Yamamoto Keisuke, Suzuki Natsumi, Wada Tadashi, Okada Tetsuya, Yoshida Hiderou, Kaufman Randal J, Mori Kazutoshi
Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto, Japan.
J Biochem. 2008 Oct;144(4):477-86. doi: 10.1093/jb/mvn091. Epub 2008 Jul 29.
Quality control of proteins in the endoplasmic reticulum (ER) is achieved by two mechanisms, the productive folding mechanism, which is assisted by a number of ER-localized molecular chaperones and folding enzymes (collectively termed ER chaperones), and the ER-associated degradation (ERAD) mechanism, by which misfolded proteins are degraded by the ubiquitin-dependent proteasome system in the cytosol. Accumulation of unfolded proteins in the ER activates the unfolded protein response (UPR), resulting in transcriptional induction of ER chaperones and ERAD components. In mammals, three signalling pathways operate for the UPR, namely the IRE1-XBP1, PERK-ATF4 and ATF6 pathways. Analysis of mouse embryonic fibroblasts deficient in UPR signalling molecule indicates that transcriptional induction of ERAD components depends on the IRE1-XBP1 pathway. However, the molecular basis of this finding remains unclear. Here, we analysed the promoter of human HRD1, which encodes an E3 ubiquitin ligase, an important component of ERAD. We found that induction of HRD1 is mediated by two cis-acting elements, a canonical ER stress response element and a novel element we designate as UPR element II. The presence of UPR element II to which XBP1 but not ATF6 directly binds explains at least in part the dependency of HRD1 induction on the IRE1-XBP1 pathway.
内质网(ER)中蛋白质的质量控制通过两种机制实现,即生产性折叠机制,该机制由多种内质网定位的分子伴侣和折叠酶(统称为内质网伴侣)协助,以及内质网相关降解(ERAD)机制,通过该机制错误折叠的蛋白质在细胞质中被泛素依赖性蛋白酶体系统降解。内质网中未折叠蛋白质的积累会激活未折叠蛋白反应(UPR),导致内质网伴侣和ERAD组分的转录诱导。在哺乳动物中,UPR有三条信号通路,即IRE1-XBP1、PERK-ATF4和ATF6通路。对缺乏UPR信号分子的小鼠胚胎成纤维细胞的分析表明,ERAD组分的转录诱导依赖于IRE1-XBP1通路。然而,这一发现的分子基础仍不清楚。在这里,我们分析了人类HRD1的启动子,它编码一种E3泛素连接酶,是ERAD的一个重要组分。我们发现HRD1的诱导由两个顺式作用元件介导,一个典型的内质网应激反应元件和一个我们命名为UPR元件II的新元件。XBP1而非ATF6直接结合的UPR元件II的存在至少部分解释了HRD1诱导对IRE1-XBP1通路的依赖性。