Qi Yuanbo, Yan Liming, Yu Caiting, Guo Xiangyang, Zhou Xin, Hu Xiaoyu, Huang Xiaofang, Rao Zihe, Lou Zhiyong, Hu Junjie
Department of Genetics and Cell Biology, College of Life Sciences, Nankai University, Tianjin 300071, China.
Tianjin Key Laboratory of Protein Sciences, Nankai University, Tianjin 300071, China.
J Cell Biol. 2016 Dec 5;215(5):621-629. doi: 10.1083/jcb.201609019.
Mitochondria undergo fusion and fission. The merging of outer mitochondrial membranes requires mitofusin (MFN), a dynamin-like GTPase. How exactly MFN mediates membrane fusion is poorly understood. Here, we determined crystal structures of a minimal GTPase domain (MGD) of human MFN1, including the predicted GTPase and the distal part of the C-terminal tail (CT). The structures revealed that a helix bundle (HB) formed by three helices extending from the GTPase and one extending from the CT closely attaches to the GTPase domain, resembling the configuration of bacterial dynamin-like protein. We show that the nucleotide-binding pocket is shallow and narrow, rendering weak hydrolysis and less dependence on magnesium ion, and that association of HB affects GTPase activity. MFN1 forms a dimer when GTP or GDP/BeF, but not GDP or other analogs, is added. In addition, clustering of vesicles containing membrane-anchored MGD requires continuous GTP hydrolysis. These results suggest that MFN tethers apposing membranes, likely through nucleotide-dependent dimerization.
线粒体进行融合与分裂。线粒体外膜的融合需要线粒体融合蛋白（MFN），一种类似发动蛋白的GTP酶。MFN究竟如何介导膜融合目前还知之甚少。在此，我们确定了人MFN1最小GTP酶结构域（MGD）的晶体结构，包括预测的GTP酶和C末端尾巴（CT）的远端部分。这些结构显示，由从GTP酶延伸出的三个螺旋和从CT延伸出的一个螺旋形成的螺旋束（HB）紧密附着于GTP酶结构域，类似于细菌类发动蛋白的结构。我们发现核苷酸结合口袋浅且窄，导致水解作用弱且对镁离子依赖性小，并且HB的结合会影响GTP酶活性。当添加GTP或GDP/BeF时，MFN1形成二聚体，而添加GDP或其他类似物时则不会。此外，含有膜锚定MGD的囊泡聚集需要持续的GTP水解。这些结果表明，MFN可能通过核苷酸依赖性二聚化来连接相邻的膜。
