A deubiquitinase negatively regulates retro-translocation of nonubiquitinated substrates.

Endoplasmic reticulum (ER) membrane-bound E3 ubiquitin ligases promote ER-associated degradation (ERAD) by ubiquitinating a retro-translocated substrate that reaches the cytosol from the ER, targeting it to the proteasome for destruction. Recent findings implicate ERAD-associated deubiquitinases (DUBs) as positive and negative regulators during ERAD, reflecting the different consequences of deubiquitinating a substrate prior to proteasomal degradation. These observations raise the question of whether a DUB can control the fate of a nonubiquitinated ERAD substrate.

Endoplasmic reticulum-dependent redox reactions control endoplasmic reticulum-associated degradation and pathogen entry.

Significance: Protein misfolding within the endoplasmic reticulum (ER) is managed by an ER quality control system that retro-translocates aberrant proteins into the cytosol for proteasomal destruction. This process, known as ER-associated degradation, utilizes the action of ER redox enzymes to accommodate the disulfide-bonded nature of misfolded proteins. Strikingly, various pathogenic viruses and toxins co-opt these redox components to reach the cytosol during entry.

The Ero1alpha-PDI redox cycle regulates retro-translocation of cholera toxin.

Cholera toxin (CT) is transported from the plasma membrane of host cells to the endoplasmic reticulum (ER) where the catalytic CTA1 subunit retro-translocates to the cytosol to induce toxicity. Our previous analyses demonstrated that the ER oxidoreductase protein disulfide isomerase (PDI) acts as a redox-dependent chaperone to unfold CTA1, a reaction postulated to initiate toxin retro-translocation. In its reduced state, PDI binds and unfolds CTA1; subsequent oxidation of PDI by Ero1alpha enables toxin release.

The E3 ubiquitin ligases Hrd1 and gp78 bind to and promote cholera toxin retro-translocation.

To cause disease, cholera toxin (CT) is transported from the cell surface to the endoplasmic reticulum (ER) lumen where the catalytic CTA1 subunit retro-translocates to the cytosol to induce pathological water secretion. Two retro-translocon components are the Derlins and ER-associated multi-spanning E3 ubiquitin ligases including Hrd1 and gp78. We demonstrated previously that Derlin-1 facilitates CTA1 retro-translocation.

Derlin-1 facilitates the retro-translocation of cholera toxin.

Cholera toxin (CT) intoxicates cells by using its receptor-binding B subunit (CTB) to traffic from the plasma membrane to the endoplasmic reticulum (ER). In this compartment, the catalytic A1 subunit (CTA1) is unfolded by protein disulfide isomerase (PDI) and retro-translocated to the cytosol where it triggers a signaling cascade, leading to secretory diarrhea. How CT is targeted to the site of retro-translocation in the ER membrane to initiate translocation is unclear.