Vps34 and TOR Kinases Coordinate mRNA Translation in the Presence or Absence of Ire1-Dependent Splicing.

In the budding yeast Saccharomyces cerevisiae, an mRNA, called , 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 mRNA by nonconventional cytosolic splicing. Removal of the intron results in translational derepression of mRNA and production of a transcription factor that activates expression 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 mRNA exon-intron junctions. Then, we show that the translational derepression of mRNA can occur independent of cytosolic splicing. These results are obtained from variants that translated an active Hac1 protein from the unspliced mRNA. Additionally, we show that the phosphatidylinositol-3-kinase Vps34 and the nutrient-sensing kinases TOR and GCN2 are key regulators of mRNA translation and consequently the ER stress responses. Collectively, our data suggest that the cytosolic splicing and the translational derepression of mRNA are coordinated by unique and parallel networks of signaling pathways.