Size‐dependent secretory protein reflux into the cytosol in association with acute endoplasmic reticulum stress

Abstract

Once secretory proteins have been targeted to the endoplasmic reticulum (ER) lumen, the proteins typically remain partitioned from the cytosol. If the secretory proteins misfold, they can be unfolded and retrotranslocated into the cytosol for destruction by the proteasome by ER-Associated protein Degradation (ERAD). Here, we report that correctly folded and targeted luminal ER fluorescent protein reporters accumulate in the cytosol during acute misfolded secretory protein stress in yeast.Photoactivation fluorescence microscopy experiments reveal that luminal reporters already localized to the ER relocalize to the cytosol, even in the absence of essential ERAD machinery. We named this process "ER reflux." Reflux appears to be regulated in a size-dependent manner for reporters. Interestingly, prior heat shock stress also prevents ER stress-induced reflux. Together, our findings establish a new ER stressregulated pathway for relocalization of small luminal secretory proteins into the cytosol, distinct from the ERAD and preemptive quality control pathways. Importantly, our results highlight the value of fully characterizing the cell biology of reporters and describe a simple modification to maintain luminal ER reporters in the ER during acute ER stress. K E Y W O R D S endoplasmic reticulum, ERAD, fluorescent protein, GFP, photoactivation, signal peptide, intracellular transport, traffic, translocation, UPR 1 | INTRODUCTION The standard model of secretory protein localization in eukaryotes holds that proteins are translated in the cytosol and then trafficked to and inserted into the endoplasmic reticulum (ER) membrane or translocated into the ER lumen in co-and posttranslational processes. 1,2 Partitioning of secretory proteins from the cytosol into the ER ensures that secretory proteins fold in the unique ER environment, interact with other partner secretory proteins, and, if appropriate, are secreted out of the ER to other organelles of the secretory pathway orinto the extracellular milieu. Secretory proteins that fail to correctly fold can be retrotranslocated from the ER lumen or ER membrane back into the cytosol followed by proteasome mediated destruction in a process termed ER associated degradation (ERAD). 3-7 Furthermore, some secretory proteins fail to enter the ER because of inefficiencies in the targeting process or sequestration of critical translocation factors. [8][9][10][11][12] Abbreviations: a.a., amino acids; bp, base pairs; D, diffusion coefficient; DTT, dithiothreitol; ER, endoplasmic reticulum; ERAD, endoplasmic reticulum associated degradation; FP, fluorescent protein; FRAP, fluorescence recovery after photobleaching; GFP, green fluorescent protein; MFI, mean fluorescence intensity; QC, quality control; sfGFP, superfolder GFP; SS, signal sequence; td, tandem dimer; Tm, tunicamycin; UPR, unfolded protein response; yemEos3.2, yeast codon optimized mEos3.2; yesfGFP, yeast codon optimized superfolder GFP.

Endoplasmic Reticulum Stress of Gut Enterocyte and Intestinal Diseases

“…Through the effect of ER stress, autophagy plays a protective role in a constitutive manner to enable the turnover of long-lived proteins, removal of damaged organelles, and misfolded proteins as a defense mechanism against pathogens ( Yang et al, 2016 ). When the accumulation of damaged proteins in the ER has exceeded the repair capacity of ERAD, portions of the organelle can be specifically targeted for large-scale degradation through autophagy ( Lajoie and Snapp, 2020 ). eIF2α phosphorylation induced by PERK works as a hotspot of stress-induced translation control ( Kouroku et al, 2007 ; Liu et al, 2010 ; Talloczy et al, 2002 ).…”

Homeostasis of the ER redox state subsequent to proteasome inhibition

“…In yeast, it was reported that ER stress caused reflux of ER proteins to the cytosol 29 , 30 . To confirm contribution of reflux of ERroGFP S4 into the cytosol to the ER reduction under proteasome inhibition, subcellular fractionation of cells was performed, resulting that most of ERroGFP S4 were localized in the ER while small portion of ERroGFP S4 were localized in the cytosol fraction (Supplementary Fig.…”

Methodologies for Measuring Protein Trafficking across Cellular Membranes

“…For example, some ER-localized GFPs were used to demonstrate that ER stress induces some small protein relocalization into the cytosol. [141,142] By using a ratiometric redox-environment reporter GFP, relative stress-dependent GFP localization to the oxidizing environment of the ER and reducing environment of the cytosol was quantified. Increasingly, large libraries of tagged or taggable proteins are also available, increasing the throughput for measuring protein localization based on fluorescent tags.…”