Membrane lipid saturation activates endoplasmic reticulum unfolded protein response transducers through their transmembrane domains

Abstract

Endoplasmic reticulum (ER) stress sensors use a related luminal domain to monitor the unfolded protein load and convey the signal to downstream effectors, signaling an unfolded protein response (UPR) that maintains compartment-specific protein folding homeostasis. Surprisingly, perturbation of cellular lipid composition also activates the UPR, with important consequences in obesity and diabetes. However, it is unclear if direct sensing of the lipid perturbation contributes to UPR activation. We found that mutant mammalian ER stress sensors, IRE1α and PERK, lacking their luminal unfolded protein stress-sensing domain, nonetheless retained responsiveness to increased lipid saturation. Lipid saturationmediated activation in cells required an ER-spanning transmembrane domain and was positively regulated in vitro by acyl-chain saturation in reconstituted liposomes. These observations suggest that direct sensing of the lipid composition of the ER membrane contributes to the UPR.lipid bilayer | membrane fluidity | integral membrane protein | palmitic acid P rotein folding homeostasis in the lumen of the endoplasmic reticulum (ER) requires matching the flux of unfolded proteins entering the organelle to the capacity of the machinery for processing this biosynthetic load. To achieve this, eukaryotes have evolved signaling pathways that couple ER perturbation (or stress) to a rectifying gene expression program referred to as the unfolded protein response (UPR). Upstream in these pathways are ER localized transmembrane proteins, such as the product of the Inositol REquiring 1 gene (IRE1) (1, 2) and Protein kinase r-like Endoplasmic Reticulum Kinase (PERK) (3), that sense a luminal stress signal(s) and convey it to downstream effectors via their enzymatic activities.The unfolded protein load in the ER regulates IRE1 and PERK activation by two complementary processes: One involves the luminal chaperone immunoglobulin heavy chain binding protein (BiP), which complexes with the luminal domain of IRE1 and PERK, maintaining the enzymes in an inactive, monomeric form. Increases in unfolded protein correlate with disruption of this repressive complex, oligomerization, and activation of the sensor (4-6). The other process involves unfolded proteins that may also engage the stress-sensing luminal domain directly and favor its dimerization and thereby downstream signaling (7,8).Perturbation of cellular lipid composition also activates the UPR. Enhanced UPR signaling has been observed in cholesterol-loaded macrophages (9), in pancreatic beta cells exposed to saturated fatty acids (10), and in the liver of mice fed a high-fat diet (11) and is believed to play a role in the lipotoxicity associated with obesity and type II diabetes mellitus. In yeast, too, lipid perturbation activates the UPR (12, 13), but the mechanisms remain obscure.In mammals, altered membrane lipid composition leads to ER calcium depletion (11,14). This is proposed to promote unfolded protein stress by interfering with calcium-dependent chaperones and enzymes requir...

Calcium-independent Phospholipase A2γ Enhances Activation of the ATF6 Transcription Factor during Endoplasmic Reticulum Stress

“…For example, perturbation of the lipid bilayer of the ER by the expression of a tail-anchored ER membrane protein resulted in the selective activation of ATF6, in the absence of IRE1␣ and PERK activation (and without ER Ca 2ϩ depletion) (42). Conversely, a recent study has demonstrated that saturation of membrane lipids promoted selective IRE1␣ and PERK activation by enhancing dimerization via their transmembrane domains (43). Defining the precise mechanism of ATF6 activation by iPLA 2 ␥ will require further investigation.…”

Approaches to imaging unfolded secretory protein stress in living cells

“…In yeast, inositol depletion can trigger UPR activation in the absence of significant changes in the ER misfolded protein burden [62]. The ability of membrane lipid perturbations to induce UPR independently of accumulation of misfolded proteins has been described using genetic and biochemical methods in both yeast and mammalian cells [25, 83]. Therefore, changes in BiP/Kar2 mobility, when coupled to other biochemical analysis, can directly assess changes in the misfolded protein burden in living cells.…”

Endoplasmic reticulum: Where nucleotide sugar transport meets cytokinin control mechanisms

“…27 It is also noteworthy, that the lipid composition of the ER membrane has been reported to contribute to the unfolded protein response. 28 We have shown that rock1 mutant plants are characterized by a strongly increased size and activity of the inflorescence meristem, indicating a negative regulatory role of ROCK1 in shoot meristem development. Elevated levels of endogenous cytokinins and cytokinin response genes in rock1 inflorescences suggest that ROCK1 regulates the cytokinin activity in meristematic cells.…”