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.CsgG | curli | amyloids | outer membrane protein | biofilm

A comprehensive guide to pilus biogenesis in Gram-negative bacteria

“…While in the periplasm, CsgA subunits can engage three possible routes: i) progress to the CsgG channel en route to the bacterial surface, ii) become proteolytic degraded, iii) remain in the periplasm where toxic fibres are neutralised by the action of CsgC binding 7 . At the OM, CsgA subunits first interact with the nonameric CsgE capping adaptor, before engaging with the CsgG diffusion channel (a 36-stranded and also nonameric β-barrel pore) 126,127 . When bound to CsgG, CsgE acts as a plug to the pre-constricted CsgG vestibule, but also as a secretion adaptor assisting CsgA subunits during their periplasmic transit 128 .…”

Solution NMR structure of CsgE: Structural insights into a chaperone and regulator protein important for functional amyloid formation

“…CsgE and CsgF are also encoded by the operon and have long been considered accessory proteins that function in concert with CsgG, the outer membrane pore responsible for the translocation of CsgA and CsgB to the outer membrane space (1,2,(36)(37)(38). CsgG forms a symmetric, nonameric, ungated, and nonselective protein secretion channel, as revealed recently (37,39). CsgF is secreted to the cell surface, binds to CsgG, and appears to be critical for the specific localization of CsgB (38).…”

Outer membrane lipoprotein biogenesis: Lol is not the end

“…CsgG functions in the transport of curli subunits [64] and Wza transports group 1 (serotype K30) capsular polysaccharides [65]. The lipids of both CsgG and Wza are anchored in the inner leaflet of OM and both proteins form oligomeric complexes with hydrophobic transmembrane domains and large periplasmic domains [66][67][68]. Each oligomeric complex features a central channel used for the secretion of the corresponding substrates [66 -68].…”