title: Structure of the nonameric bacterial amyloid secretion channel.
abstract: Various strains of bacteria are able to produce a unique class of functional amyloids termed curli, which are critical for biofilm formation, host cell adhesion, and colonization of inert surfaces. Curli are secreted via the type VIII bacterial secretion system, and they share biochemical and structural characteristics with amyloid fibers that have been implicated in deleterious disease in humans. Here, we report the crystal structure of Escherichia coli CsgG, which is an essential lipoprotein component of the type VIII secretion system and which forms a secretion channel in the bacterial outer membrane for transporting curli subunits. CsgG forms a crown-shaped, symmetric nonameric channel that spans the outer membrane via a 36-strand β-barrel, with each subunit contributing four β-strands. This nonameric complex contains a central channel with a pore located at the middle. The eyelet of the pore is ∼12 Å in diameter and is lined with three stacked nine-residue rings consisting of Tyr-66, Asn-70, or Phe-71. Our structure-based functional studies suggest that Tyr-66 and Phe-71 residues function as gatekeepers for the selective secretion of curli subunits. Our study describes in detail, to our knowledge, the first core structure of the type VIII bacterial secretion machinery. Importantly, our structural analysis suggests that the curli subunits are secreted via CsgG across the bacterial outer membrane in an unfolded form.
authors.affiliation: National Laboratory of Biomacromolecules, National Center of Protein Science-Beijing, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China;
authors.forename: Baohua
authors.initials: B
authors.lastname: Cao
authors.affiliation: National Laboratory of Biomacromolecules, National Center of Protein Science-Beijing, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China; School of Life Sciences, University of Science and Technology of China, Hefei 230027, Anhui, China; and.
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhao
authors.affiliation: National Laboratory of Biomacromolecules, National Center of Protein Science-Beijing, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China; University of Chinese Academy of Sciences, Beijing 100101, China.
authors.forename: Yongjun
authors.initials: Y
authors.lastname: Kou
authors.affiliation: National Laboratory of Biomacromolecules, National Center of Protein Science-Beijing, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China;
authors.forename: Dongchun
authors.initials: D
authors.lastname: Ni
authors.affiliation: National Laboratory of Biomacromolecules, National Center of Protein Science-Beijing, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China;
authors.forename: Xuejun Cai
authors.initials: XC
authors.lastname: Zhang
authors.affiliation: National Laboratory of Biomacromolecules, National Center of Protein Science-Beijing, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China; yihuahuang@sun5.ibp.ac.cn.
authors.forename: Yihua
authors.initials: Y
authors.lastname: Huang
mesh_terms: D058947:Bacterial Secretion Systems / Q000235:genetics*; D015153:Blotting, Western; D002850:Chromatography, Gel; D003460:Crystallization; D004252:DNA Mutational Analysis; D017931:DNA Primers / Q000235:genetics; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D055786:Gene Knockout Techniques; D008074:Lipoproteins / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D008854:Microscopy, Electron; D008958:Models, Molecular; D016296:Mutagenesis; D016133:Polymerase Chain Reaction; D011487:Protein Conformation
chemical_list: D058947:Bacterial Secretion Systems; C534546:CsgG protein, E coli; D017931:DNA Primers; D029968:Escherichia coli Proteins; D008074:Lipoproteins
keywords: CsgG; amyloids; biofilm; curli; outer membrane protein