title: Structural insights into the secretin translocation channel in the type II secretion system.
abstract: 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.
authors.affiliation: School of Life Sciences, Tsinghua University, Beijing, China.|Advanced Innovation Center for Structural Biology, Tsinghua University, Beijing, China.|Ministry of Education Key Laboratory of Protein Sciences, Tsinghua University, Beijing, China.
authors.forename: Zhaofeng
authors.initials: Z
authors.lastname: Yan
authors.affiliation: School of Life Sciences, Tsinghua University, Beijing, China.|Advanced Innovation Center for Structural Biology, Tsinghua University, Beijing, China.|Ministry of Education Key Laboratory of Protein Sciences, Tsinghua University, Beijing, China.|Tsinghua-Peking Joint Center for Life Sciences, Beijing, China.
authors.forename: Meng
authors.initials: M
authors.lastname: Yin
authors.affiliation: Life Sciences Institute and Innovation Center for Cell Signaling Network, Zhejiang University, Hangzhou, China.
authors.forename: Dandan
authors.initials: D
authors.lastname: Xu
authors.affiliation: Life Sciences Institute and Innovation Center for Cell Signaling Network, Zhejiang University, Hangzhou, China.
authors.forename: Yongqun
authors.initials: Y
authors.lastname: Zhu
authors.affiliation: School of Life Sciences, Tsinghua University, Beijing, China.|Advanced Innovation Center for Structural Biology, Tsinghua University, Beijing, China.|Ministry of Education Key Laboratory of Protein Sciences, Tsinghua University, Beijing, China.|Tsinghua-Peking Joint Center for Life Sciences, Beijing, China.
authors.forename: Xueming
authors.initials: X
authors.lastname: Li
mesh_terms: D000595:Amino Acid Sequence; D017124:Conserved Sequence; D020285:Cryoelectron Microscopy; D004926:Escherichia coli / Q000648:ultrastructure; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000648:ultrastructure*; D015640:Ion Channel Gating; D008958:Models, Molecular; D018272:Porins / Q000648:ultrastructure*; D000072757:Protein Conformation, beta-Strand; D054730:Protein Interaction Domains and Motifs; D020836:Protein Structure, Quaternary; D012633:Secretin / Q000737:chemistry; D000069178:Type II Secretion Systems / Q000737:chemistry / Q000648:ultrastructure*
chemical_list: D029968:Escherichia coli Proteins; C540076:GspD protein, E coli; D018272:Porins; D000069178:Type II Secretion Systems; D012633:Secretin