The cryo-EM structure of the human ERAD retrotranslocation complex

Selenoprotein K at the intersection of cellular pathways

Selenoprotein K (selenok) is linked to the integrated stress response, which helps cells combat stressors and regain normal function. The selenoprotein contains numerous protein interaction hubs and post-translational modification sites and is involved in protein palmitoylation, vesicle trafficking, and the resolution of ER stress. Anchored to the endoplasmic reticulum (ER) membrane, selenok interacts with protein partners to influence their stability, localization, and trafficking, impacting various cellular functions such as calcium homeostasis, cellular migration, phagocytosis, gene expression, and immune response. Consequently, selenok expression level is linked to cancer and neurodegenerative diseases. Because it contains the reactive amino acid selenocysteine, selenok is likely to function as an enzyme. However, highly unusual for enzymes, the protein segment containing the selenocysteine lacks a stable secondary or tertiary structure, yet it includes multiple interaction sites for protein partners and post-translational modifications. Currently, the reason(s) for the presence of the rare selenocysteine in selenok is not known. Furthermore, of selenok's numerous interaction sites, only some have been sufficiently characterized, leaving many of selenok's potential protein partners to be discovered. In this review, we explore selenok's role in various cellular pathways and its impact on human health, thereby highlighting the links between its diverse cellular functions.

Elucidation of the late steps in the glycan-dependent ERAD of soluble misfolded glycoproteins

The endoplasmic reticulum (ER) utilizes ER-associated degradation (ERAD), a highly conserved eukaryotic pathway, to eliminate misfolded or unassembled proteins and maintain protein homeostasis in cells. The clearance of misfolded glycoproteins involves several distinct steps, including the recognition of a specific glycan signal, retrotranslocation to the cytosol, and subsequent degradation of the misfolded protein by the ubiquitin proteasome system. Confocal microscopy was used to track the fate of a well-characterized ERAD substrate via a self-complementing split fluorescent protein assay. The results demonstrate that a misfolded variant of the STRUBBELIG (SUB) extracellular protein domain (SUBEX-C57Y) is retrotranslocated to the cytosol when transiently expressed in Nicotiana benthamiana leaf epidermal cells. Retrotranslocation requires a protein domain with a lesion that is exposed in the lumen of the ER, N-glycan trimming by α-mannosidases, HRD1-mediated ubiquitination, and the ATPase function of CDC48. The retrotranslocated SUBEX-C57Y ERAD substrate undergoes deglycosylation, and proteasomal degradation is blocked by a catalytically inactive cytosolic peptide N-glycanase. These findings define distinct aspects of ERAD that have been elusive until now and may represent the default pathway for degrading misfolded glycoproteins in plants.

Biological mechanisms and clinical significance of endoplasmic reticulum oxidoreductase 1 alpha (ERO1α) in human cancer

A firm link between endoplasmic reticulum (ER) stress and tumors has been wildly reported. Endoplasmic reticulum oxidoreductase 1 alpha (ERO1α), an ER-resident thiol oxidoreductase, is confirmed to be highly upregulated in various cancer types and associated with a significantly worse prognosis. Of importance, under ER stress, the functional interplay of ERO1α/PDI axis plays a pivotal role to orchestrate proper protein folding and other key processes. Multiple lines of evidence propose ERO1α as an attractive potential target for cancer treatment. However, the unavailability of specific inhibitor for ERO1α, its molecular inter-relatedness with closely related paralog ERO1β and the tightly regulated processes with other members of flavoenzyme family of enzymes, raises several concerns about its clinical translation. Herein, we have provided a detailed description of ERO1α in human cancers and its vulnerability towards the aforementioned concerns. Besides, we have discussed a few key considerations that may improve our understanding about ERO1α in tumors.