School of Life Sciences, Tsinghua University, Beijing, China.
Advanced Innovation Center for Structural Biology, Tsinghua University, Beijing, China.
Nat Struct Mol Biol. 2017 Feb;24(2):177-183. doi: 10.1038/nsmb.3350. Epub 2017 Jan 9.
The secretin GspD of the type II secretion system (T2SS) forms a channel across the outer membrane in Gram-negative bacteria to transport substrates from the periplasm to the extracellular milieu. The lack of an atomic-resolution structure of the GspD channel hinders the investigation of substrate translocation mechanism of T2SS. Here we report cryo-EM structures of two GspD channels (∼1 MDa), from Escherichia coli K12 and Vibrio cholerae, at ∼3 Å resolution. The structures reveal a pentadecameric channel architecture, wherein three rings of GspD N domains form the periplasmic channel. The secretin domain constitutes a novel double β-barrel channel, with at least 60 β-strands in each barrel, and is stabilized by S domains. The outer membrane channel is sealed by β-strand-enriched gates. On the basis of the partially open state captured, we proposed a detailed gate-opening mechanism. Our structures provide a structural basis for understanding the secretin superfamily and the mechanism of substrate translocation in T2SS.
革兰氏阴性菌中的 II 型分泌系统（T2SS）的分泌蛋白 GspD 形成穿过外膜的通道，将底物从周质空间转运到细胞外环境。缺乏 GspD 通道的原子分辨率结构阻碍了对 T2SS 底物转运机制的研究。在此，我们报道了来自大肠杆菌 K12 和霍乱弧菌的两个 GspD 通道（约 1 MDa）的冷冻电镜结构，分辨率约为 3 Å。这些结构揭示了一个十五聚体通道结构，其中 GspD N 结构域的三个环形成周质通道。分泌蛋白结构域构成了一种新颖的双β-桶通道，每个桶中至少有 60 个β-链，由 S 结构域稳定。外膜通道由富含β-链的门封闭。基于捕获到的部分开放状态，我们提出了一个详细的门开启机制。我们的结构为理解分泌蛋白超家族和 T2SS 中底物转运机制提供了结构基础。
