Structure of the nonameric bacterial amyloid secretion channel

Significance

Numerous bacteria produce a unique class of “functional” amyloids termed curli that are important for the fitness of the organism by mediating biofilm formation, host cell adhesion, and colonization on inert surfaces. Here, we report the crystal structure of CsgG, a lipoprotein that forms a secretion channel for curli subunits in the outer membrane. Each CsgG monomer is composed of four β-strands that span the outer membrane. Nine CsgG monomers together form a large, 36-stranded β-barrel with a central secretion channel. The channel is restricted by stacked rings within the pore, formed from inwardly protruding residues. The CsgG structure may provide a template for the development of antibiotics aimed at attenuating biofilm formation.

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.