Newman Laura E, Schiavon Cara R, Turn Rachel E, Kahn Richard A
Department of Biochemistry, Emory University School of Medicine, Atlanta, GA, USA.
Cell Logist. 2017 Jun 23;7(3):e1340104. doi: 10.1080/21592799.2017.1340104. eCollection 2017.
Mitochondria are essential, dynamic organelles that regularly undergo both fusion and fission in response to cellular conditions, though mechanisms of the regulation of their dynamics are incompletely understood. We provide evidence that increased activity of the small GTPase ARL2 is strongly correlated with an increase in fusion, while loss of ARL2 activity results in a decreased rate of mitochondrial fusion. Strikingly, expression of activated ARL2 can partially restore the loss of fusion resulting from deletion of either mitofusin 1 (MFN1) or mitofusin 2 (MFN2), but not deletion of both. We only observe the full effects of ARL2 on mitochondrial fusion when it is present in the intermembrane space (IMS), as constructs driven to the matrix or prevented from entering mitochondria are essentially inactive in promoting fusion. Thus, ARL2 is the first regulatory (small) GTPase shown to act inside mitochondria or in the fusion pathway. Finally, using high-resolution, structured illumination microscopy (SIM), we find that ARL2 and mitofusin immunoreactivities present as punctate staining along mitochondria that share a spatial convergence in fluorescence signals. Thus, we propose that ARL2 plays a regulatory role in mitochondrial fusion, acting from the IMS and requiring at least one of the mitofusins in their canonical role in fusion of the outer membranes.
线粒体是重要的动态细胞器，会根据细胞状况定期进行融合和裂变，不过其动力学调节机制尚未完全明确。我们提供的证据表明，小GTP酶ARL2活性增加与融合增加密切相关，而ARL2活性丧失则导致线粒体融合速率降低。引人注目的是，激活的ARL2表达可部分恢复因线粒体融合蛋白1（MFN1）或线粒体融合蛋白2（MFN2）缺失而导致的融合丧失，但不能恢复两者均缺失时的融合丧失。只有当ARL2存在于膜间隙（IMS）时，我们才观察到其对线粒体融合的全部作用，因为驱动至基质或阻止进入线粒体的构建体在促进融合方面基本无活性。因此，ARL2是首个被证明在线粒体内或融合途径中起作用的调节性（小）GTP酶。最后，使用高分辨率结构光照显微镜（SIM），我们发现ARL2和线粒体融合蛋白的免疫反应性沿线粒体呈点状染色，荧光信号在空间上汇聚。因此，我们提出ARL2在线粒体融合中起调节作用，从IMS发挥作用，且在外膜融合的经典作用中至少需要一种线粒体融合蛋白。
