FIGURE 13-22 Modifications of N-linked oligosaccharides are used to monitor folding and for quality control. After three glucose residues are removed from N-linked oligosaccharides in the ER, a single glucose can be re-added by a glucosyltransferase to form Glc₁Man₉(GlcNAc)₂ (see Figure 13-18, step 3a). This modified N-linked oligosaccharide binds the lectins calnexin (CNX) and calreticulin (CRT) for retention in the ER and engagement of folding chaperones. Proteins that cannot fold properly, and are therefore retained in the ER for longer times, undergo mannose trimming by ER α-mannosidases to form Man_5–6(GlcNAc)₂, which is recognized by OS-9. Recognition by OS-9 leads to dislocation of the misfolded protein out of the ER, ubiquitinylation, and degradation by proteasomes.