The Rab GTPase Ypt1p governs the activation of Unfolded Protein Response (UPR) in Saccharomyces cerevisiae by promoting the preferential nuclear degradation of pre-HAC1 mRNA

Induction of unfolded protein response (UPR) involves activation of transcription factor Hac1p that facilitates the transactivation of genes encoding ER-chaperones. Hac1p is encoded by HAC1 pre-mRNA harboring an intron and a bipartite element (BE) at its 3'-UTR. This precursor RNA undergoes a reversible and differential intra-nuclear mRNA decay by the nuclear exosome/CTEXT at various phases of UPR. In this investigation, using a combination of genetic, and biochemical approach, the Rab-GTPase Ypt1p is demonstrated to control UPR signaling dynamics. Regulation of UPR by Ypt1p relies on its characteristic nuclear localization in absence of ER-stress resulting in its strong association with pre-HAC1 mRNA at its 3'-UTR that promotes sequential recruitments of Nrd1-Nab3p-Sen1p (NNS) complex [->] CTEXT [->] the nuclear exosome onto the pre-HAC1 mRNA that is accompanied by its rapid and selective nuclear decay. This accelerated 3'[->]5' mRNA decay produces a pre-HAC1 mRNA pool lacking the functional BE thus causing its inefficient targeting to Ire1p foci leading to their diminished splicing and translation. ER stress triggers a rapid relocalization of Ypt1p to the cytoplasm with its consequent dissociation from pre-HAC1 mRNA thereby causing a decreased recruitment of NNS/exosome/CTEXT to precursor HAC1 RNA leading to its diminished 3'[->]5' degradation by the exosome. This diminished decay produces an increased abundance of pre-HAC1 mRNA population with intact functional BE leading to its enhanced recruitment to Ire1p foci that is followed by its increased splicing and translation. This enhanced translation produces a huge burst of Hac1p that rapidly transactivates the genes encoding ER-chaperones.