Protein Synthesis and Degradation
The kinase PERK represses translation of the G-protein–coupled receptor LGR5 and receptor tyrosine kinase ERBB3 during ER stress in cancer cells
As a branch of the unfolded protein response, protein kinase R-like endoplasmic reticulum kinase (PERK) represses global translation in response to endoplasmic reticulum (ER) stress. This pathophysiological condition is associated with the tumor microenvironment in cancer. Previous findings in our lab have suggested that PERK selectively represses translation of some mRNAs, but this possibility awaits additional investigation. In this study, we show that a stem-cell marker protein, leucine-rich repeat-containing G-protein–coupled receptor 5 (LGR5), is rapidly depleted in colon cancer cells during ER stress, an effect that depended on the PERK-mediated translational repression.SDF2-like protein 1 (SDF2L1) regulates the endoplasmic reticulum localization and chaperone activity of ERdj3 protein
Molecular chaperones facilitate protein folding by associating with nascent polypeptides, thereby preventing protein misfolding and aggregation. Endoplasmic reticulum (ER) chaperone BiP, the sole HSP70 chaperone in the ER, is regulated by HSP40 chaperones, including ER-resident protein ERdj3 (DNAJB11). ERdj3 lacks an ER retrieval signal, is secreted under ER stress conditions, and functions as a chaperone in the extracellular space, but how its secretion is regulated remains unclear. We recently showed that ERdj3 forms a complex with ER-resident stromal cell–derived factor 2 (SDF2) and SDF2L1 (SDF2-like protein 1) and thereby prevents protein aggregation during the BiP chaperone cycle.Ribosome depurination by ricin leads to inhibition of endoplasmic reticulum stress–induced HAC1 mRNA splicing on the ribosome
Ricin undergoes retrograde transport to the endoplasmic reticulum (ER), and ricin toxin A chain (RTA) enters the cytosol from the ER. Previous reports indicated that RTA inhibits activation of the unfolded protein response (UPR) in yeast and in mammalian cells. Both precursor (preRTA) and mature form of RTA (mRTA) inhibited splicing of HAC1u (u for uninduced) mRNA, suggesting that UPR inhibition occurred on the cytosolic face of the ER. Here, we examined the role of ribosome binding and depurination activity on inhibition of the UPR using mRTA mutants.The endoplasmic reticulum (ER) chaperone BiP is a master regulator of ER functions: Getting by with a little help from ERdj friends
The endoplasmic reticulum (ER) represents the entry point into the secretory pathway where nascent proteins encounter a specialized environment for their folding and maturation. Inherent to these processes is a dedicated quality-control system that detects proteins that fail to mature properly and targets them for cytosolic degradation. An imbalance in protein folding and degradation can result in the accumulation of unfolded proteins in the ER, resulting in the activation of a signaling cascade that restores proper homeostasis in this organelle.Role for ribosome-associated complex and stress-seventy subfamily B (RAC-Ssb) in integral membrane protein translation
Targeting of most integral membrane proteins to the endoplasmic reticulum is controlled by the signal recognition particle, which recognizes a hydrophobic signal sequence near the protein N terminus. Proper folding of these proteins is monitored by the unfolded protein response and involves protein degradation pathways to ensure quality control. Here, we identify a new pathway for quality control of major facilitator superfamily transporters that occurs before the first transmembrane helix, the signal sequence recognized by the signal recognition particle, is made by the ribosome.Lysine trimethylation regulates 78-kDa glucose-regulated protein proteostasis during endoplasmic reticulum stress
The up-regulation of chaperones such as the 78-kDa glucose-regulated protein (GRP78, also referred to as BiP or HSPA5) is part of the adaptive cellular response to endoplasmic reticulum (ER) stress. GRP78 is widely used as a marker of the unfolded protein response, associated with sustained ER stress. Here we report the discovery of a proteostatic mechanism involving GRP78 trimethylation in the context of ER stress. Using mass spectrometry–based proteomics, we identified two GRP78 fractions, one homeostatic and one induced by ER stress.Function of inhibitor of Bruton's tyrosine kinase isoform α (IBTKα) in nonalcoholic steatohepatitis links autophagy and the unfolded protein response
Nonalcoholic fatty liver disease (steatosis) is the most prevalent liver disease in the Western world. One of the advanced pathologies is nonalcoholic steatohepatitis (NASH), which is associated with induction of the unfolded protein response (UPR) and disruption of autophagic flux. However, the mechanisms by which these processes contribute to the pathogenesis of human diseases are unclear. Herein, we identify the α isoform of the inhibitor of Bruton's tyrosine kinase (IBTKα) as a member of the UPR, whose expression is preferentially translated during endoplasmic reticulum (ER) stress.Nuclear Matrix Protein 4 Is a Novel Regulator of Ribosome Biogenesis and Controls the Unfolded Protein Response via Repression of Gadd34 Expression
The unfolded protein response (UPR) maintains protein homeostasis by governing the processing capacity of the endoplasmic reticulum (ER) to manage ER client loads; however, key regulators within the UPR remain to be identified. Activation of the UPR sensor PERK (EIFAK3/PEK) results in the phosphorylation of the α subunit of eIF2 (eIF2α-P), which represses translation initiation and reduces influx of newly synthesized proteins into the overloaded ER. As part of this adaptive response, eIF2α-P also induces a feedback mechanism through enhanced transcriptional and translational expression of Gadd34 (Ppp1r15A),which targets type 1 protein phosphatase for dephosphorylation of eIF2α-P to restore protein synthesis.Evidence That Base-pairing Interaction between Intron and mRNA Leader Sequences Inhibits Initiation of HAC1 mRNA Translation in Yeast
Background: Hac1 protein, encoded by a cytoplasmically spliced mRNA, activates the unfolded protein response to maintain cellular protein homeostasis and alleviate endoplasmic reticulum stress.Results: Under non-stress conditions, translation initiation on the HAC1 mRNA is repressed.Conclusion: Base-pairing interaction between the 5′ leader and intron represses translation initiation on the HAC1 mRNA.Significance: A unique mechanism of intron-mediated inhibition of ribosomal scanning.
