Abstract
HAC1 encodes a key transcription factor that transmits the unfolded protein response (UPR) from the endoplasmic reticulum (ER) to the nucleus and regulates downstream UPR genes in Saccharomyces cerevisiae. In response to the accumulation of unfolded proteins in the ER, Ire1p oligomers splice HAC1 pre-mRNA (HAC1u) via a non-conventional process and allow the spliced HAC1 (HAC1i) to be translated efficiently. However, leaky splicing and translation of HAC1u may occur in non-UPR cells to induce undesirable UPR. To control accidental UPR activation, multiple fail-safe mechanisms have been proposed to prevent leaky HAC1 splicing and translation and to facilitate rapid degradation of translated Hac1up and Hac1ip. Among proposed regulatory mechanisms is a degron sequence encoded at the 5′ end of the HAC1 intron that silences Hac1up expression. To investigate the necessity of an intron-encoded degron sequence that specifically targets Hac1up for degradation, we employed publicly available transcriptomic data to quantify leaky HAC1 splicing and translation in UPR-induced and non-UPR cells. As expected, we found that HAC1u is only efficiently spliced into HAC1i and efficiently translated into Hac1ip in UPR-induced cells. However, our analysis of ribosome profiling data confirmed frequent occurrence of leaky translation of HAC1u regardless of UPR induction, demonstrating the inability of translation fail-safe to completely inhibit Hac1up production. Additionally, among 32 yeast HAC1 surveyed, the degron sequence is highly conserved by Saccharomyces yeast but is poorly conserved by all other yeast species. Nevertheless, the degron sequence is the most conserved HAC1 intron segment in yeasts. These results suggest that the degron sequence may indeed play an important role in mitigating the accumulation of Hac1up to prevent accidental UPR activation in the Saccharomyces yeast.
Highlights
Folding of nascent proteins in the endoplasmic reticulum (ER) is an error-prone process in eukaryotic cells, and the accumulation of unfolded/misfolded proteins can lead to lethal consequences if left unregulated [1]
The HAC1 50 and 30 splice site information of Saccharomyces cerevisiae, Candida orthopsilosis, Candida albicans, Lodderomyces elongisporus, Clavispora lusitaniae, Scheffersomyces stipitis, Meyerozyma guilliermondii, and Debaryomyces hansenii were obtained from Iracane et al [47] and those of Yarrowia lipolytica, Candida glabrata, Lachancea thermotolerans, Eremothecium gossypii, Kluyveromyces lactis, Zygosaccharomyces rouxii, Saccharomyces paradoxus, Naumovozyma castellii, and Candida dubliniensis were obtained from Hooks et al [25]
To determine a putative degron sequence in the other 31 yeast species surveyed in this study, we manually identified the first in-frame stop codon (STOP1) in the intron of their HAC1 genes such that the nucleotide sequence of the truncated HAC1u, from start codon until
Summary
Folding of nascent proteins in the endoplasmic reticulum (ER) is an error-prone process in eukaryotic cells, and the accumulation of unfolded/misfolded proteins can lead to lethal consequences if left unregulated [1]. In response to the increase of unfolded/misfolded proteins, yeasts have evolved a complex Ire1p+Hac1p-mediated signaling pathway to tightly control protein folding [2,3]. This unfolded protein response (UPR) increases ER folding capacity by increasing the production of specific chaperon proteins, reducing folding load by reducing global translation activity and triggers apoptosis when UPR fails to restore ER homeostasis. HAC1 pre-mRNA (HAC1u ) is Microorganisms 2021, 9, 620. 9, 620 x FOR PEER REVIEW Microorganisms 22 of of 18. Ire1p oligomers to u splicing is that it is not spliced u to undergo splicing
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have