Abstract Rot1 is an essential yeast protein originally shown to be implicated in such diverse processes such as β-1,6-glucan synthesis, actin cytoskeleton dynamics, or lysis of autophagic bodies. More recently also a role as a molecular chaperone has been discovered. Here we report that Rot1 interacts in a synthetic manner with Ost3, one of the nine subunits of the oligosaccharyltransferase complex, the key enzyme of N-glycosylation. Deletion of OST3 in the rot1-1 mutant causes a temperature sensitive phenotype as well as sensitivity towards compounds interfering with cell wall biogenesis such as Calcofluor White, caffeine, Congo Red and hygromycin B, whereas deletion of OST6, a functional homolog of OST3, has no effect. Oligosaccharyltransferase activity in vitro determined in membranes from rot1-1ost3∆ cells was found to be decreased to 45% compared to wild-type membranes, and model glycoproteins of N-glycosylation, like carboxypeptidase CPY, Gas1 or DPAP B, displayed an underglycosylation pattern. By affinity chromatography a physical interaction between Rot1 and Ost3 was demonstrated. Moreover, Rot1 was found to be involved also in the Omannosylation process, as glycosylation of distinct glycoproteins of this type were affected as well. Altogether the data extend the role of Rot1 as a chaperone required to ensure proper glycosylation
