A stress assembly that confers cell viability by preserving ERES components during amino-acid starvation

Nutritional restriction leads to protein translation attenuation that results in the storage and degradation of free mRNAs in cytoplasmic assemblies. In this study, we show in Drosophila S2 cells that amino-acid starvation also leads to the inhibition of another major anabolic pathway, the protein transport through the secretory pathway, and to the formation of a novel reversible non-membrane bound stress assembly, the Sec body that incorporates components of the ER exit sites. Sec body formation does not depend on membrane traffic in the early secretory pathway, yet requires both Sec23 and Sec24AB. Sec bodies have liquid droplet-like properties, and they act as a protective reservoir for ERES components to rebuild a functional secretory pathway after re-addition of amino-acids acting as a part of a survival mechanism. Taken together, we propose that the formation of these structures is a novel stress response mechanism to provide cell viability during and after nutrient stress

Phospho-Rasputin Stabilization by Sec16 Is Required for Stress Granule Formation upon Amino Acid Starvation

Most cellular stresses induce protein translation inhibition and stress granule formation. Here, using Drosophila S2 cells, we investigate the role of G3BP/Rasputin in this process. In contrast to arsenite treatment, where dephosphorylated Ser142 Rasputin is recruited to stress granules, we find that, upon amino acid starvation, only the phosphorylated Ser142 form is recruited. Furthermore, we identify Sec16, a component of the endoplasmic reticulum exit site, as a Rasputin interactor and stabilizer. Sec16 depletion results in Rasputin degradation and inhibition of stress granule formation. However, in the absence of Sec16, pharmacological stabilization of Rasputin is not enough to rescue the assembly of stress granules. This is because Sec16 specifically interacts with phosphorylated Ser142 Rasputin, the form required for stress granule formation upon amino acid starvation. Taken together, these results demonstrate that stress granule formation is fine-tuned by specific signaling cues that are unique to each stress. These results also expand the role of Sec16 as a stress response protein

TORC2 mediates the heat stress response in Drosophila by promoting the formation of stress granules

The kinase TORis found in two complexes, TORC1,which is involved in growth control, and TORC2, whose roles are less well defined. Here, we asked whether TORC2 has a role in sustaining cellular stress. We show that TORC2 inhibition in Drosophila melanogaster leads to a reduced tolerance to heat stress, whereas sensitivity to other stresses is not affected. Accordingly, we show that upon heat stress, both in the animal and Drosophila cultured S2 cells, TORC2 is activated and is required for maintaining the level of its known target, Akt1 (also known as PKB). We show that the phosphorylation of the stress-activated protein kinases is not modulated by TORC2 nor is the heat-induced upregulation of heat-shock proteins. Instead, we show, both in vivo and in cultured cells, that TORC2 is required for the assembly of heat-induced cytoprotective ribonucleoprotein particles, the pro-survival stress granules. These granules are formed in response to protein translation inhibition imposed by heat stress that appears to be less efficient in the absence of TORC2 function.We propose that TORC2 mediates heat resistance in Drosophila by promoting the cell autonomous formation of stress granules

Phospho-rasputin stabilization by Sec16 is required for stress granule formation upon amino acid starvation

Most cellular stresses induce protein translation inhibition and stress granule formation. Here, using Drosophila S2 cells, we investigate the role of G3BP/Rasputin in this process. In contrast to arsenite treatment, where dephosphorylated Ser142 Rasputin is recruited to stress granules, we find that, upon amino acid starvation, only the phosphorylated Ser142 form is recruited. Furthermore, we identify Sec16, a component of the endoplasmic reticulum exit site, as a Rasputin interactor and stabilizer. Sec16 depletion results in Rasputin degradation and inhibition of stress granule formation. However, in the absence of Sec16, pharmacological stabilization of Rasputin is not enough to rescue the assembly of stress granules. This is because Sec16 specifically interacts with phosphorylated Ser142 Rasputin, the form required for stress granule formation upon amino acid starvation. Taken together, these results demonstrate that stress granule formation is fine-tuned by specific signaling cues that are unique to each stress. These results also expand the role of Sec16 as a stress response protein