Specificity and Regulation of the Endoplasmic Reticulum‐Associated Degradation Machinery

Abstract

The endoplasmic reticulum-associated degradation (ERAD) machinery selects native and misfolded polypeptides for dislocation across the ER membrane and proteasomal degradation. Regulated degradation of native proteins is an important aspect of cell physiology. For example, it contributes to the control of lipid biosynthesis, calcium homeostasis and ERAD capacity by setting the turnover rate of crucial regulators of these pathways. In contrast, degradation of native proteins has pathologic relevance when caused by viral or bacterial infections, or when it occurs as a consequence of dysregulated ERAD activity. The efficient disposal of misfolded proteins prevents toxic depositions and persistent sequestration of molecular chaperones that could induce cellular stress and perturb maintenance of cellular proteostasis. In the first section of this review, we survey the available literature on mechanisms of selection of native and non-native proteins for degradation from the ER and on how pathogens hijack them. In the second section, we highlight the mechanisms of ERAD activity adaptation to changes in the ER environment with a particular emphasis on the post-translational regulatory mechanisms collectively defined as ERAD tuning. The cellular proteome is mostly synthesized by cytosolic ribosomes to operate in the cytosol and, upon appropriate targeting, in various intracellular organelles or in the extracellular space. Cellular compartments where protein folding occurs [e.g. the cytosol, mitochondria, the endoplasmic reticulum (ER)] contain two classes of non-native polypeptides: (i) newly synthesized polypeptide chains that must be assisted by folding chaperones and enzymes to attain the native mono-or oligomeric structure; (ii) terminally misfolded conformers that must efficiently be degraded to prevent the formation of toxic deposits and the persistent sequestration of chaperones that could eventually inhibit the cellular protein folding capacity and elicit stress (1-3) ( Figure 1A). The distinction between the two classes of non-native chains, one to be preserved, the second to be cleared from the folding compartment, is not an easy task for the cellular quality control machineries. Selection for disposal might be a stochastic process: the longer the persistency of structural defects in the ER, the greater is the probability to be selected for destruction. Therefore, mutations that delay folding may channel the polypeptide into destructive pathways, even if they do not compromise the function of the mutated protein.In the ER, non-native, but also native proteins might be selected for degradation by components of the endoplasmic reticulum-associated degradation (ERAD) machinery that deliver them at dislocation sites embedded in the ER membrane. Dislocation sites consist of a multitude of luminal and membrane-bound specialized ER-resident proteins as well as a number of cytosolic factors insuring dislocation across the ER membrane, poly-ubiquitylation and disposal of ERAD substrates by 26S proteasomes (Specificity of...

Promotion of Endoplasmic Reticulum-Associated Degradation of Procathepsin D by Human Herpesvirus 8-Encoded Viral Interleukin-6

“…Involvement in vIL-6-induced destabilization of pCatD of a selection of ERAD-associated proteins was tested by using shRNA-mediated depletion in appropriately transfected HEK293T cells. shRNA sequences targeting SEL1L, HRD1, and Derlin-1 (translocon proteins); OS9 and XTP3-B (lectins); BiP (chaperone); and ERdj5 (cochaperone) (19,21) were previously validated in HEK293T cells (19); we confirmed their activities in these cells (Fig. 3B).…”

“…vIL-6 coexpression increased mono-and polyubiquitination of pCatD-CBD (Fig. 2C), providing additional evidence of vIL-6-induced destabilization of pCatD via ER-associated proteasomal degradation (ERAD) (21). show ␤-actin-normalized pCatD levels after cycloheximide treatment relative to time zero levels (set at 1).…”

“…Current knowledge of ERAD pathways and components indicates that there is a set of lectins, chaperones, denaturing protein disulfide isomerases and associated reductases, and translocon complex proteins that are involved (Fig. 3A); there is some redundancy in function but also a degree of substrate specificity (19,21). Luminal ("soluble") substrates are typically processed via Derlin-associated SEL1L and HRD1 E3 ligase complexes to mediate ER-to-cytosol translocation and ubiquitination of substrates for proteasomal degradation.…”

“…Luminal ("soluble") substrates are typically processed via Derlin-associated SEL1L and HRD1 E3 ligase complexes to mediate ER-to-cytosol translocation and ubiquitination of substrates for proteasomal degradation. Membrane-associated ER proteins appear to be processed preferentially through alternative pathways involving other translocon complexes and associated E3 ligases, such as gp78 (19,21,22). Involvement in vIL-6-induced destabilization of pCatD of a selection of ERAD-associated proteins was tested by using shRNA-mediated depletion in appropriately transfected HEK293T cells.…”

PrP-containing aggresomes are cytosolic components of an ER quality control mechanism

“…In the ER, the polypeptide is processed by a highly ordered set of components that remove its signal peptide and promote the correct folding of the molecule (Ruggiano et al, 2014). Despite the activity of these ERresident mechanisms, a substantial fraction of the nascent polypeptides fail to attain their desired spatial conformation (Schubert et al, 2000), retro-translocate to the cytosol and are designated for degradation by the ER-associated degradation (ERAD) machinery (Merulla et al, 2013) or by autophagy (Suntharalingam et al, 2012). Occasionally, subsets of aggregation-prone proteins evade quality control surveillance and form potentially hazardous, insoluble aggregates.…”