N-linked sugar-regulated protein folding and quality control in the ER

Abstract

Asparagine-linked glycans (N-glycans) are displayed on the majority of proteins synthesized in the endoplasmic reticulum (ER). Removal of the outermost glucose residue recruits the lectin chaperone malectin possibly involved in a first triage of defective polypeptides. Removal of a second glucose promotes engagement of folding and quality control machineries built around the ER lectin chaperones calnexin (CNX) and calreticulin (CRT) and including oxidoreductases and peptidyl-prolyl isomerases. Deprivation of the last glucose residue dictates the release of N-glycosylated polypeptides from the lectin chaperones. Correctly folded proteins are authorized to leave the ER. Non-native polypeptides are recognized by the ER quality control key player UDP-glucose glycoprotein glucosyltransferase 1 (UGT1), re-glucosylated and re-addressed to the CNX/CRT chaperone binding cycle to provide additional opportunity for the protein to fold in the ER. Failure to attain the native structure determines the selection of the misfolded polypeptides for proteasome-mediated degradation.

Keywords: Calnexin; Calreticulin; Endoplasmic reticulum; N-glycosylation; Protein folding and quality control; UDP-glucose glycoprotein glucosyltransferase 1.

Figure 1. N-glycan composition and processing

The pre-formed oligosaccharide is composed of three glucoses (dark grey circles), nine mannoses (light grey circles) and two N-acetylglucosamines (black squares). The oligosaccharide branches are shown with A, B and C. The oligosaccharide processing enzymes are listed in the figure and their action is shown. The type of glycosidic bond is shown in color.

Figure 2. Scheme of the CNX/CRT cycle

Upon addition of the 14-subunits oligosaccharide catalyzed by the OST complex (step 1), the first glucose is removed by α-glucosidase I (step 2). Di-glucosylated polypeptides associate with malectin. α-glucosidase II cleaves the second glucose from the glycan (step 3), generating mono-glucosylated polypeptides, which interact with CNX and CRT. Binding and release from lectin chaperones could occur (step 4). The release from the ER lectins is determined by the second cleavage by α-glucosidase II, which removes the last glucose residue (step 5). Re-glucosylation by UGT1 dictates the re-association of the polypeptides with CNX or CRT (step 6). Correctly folded polypeptides are exported from the ER (step 7). Terminally misfolded proteins are further processed by mannosidases (step 8) and eventually retrotranslocated for proteasomal degradation (step 9). The substrate-binding and catalytic domains of UGT1 are shown with s and c, respectively.

Figure 3. Disulfide and peptidyl-prolyl bonds

Mechanisms of disulfide bond oxidation, reduction and isomerization catalyzed by PDI proteins (upper panel). Mechanism of peptidyl-prolyl cis-trans isomerization catalyzed by PPI proteins (lower panel).