Upon endoplasmic reticulum (ER) stress, activation of the ER-resident transmembrane protein kinase/endoribonuclease inositol-requiring enzyme 1 (IRE1) initiates a key branch of the unfolded protein response (UPR) through unconventional splicing generation of the transcription factor X-box-binding protein 1 (XBP1s). Activated IRE1 can form large clusters/foci, whose exact dynamic architectures and functional properties remain largely elusive. Here we report that, in mammalian cells, formation of IRE1α clusters is an ER membrane-bound phase separation event that is coupled to the assembly of stress granules (SGs). In response to different stressors, IRE1α clusters are dynamically tethered to SGs at the ER. The cytosolic linker portion of IRE1α possesses intrinsically disordered regions and is essential for its condensation with SGs. Furthermore, disruption of SG assembly abolishes IRE1α clustering and compromises XBP1 mRNA splicing, and such IRE1α–SG coalescence engenders enrichment of the biochemical components of the pro-survival IRE1α–XBP1 pathway during ER stress. Our findings unravel a phase transition mechanism for the spatiotemporal assembly of IRE1α–SG condensates to establish a more efficient IRE1α machinery, thus enabling higher stress-handling capacity.

在内质网 （ER） 应激时，内质网驻留跨膜蛋白激酶/核酸内切酶肌醇需求酶 1 （IRE1） 的激活通过转录因子 X 盒结合蛋白 1 （XBP1） 的非常规剪接生成启动未折叠蛋白反应 （UPR） 的关键分支。激活的 IRE1 可以形成大的簇/病灶，其确切的动态结构和功能特性在很大程度上仍然难以捉摸。在这里，我们报道了在哺乳动物细胞中，IRE1α 簇的形成是 ER 膜结合的相分离事件，它与应激颗粒 （SG） 的组装耦合。为了响应不同的压力源，IRE1α 簇在 ER 与 SG 动态相连。IRE1α 的胞质接头部分具有固有的无序区域，对于它与 SG 的缩合至关重要。此外，SG 组装的破坏消除了 IRE1α 聚集并损害了 XBP1 mRNA 剪接，并且这种 IRE1α-SG 结合导致在 ER 应激期间富集促存活 IRE1α-XBP1 通路的生化成分。我们的研究结果揭示了 IRE1α-SG 凝聚物时空组装的相变机制，以建立更高效的 IRE1α 机制，从而实现更高的应力处理能力。