Mori K, Kawahara T, Yoshida H, Yanagi H, Yura T
HSP Research Institute, Kyoto Research Park, Shimogyo-ku, Japan.
Genes Cells. 1996 Sep;1(9):803-17. doi: 10.1046/j.1365-2443.1996.d01-274.x.
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).
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.
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.
内质网（ER）中未折叠蛋白的积累会触发位于内质网的分子伴侣和折叠酶的转录诱导。因此，真核细胞拥有一条从内质网到细胞核的细胞内信号通路，称为未折叠蛋白反应（UPR）通路。在酿酒酵母中，这种诱导是由顺式作用的未折叠蛋白反应元件（UPRE）介导的，人们认为该元件可被一种或多种转录因子识别。
广泛的突变分析表明，UPRE包含一个间隔一个核苷酸的部分回文序列（CAGCGTG），该序列对其功能至关重要。然后，我们通过酵母单杂交筛选克隆了ERN4（可能与HAC1相同）基因，其中GAL4-ERN4融合基因组成性激活UPR通路。ERN4基因编码一种碱性亮氨酸拉链蛋白（Ern4p），它在体外特异性结合UPRE并在体内激活转录。缺乏Ern4p的细胞无法诱导所测试的五个靶基因中的任何一个的转录，并且对内质网应激和生长所需的肌醇表现出敏感性。
我们得出结论，Ern4p代表了负责UPR导致内质网定位应激蛋白诱导的推定转录因子（UPRF）的主要成分。
