Quality control in biosynthetic pathways of N-linked glycoproteins in the yeast endoplasmic reticulum

The initial steps in N-glycosylation involve the synthesis of dolichol-link ed Glc(3)Man(9)GlcNAc(2) oligosaccharides and the transfer of these oligosa ccharides to nascent polypeptides. These processes take place at the membra ne of the endoplasmic reticulum (ER) and are conserved among eukaryotes. On ce transferred to the protein the N-linked oligosaccharides are immediately trimmed by glycosidases located in the ER. This review focuses on the N-li nked glycosylation pathway in the ER of Saccharomyces cerevisiae and Schizo saccharomyces pombe. In particular, we outline how yeast cells ensure that only completely assembled lipid-linked oligosaccharides are transferred to nascent polypeptides. We will discuss the oligosaccharide trimming of glyco proteins with respect to glycoprotein quality control and degradation, focu sing on the two different quality control mechanisms of S. cerevisiae and S . pombe.