Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan.
Genes Cells. 2013 Apr;18(4):288-301. doi: 10.1111/gtc.12035. Epub 2013 Feb 6.
In eukaryotic cells under nonstressed conditions, the endoplasmic reticulum (ER)-located molecular chaperone BiP is associated with an ER-membrane protein Ire1 to inhibit its self-association. While ER stress leads Ire1 to form transiently BiP-unbound clusters, which strongly evoke the unfolded protein response (UPR), here we propose an alternative activation status of Ire1. When yeast cells are physiologically ER-stressed by inositol depletion for a prolonged time, the UPR is weakly activated in a sustained manner after a transient peak of activation. During persistent stress, Ire1 foci disappear, while Ire1 continues to be self-associated. Under these conditions, Ire1 may be activated as a homo-dimer, as it shows considerable activity even when carrying the W426A mutation, which allows Ire1 to form homo-dimers but not clusters. Unlike the Ire1 clusters, the nonclustered active form seems to be associated with BiP. An Ire1 mutant not carrying the BiP-association site continued to form clusters and to be activated strongly even after long-term stress. Similar observations were obtained when cells were ER-stressed by dithiothreitol. We thus propose that upon persistent ER stress, Ire1 is weakly and continuously activated in a nonclustered form through its (re)association with BiP, which disperses the Ire1 clusters.
在非应激条件下的真核细胞中，内质网（ER）定位的分子伴侣 BiP 与 ER 膜蛋白 Ire1 相关联，以抑制其自身缔合。虽然 ER 应激导致 Ire1 形成瞬时的 BiP 非结合簇，强烈引发未折叠蛋白反应（UPR），但在这里我们提出了 Ire1 的另一种激活状态。当酵母细胞因肌醇耗竭而在生理上受到长时间的 ER 应激时，UPR 在短暂激活峰值后以持续的方式弱激活。在持续的应激下，Ire1 焦点消失，而 Ire1 继续自我缔合。在这些条件下，Ire1 可能作为同二聚体被激活，因为即使携带允许 Ire1 形成同二聚体而不是簇的 W426A 突变，它仍显示出相当大的活性。与 Ire1 簇不同，无聚集的活性形式似乎与 BiP 相关联。不携带 BiP 结合位点的 Ire1 突变体即使在长期应激后仍继续形成簇并被强烈激活。当细胞通过二硫苏糖醇（DTT）受到 ER 应激时，也得到了类似的观察结果。因此，我们提出，在持续的 ER 应激下，Ire1 通过与其（重新）结合 BiP 以无聚簇的形式弱且持续地被激活，从而分散 Ire1 簇。
