The Mammalian Endoplasmic Reticulum-Associated Degradation System

The endoplasmic reticulum (ER) is the site of synthesis for nearly one-third of the eukaryotic proteome and is accordingly endowed with specialized machinery to ensure that proteins deployed to the distal secretory pathway are correctly folded and assembled into native oligomeric complexes. Proteins failing to meet this conformational standard are degraded by ER-associated degradation (ERAD), a complex process through which folding-defective proteins are selected and ultimately degraded by the ubiquitin-proteasome system. ERAD proceeds through four tightly coupled steps involving substrate selection, dislocation across the ER membrane, covalent conjugation with polyubiquitin, and proteasomal degradation. The ERAD machinery shows a modular organization with central ER membrane-embedded ubiquitin ligases linking components responsible for recognition in the ER lumen to the ubiquitin-proteasome system in the cytoplasm. The core ERAD machinery is highly conserved among eukaryotes and much of our basic understanding of ERAD organization has been derived from genetic and biochemical studies of yeast. In this article we discuss how the core ERAD machinery is organized in mammalian cells.The endoplasmic reticulum (ER) is the entry portal to the secretory pathway and is comprised of a specialized oxidative environment in which nascent polypeptides fold and assemble into native structures with the aid of a unique set of molecular chaperones, folding catalysts, and posttranslational modifications (Helenius and Aebi 2004). An estimated one-third of the mammalian genome encodes proteins destined for the secretory pathway. The ER folding apparatus must therefore be able to accommodate substrates that are highly diverse in terms of structure, oligomeric state, and folding rate. This diversity requires stringent quality control systems to maintain biosynthetic fidelity and to prevent the accumulation or deployment of misfolded proteins that can cause proteotoxicity. The importance of these systems is evidenced by the large number of human diseases that are linked to protein misfolding in the secretory pathway (Guerriero and Brodsky 2012).ER-associated degradation (ERAD) is the temporally and spatially coordinated surveillance process charged with clearance of aberrant proteins in the ER. Much of what is known about this system has come from studies that have exploited genetic analysis in yeast (reviewed in Vembar and Brodsky 2008; Xie and Ng 2010). The essential features of ERAD are highly conserved among eukaryotes; however, because of the much larger proteome and multicellular lifestyle, the ERAD system in metazoans is considerably more complex than in fungi. In this article, we review the organization and function of the ERAD pathway in mammalian cells.In ERAD, proteins that have been biosynthetically integrated into the ER membrane or translocated into the lumen are ultimately degraded by the ubiquitin-proteasome system (UPS). This imposes a fundamental topological constraint, in that the substrates are not initially present in the same compartment as the proteolytic system that degrades them. Thus the ERAD system necessarily spans the ER bilayer, and degradation must be mechanistically coupled to transfer (dislocation) of substrates to the cytoplasm. ERAD can be envisioned as encompassing four distinct, coupled steps (Fig. 1): (1) substrate recognition; (2) dislocation across the lipid bilayer; (3) addition (and subsequent removal) of polyubiquitin adducts; and (4) degradation by the 26S proteasome.Open in a separate windowFigure 1.Key steps in ERAD. ERAD occurs through a series of temporally ordered steps, which include: Step 1—Recognition: Molecular chaperones and lectins within the ER lumen interact with incompletely folded or unassembled clients. These factors link substrate recognition to the dislocation machinery by binding to membrane-embedded adaptors. Step 2—Dislocation: Substrates are dislocated across the bilayer presumably through proteinaceous pores (dislocons), via a process coupled to the energy derived from ATP hydrolysis by VCP/p97. Step 3—Ubiquitination: On gaining access to the cytosol, substrates are polyubiquitinated by E3 ligases. Step 4—Degradation: Ubiquitinated substrates are degraded by cytosolic 26S proteasomes.