Structural and mechanistic insights into the bacterial amyloid secretion channel CsgG

CsgG and CgsE form an encaging translocon for selective, iterative diffusion of curli subunits across the non-energized bacterial outer membrane. Amyloid transporter CsgG structure This manuscript reports the atomic structure of the bacterial amyloid transporter CsgG, an outer-membrane lipoprotein that forms the translocation channel for curli (amyloid fibre) subunits that once secreted, polymerize into cross β-fibres that mediate biofilm formation. Curli fibres constitute the major protein component of the extracellular matrix in biofilms formed by Bacteroidetes and Proteobacteria. The CsgG structure reveals a 36-stranded β-barrel that forms a channel spanning the membrane bilayer and suggests a potential mechanism for guiding substrates through the secretion pore via a channel constriction. Curli are functional amyloid fibres that constitute the major protein component of the extracellular matrix in pellicle biofilms formed by Bacteroidetes and Proteobacteria (predominantly of the α and γ classes) 1 , 2 , 3 . They provide a fitness advantage in pathogenic strains and induce a strong pro-inflammatory response during bacteraemia 1 , 4 , 5 . Curli formation requires a dedicated protein secretion machinery comprising the outer membrane lipoprotein CsgG and two soluble accessory proteins, CsgE and CsgF 6 , 7 . Here we report the X-ray structure of Escherichia coli CsgG in a non-lipidated, soluble form as well as in its native membrane-extracted conformation. CsgG forms an oligomeric transport complex composed of nine anticodon-binding-domain-like units that give rise to a 36-stranded β-barrel that traverses the bilayer and is connected to a cage-like vestibule in the periplasm. The transmembrane and periplasmic domains are separated by a 0.9-nm channel constriction composed of three stacked concentric phenylalanine, asparagine and tyrosine rings that may guide the extended polypeptide substrate through the secretion pore. The specificity factor CsgE forms a nonameric adaptor that binds and closes off the periplasmic face of the secretion channel, creating a 24,000 Å 3 pre-constriction chamber. Our structural, functional and electrophysiological analyses imply that CsgG is an ungated, non-selective protein secretion channel that is expected to employ a diffusion-based, entropy-driven transport mechanism.