Quality control of glycoprotein folding and ERAD: the role of N-glycan handling, EDEM1 and OS-9

Protein N-glycosylation and quality control of protein folding as well as the connected ER-associated degradation of misfolded glycoproteins (ERAD) are not only evolutionary highly conserved but also functionally linked. It is now established that particular N-glycan structures which result from processing reactions by exo-glycosidases in the ER are of importance for glycoprotein folding and for ERAD. Thus, mono-glucosylated N-glycan intermediates harbor structural information which is important for promoting glycoprotein folding. On the other hand, specific mannose-trimmed N-glycans harbor structural information for routing misfolded glycoproteins to ERAD. In this review, we summarize current knowledge concerning the role played by glucosidases I and II, in concert with the bifunctional glucosyltransferase and calnexin/calreticulin in glycoprotein folding, the role of conventional ER mannosidase I in concert with the mannosidase EDEM1 in handling and routing of misfolded glycoproteins, and how the bifunctional OS-9 provides a link to the ER dislocon for degradation.