Abstract
HAC1 encodes a transcription factor that is the central effector of the unfolded protein response (UPR) in budding yeast. When the UPR is inactive, HAC1 mRNA is stored as an unspliced isoform in the cytoplasm and no Hac1 protein is detectable. Intron removal is both necessary and sufficient to relieve the post-transcriptional silencing of HAC1 mRNA, yet the precise mechanism by which the intron prevents Hac1 protein accumulation has remained elusive. Here, we show that a combination of inhibited translation initiation and accelerated protein degradation-both dependent on the intron-prevents the accumulation of Hac1 protein when the UPR is inactive. Functionally, both components of this fail-safe silencing mechanism are required to prevent ectopic production of Hac1 protein and concomitant activation of the UPR. Our results provide a mechanistic understanding of HAC1 regulation and reveal a novel strategy for complete post-transcriptional silencing of a cytoplasmic mRNA.
Highlights
The unfolded protein response (UPR) is a eukaryotic stress response pathway that is activated when unfolded proteins accumulate in the endoplasmic reticulum (ER) lumen (Gardner et al, 2013)
Based on the sedimentation of HAC1 mRNPs, we predicted that the mRNA would generate a large number of ribosome-protected fragments, in a quantity only ~2-fold fewer than abundant mRNAs
Rather than providing evidence for stalled ribosomes on HAC1u mRNA, instead these observations suggest that either ribosomes are stalled on the mRNA in a closely packed configuration that prevents nuclease cleavage between ribosomes, which would eliminate the ~28-nucleotide fragments that are sequenced in the ribosome-profiling method (Figure 1C, middle); or that there are not stalled ribosomes on HAC1u mRNA (Figure 1C, bottom)
Summary
The unfolded protein response (UPR) is a eukaryotic stress response pathway that is activated when unfolded proteins accumulate in the endoplasmic reticulum (ER) lumen (Gardner et al, 2013). After the exons are joined by tRNA ligase, the resulting spliced mRNA (denoted HAC1i; ‘i’ for UPR ‘induced’) is translated into Hac1ip. This active transcription factor is imported into the nucleus, where it activates the expression of UPR target genes involved in restoring protein-folding homeostasis in the ER (Chapman et al, 1998). Intron removal is both necessary and sufficient to relieve the posttranscriptional silencing of HAC1 that otherwise prevents Hac1p accumulation (Chapman and Walter, 1997)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
