In the budding yeast Saccharomyces cerevisiae an mRNA, called HAC1, exists in a translationally repressed form in the cytoplasm. Under conditions of cellular stress, such as when unfolded proteins accumulate inside the endoplasmic reticulum (ER), an RNase Ire1 removes an intervening sequence (intron) from the HAC1 mRNA by non-conventional cytosolic splicing. Removal of the intron results in translational de-repression of HAC1 mRNA and production of a transcription factor that activates expressions of many enzymes and chaperones to increase the protein-folding capacity of the cell. Here, we show that Ire1-mediated RNA cleavage requires Watson-Crick base pairs in two RNA hairpins, which are located at the HAC1 mRNA exon-intron junctions. Then, we show that the translational de-repression of HAC1 mRNA can occur independent of cytosolic splicing. These results are obtained from HAC1 variants that translated an active Hac1 protein from the un-spliced mRNA. Additionally, we show that the phosphatidylinositol-3-kinase Vps34 and the nutrient-sensing kinases TOR and GCN2 are key regulators of HAC1 mRNA translation and consequently the ER stress responses. Collectively, our data suggest that the cytosolic splicing and the translational de-repression of HAC1 mRNA are coordinated by unique and parallel networks of signaling pathways.

中文翻译：

---

Vps34 和 TOR 激酶在存在或不存在 Ire1 依赖性剪接的情况下协调 HAC1 mRNA 翻译

在出芽酵母酿酒酵母中，一种称为HAC1的 mRNA以翻译抑制形式存在于细胞质中。在细胞应激条件下，例如当未折叠蛋白在内质网 (ER) 内积聚时，RNase Ire1通过非常规胞质剪接从HAC1 mRNA 中去除插入序列（内含子）。去除内含子导致HAC1 mRNA 的翻译去抑制和转录因子的产生，该因子激活许多酶和分子伴侣的表达以增加细胞的蛋白质折叠能力。在这里，我们表明 Ire1 介导的 RNA 切割需要位于HAC1 的两个 RNA 发夹中的 Watson-Crick 碱基对mRNA 外显子-内含子连接。然后，我们表明HAC1 mRNA的翻译去抑制可以独立于细胞质剪接而发生。这些结果来自从未剪接的 mRNA 翻译活性 Hac1 蛋白的HAC1变体。此外，我们表明磷脂酰肌醇 3-激酶 Vps34 和营养感应激酶 TOR 和GCN2是HAC1 mRNA 翻译的关键调节剂，因此是 ER 应激反应。总的来说，我们的数据表明，HAC1 mRNA的胞质剪接和翻译去抑制是由独特且平行的信号通路网络协调的。