditions related to the biosynthesis of dolichol-linked oligosaccharides and ubiquitin ligase, but is canceled under conditions promoting protein unfolding. These results suggest a possible role of free N-glycans in protein folding.