Ubiquitin and ubiquitin-like modifications in the endoplasmic reticulum stress response

The endoplasmic reticulum (ER) is a cellular organelle frequently subjected to stress under both physiological and pathological circumstances, associated with the accumulation of mis/unfolded proteins in its lumen. To cope with this stress, cells have evolved an adaptive program called the unfolded protein response (UPR), whose primary function is to restore ER proteostasis. When the stress is prolonged, the UPR can also trigger cell death. The UPR controls multiple machineries involved in pre-emptive quality control (QC) of proteins prior to ER entry, ribosome-associated QC, protein folding within the ER, protein degradation through various processes, and export from the ER for secretion. Because the UPR and the machineries it controls play fundamental roles in determining cell fate, they are finely regulated, including through post-translational modifications (PTMs). In this review, we focus on the role of the ubiquitin and ubiquitin-like PTMs in the regulation and mediation of ER proteostasis. We specifically focus on three core processes: the UPR, ER-associated ribosome QC and ER-associated degradation. Lastly, we briefly discuss how Ub and Ubl also control the integrated stress response and the formation of inter-organelle membrane contact sites and thus act as general regulators of responses to cellular stresses beyond ER proteotoxicity.