Structural insights into the secretin translocation channel in the type II secretion system.

Zhaofeng Yan, Meng Yin, Dandan Xu, Yongqun Zhu, Xueming Li

Nature structural & molecular biology 2017 Feb

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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.

Citation

Zhaofeng Yan, Meng Yin, Dandan Xu, Yongqun Zhu, Xueming Li. Structural insights into the secretin translocation channel in the type II secretion system. Nature structural & molecular biology. 2017 Feb;24(2):177-183