Lajoie Patrick, Snapp Erik L
Department of Anatomy and Cell Biology, The University of Western Ontario, London, Ontario, Canada.
Janelia Research Campus Ashburn, Virginia, USA.
Traffic. 2020 Jun;21(6):419-429. doi: 10.1111/tra.12729. Epub 2020 Apr 13.
Once secretory proteins have been targeted to the endoplasmic reticulum (ER) lumen, the proteins typically remain partitioned from the cytosol. If the secretory proteins misfold, they can be unfolded and retrotranslocated into the cytosol for destruction by the proteasome by ER-Associated protein Degradation (ERAD). Here, we report that correctly folded and targeted luminal ER fluorescent protein reporters accumulate in the cytosol during acute misfolded secretory protein stress in yeast. Photoactivation fluorescence microscopy experiments reveal that luminal reporters already localized to the ER relocalize to the cytosol, even in the absence of essential ERAD machinery. We named this process "ER reflux." Reflux appears to be regulated in a size-dependent manner for reporters. Interestingly, prior heat shock stress also prevents ER stress-induced reflux. Together, our findings establish a new ER stress-regulated pathway for relocalization of small luminal secretory proteins into the cytosol, distinct from the ERAD and preemptive quality control pathways. Importantly, our results highlight the value of fully characterizing the cell biology of reporters and describe a simple modification to maintain luminal ER reporters in the ER during acute ER stress.
一旦分泌蛋白被靶向运输到内质网（ER）腔，这些蛋白通常会与细胞质分隔开来。如果分泌蛋白错误折叠，它们可以被展开并通过内质网相关蛋白降解（ERAD）逆向转运到细胞质中，由蛋白酶体进行降解。在此，我们报告称，在酵母急性错误折叠分泌蛋白应激期间，正确折叠并靶向运输到内质网腔的荧光蛋白报告基因会在细胞质中积累。光激活荧光显微镜实验表明，即使在缺乏必需的ERAD机制的情况下，已经定位于内质网的腔内报告基因也会重新定位于细胞质中。我们将这个过程命名为“内质网反流”。对于报告基因来说，反流似乎以大小依赖的方式受到调节。有趣的是，先前的热休克应激也能防止内质网应激诱导的反流。总之，我们的研究结果建立了一种新的内质网应激调节途径，用于将小的腔内分泌蛋白重新定位到细胞质中，这与ERAD和抢先质量控制途径不同。重要的是，我们的结果突出了全面表征报告基因细胞生物学特性的价值，并描述了一种简单的修饰方法，以在急性内质网应激期间将内质网腔内报告基因维持在内质网中。
