<b>Signalling from endoplasmic reticulum to nucleus: transcription factor with a basic‐leucine zipper motif is required for the unfolded protein‐response pathway</b>

TLDR

Accumulation of unfolded proteins in the ER activates the unfolded protein‑response (UPR) pathway, inducing chaperone and folding enzyme genes through the UPRE cis‑acting element that is recognized by transcription factor(s) in yeast. We cloned the ERN4 (HAC1) gene via yeast one‑hybrid screening, generating a GAL4‑ERN4 fusion that constitutively activates the UPR pathway. ERN4 encodes the basic‑leucine‑zipper protein Ern4p, which binds the essential CAGCGTG palindrome in UPRE, activates transcription of UPR target genes, and is required for their induction; loss of Ern4p abolishes UPR gene expression and confers ER‑stress sensitivity and inositol dependence.

Abstract

Background : Accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers the transcriptional induction of molecular chaperones and folding enzymes localized in the ER. Thus, eukaryotic cells possess an intracellular signalling pathway from the ER to the nucleus, called the unfolded protein‐response (UPR) pathway. In Saccharomyces cerevisiae, such induction is mediated by the cis ‐acting unfolded protein‐response element (UPRE) which has been thought to be recognized by one or more transcription factor(s). Results : Extensive mutational analysis revealed that UPRE contains a partial palindrome with a spacer of one nucleotide (CAGCGTG) that is essential for its function. We then cloned the ERN4 (presumably identical with HAC1 ) gene using yeast one‐hybrid screening, in which the GAL4‐ERN4 fusion gene constitutively activates the UPR pathway. The ERN4 gene encodes a basic‐leucine zipper protein (Ern4p) that specifically binds to UPRE in vitro and activates transcription in vivo . Cells lacking Ern4p are unable to induce transcription of any of the five target genes tested and exhibit sensitivity to ER stress and inositol requirement for growth. Conclusion : We concluded that Ern4p represents a major component of the putative transcription factor (UPRF) responsible for the UPR leading to the induction of ER‐localized stress proteins.