Stress granules (SGs) are membraneless organelles composed of mRNAs and RNA binding proteins which undergo assembly in response to stress-induced inactivation of translation initiation. In general, SG recruitment is limited to a subpopulation of a given mRNA species and RNA-seq analyses of purified SGs revealed that signal sequence-encoding (i.e., endoplasmic reticulum [ER]-targeted) transcripts are significantly underrepresented, consistent with prior reports that ER localization can protect mRNAs from SG recruitment. Using translational profiling, cell fractionation, and single molecule mRNA imaging, we examined SG biogenesis following activation of the unfolded protein response (UPR) by 1,4-dithiothreitol (DTT) and report that gene-specific subsets of cytosolic and ER-targeted mRNAs can be recruited into SGs. Furthermore, we demonstrate that SGs form in close proximity to or directly associated with the ER membrane. ER-associated SG assembly was also observed during arsenite stress, suggesting broad roles for the ER in SG biogenesis. Recruitment of a given mRNA into SGs required stress-induced translational repression, though translational inhibition was not solely predictive of an mRNA's propensity for SG recruitment. SG formation was prevented by the transcriptional inhibitors actinomycin D or triptolide, suggesting a functional link between gene transcriptional state and SG biogenesis. Collectively these data demonstrate that ER-targeted and cytosolic mRNAs can be recruited into ER-associated SGs and this recruitment is sensitive to transcriptional inhibition. We propose that newly transcribed mRNAs exported under conditions of suppressed translation initiation are primary SG substrates, with the ER serving as the central subcellular site of SG formation.

中文翻译：

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将内质网靶向和胞质 mRNA 募集到膜相关应激颗粒中

应激颗粒 (SG) 是由 mRNA 和 RNA 结合蛋白组成的无膜细胞器，它们会响应应激诱导的翻译起始失活而进行组装。一般来说，SG 招募仅限于给定 mRNA 种类的亚群，并且纯化 SG 的 RNA-seq 分析表明信号序列编码（即内质网 [ER] 靶向）转录物的代表性明显不足，这与之前的报道一致ER 定位可以保护 mRNA 免受 SG 招募。使用翻译分析、细胞分级分离和单分子 mRNA 成像，我们检查了 1,4-二硫苏糖醇 (DTT) 激活未折叠蛋白反应 (UPR) 后 SG 的生物发生，并报告了胞浆和 ER 靶向 mRNA 的基因特异性子集可以被招募到 SG。此外，我们证明 SG 形成于内质网膜附近或直接与其相关。在亚砷酸盐胁迫期间也观察到了与 ER 相关的 SG 组装，这表明 ER 在 SG 生物发生中发挥着广泛的作用。将给定的 mRNA 招募到 SG 中需要应激诱导的翻译抑制，尽管翻译抑制并不能单独预测 mRNA 招募 SG 的倾向。转录抑制剂放线菌素 D 或雷公藤甲素可阻止 SG 形成，这表明基因转录状态与 SG 生物发生之间存在功能联系。总的来说，这些数据表明，ER 靶向和胞质 mRNA 可以被募集到 ER 相关的 SG 中，并且这种募集对转录抑制敏感。我们认为，在翻译起始受到抑制的条件下输出的新转录的 mRNA 是主要的 SG 底物，而 ER 是 SG 形成的中心亚细胞位点。