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-linked Glc(3)Man(9)GlcNAc(2) oligosaccharides and the transfer of these oligosaccharides to nascent polypeptides. These processes take place at the membrane of the endoplasmic reticulum (ER) and are conserved among eukaryotes. Once transferred to the protein the N-linked oligosaccharides are immediately trimmed by glycosidases located in the ER. This review focuses on the N-linked glycosylation pathway in the ER of Saccharomyces cerevisiae and Schizosaccharomyces 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 glycoproteins with respect to glycoprotein quality control and degradation, focusing on the two different quality control mechanisms of S. cerevisiae and S. pombe.