ERAD and ERAD tuning: disposal of cargo and of ERAD regulators from the mammalian ER

Abstract

The endoplasmic reticulum (ER) is the site of maturation for secretory and membrane proteins in eukaryotic cells. Unsuccessful folding attempts are eventually interrupted and most folding-defective polypeptides are dislocated across the ER membrane and degraded by cytosolic proteasomes in a complex series of events collectively defined as ER-associated degradation (ERAD). Uncontrolled ERAD activity might prematurely interrupt ongoing folding programs. At steady state, this is prevented by ERAD tuning, that is, the removal of select ERAD regulators from the ER and their degradation by proteasomes and by endo-lysosomal proteases. In Coronaviruses infected cells, the formation of LC3-I coated vesicles containing ERAD regulators cleared from the ER lumen is co-opted to anchor viral replication and transcription complexes to ER-derived membranes.

Figure 1

The N-linked oligosaccharide structure. The core oligosaccharide is added onto side chains of asparagine residues in a specific sequon (N = asparagine, X = any amino acid but proline, S/T = serine or threonine). It is composed of three glucose (triangles), nine mannose (circles) and two N-acetylglucosamine (squares) residues. Removable α1,2-linked mannose residues are shown in dark green. Letters a-n are assigned to each saccharide and A–C define the oligosaccharide branch. The linkage between individual saccharides is shown.

Figure 2

Folding and ERAD pathways in the mammalian ER lumen. Newly synthesized glycopolypeptides associate with the lectin chaperones calnexin (CNX) and calreticulin (CRT). The oxidoreductase ERp57 catalyzes formation of native disulfide bonds (step 1). Upon release from CNX and CRT, the glucose l and the mannose i are removed by the exoglucosidase II (GII) and the ERManI, respectively. Native glycopolypeptides, in some cases under the assistance of specialized cargo lectins, are secreted in coat protein complex II (COPII)-coated vesicles and are transported at their final destination (step 2). Non-native glycopolypeptides are retained in the CNX chaperone system by the UGT1 that adds-back one glucose residue on the mannose residue g (step 3). Extensive de-mannosylation irreversibly extracts terminally misfolded polypeptides from the CNX cycle (step 4). Pathways directing ERAD substrates to dislocation sites at the ER membrane obligatorily rely on OS-9/XTP3-B, CyPB (for substrates containing peptidyl-prolyl bonds in the cis configuration), SEL1L and HRD1 only for ERAD-L_S proteins (step 5L_S). ERAD-L_M substrates may engage multiple pathways (steps 5L_M). Dislocation across the ER membrane occurs through an elusive proteinaceous channel (?). At the cytosolic face of the ER membrane ERAD substrates are poly-ubiquitylated, de-glycosylated and degraded by 26S-proteasomes (step 6).

Figure 3

ERAD tuning. Many ERAD regulators are short-living proteins at steady state. Some of them are degraded with the intervention of cytosolic proteasomes (e.g. SEL1L and HERP). The selective removal of EDEM1 and OS-9 from the ER can be subdivided in three steps. (1) Association with an elusive receptor allows segregation of EDEM1, OS-9 and possibly other ERAD factors (EF) from conventional, long living ER-resident chaperones (in grey); (2) the ERAD regulators exit the ER in small, LC3-I-coated vesicles, the EDEMosomes; (3) EDEMosomes deliver their content to endosomal/lysosomal compartments for disposal.