Walter Franziska, D'Orsi Beatrice, Jagannathan Anagha, Dussmann Heiko, Prehn Jochen H M
Department of Physiology and Medical Physics, Royal College of Surgeons in Ireland University of Medicine and Health Sciences, Dublin, Ireland.
SFI FutureNeuro Research Centre, Royal College of Surgeons in Ireland University of Medicine and Health Sciences, Dublin, Ireland.
Front Cell Dev Biol. 2022 Aug 15;10:915065. doi: 10.3389/fcell.2022.915065. eCollection 2022.
The Bcl-2 family proteins BAK and BAX control the crucial step of pore formation in the mitochondrial outer membrane during intrinsic apoptosis. Bcl-2-related ovarian killer (BOK) is a Bcl-2 family protein with a high sequence similarity to BAK and BAX. However, intrinsic apoptosis can proceed in the absence of BOK. Unlike BAK and BAX, BOK is primarily located on the endoplasmic reticulum (ER) and Golgi membranes, suggesting a role for BOK in regulating ER homeostasis. In this study, we report that BOK is required for a full ER stress response. Employing previously characterized fluorescent protein-based ER stress reporter cell systems, we show that BOK-deficient cells have an attenuated response to ER stress in all three signaling branches of the unfolded protein response. Fluo-4-based confocal Ca imaging revealed that disruption of ER proteostasis in BOK-deficient cells was not linked to altered ER Ca levels. Fluorescence recovery after photobleaching (FRAP) experiments using GRP78/BiP-eGFP demonstrated that GRP78 motility was significantly lower in BOK-deficient cells. This implied that less intraluminal GRP78 was freely available and more of the ER chaperone bound to unfolded proteins. Collectively, these experiments suggest a new role for BOK in the protection of ER proteostasis and cellular responses to ER stress.
Bcl-2家族蛋白BAK和BAX在线粒体凋亡过程中控制线粒体外膜孔形成的关键步骤。Bcl-2相关卵巢杀手（BOK）是一种与BAK和BAX具有高度序列相似性的Bcl-2家族蛋白。然而，在没有BOK的情况下，内源性凋亡仍可进行。与BAK和BAX不同，BOK主要位于内质网（ER）和高尔基体膜上，这表明BOK在调节内质网稳态中发挥作用。在本研究中，我们报告BOK是完整内质网应激反应所必需的。利用先前表征的基于荧光蛋白的内质网应激报告细胞系统，我们发现BOK缺陷细胞在未折叠蛋白反应的所有三个信号分支中对内质网应激的反应减弱。基于Fluo-4的共聚焦钙成像显示，BOK缺陷细胞中内质网蛋白质稳态的破坏与内质网钙水平的改变无关。使用GRP78/BiP-eGFP进行的光漂白后荧光恢复（FRAP）实验表明，BOK缺陷细胞中GRP78的运动性显著降低。这意味着腔内可自由获得的GRP78较少，更多的内质网伴侣蛋白与未折叠蛋白结合。总的来说，这些实验表明BOK在保护内质网蛋白质稳态和细胞对内质网应激的反应中具有新的作用。
