<scp>ER</scp> ‐localized translational control as a nexus for proteostasis

Maintenance of proteostasis is essential for cellular and organismal homeostasis, and disruption of protein quality control (QC) networks underlies numerous human diseases. The endoplasmic reticulum (ER) functions as a central organelle for the synthesis, folding, maturation, and trafficking of secretory and membrane proteins, and serves as a central hub of intracellular proteostasis. Recent studies have established that the ER membrane serves not only as a site of protein translocation but also as a dynamic platform integrating translational regulation, RNA surveillance, and multiple QC pathways. During ER-associated translation, cells continuously monitor ribosome dynamics, mRNA integrity, nascent-chain folding, and transmembrane protein insertion processes to prevent the accumulation of aberrant proteins. These surveillance systems include the PKR-like ER kinase (PERK)-mediated integrated stress response (ISR), regulated IRE1-dependent decay (RIDD), nonsense-mediated mRNA decay (NMD), RNA silencing, ribosome-associated QC (RQC), ubiquitin-fold modifier 1 conjugation (UFMylation), ER-phagy, and ER stress-induced pre-emptive QC (ERpQC). Although these pathways were originally characterized independently, increasing evidence indicates that they function cooperatively on or near the ER membrane to coordinate translational attenuation, mRNA degradation, ribosome recycling, nascent-chain elimination, and organelle remodeling. In particular, UFMylation has emerged as a central mechanism linking ER-associated RQC, translocation-associated QC (TAQC), and ER-phagy. Dysfunction of these ER-localized translational QC pathways contributes to neurodegeneration, inflammation, fibrosis, cancer, and aging-related disorders. In this review, we summarize recent advances in ER-localized translational control and discuss how integrated QC networks on the ER membrane maintain proteostasis and influence disease pathogenesis.