Chapter 14 : Curli Biogenesis: Bacterial Amyloid Assembly by the Type VIII Secretion Pathway

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Curli are extracellular proteinaceous fibers made by Gram-negative bacteria. urli-pecific enes () are primarily found in and ( ). The main function of curli fibers is associated with a sedimentary lifestyle and multicellular behavior in biofilms, as they form scaffolds that provide adhesive and structural support to the community ( ). In certain pathogenic bacteria, curli have also been implicated in host colonization, innate response activation, and cell invasion ( ).

Citation: Bhoite S, van Gerven N, Chapman M, Remaut H. 2019. Curli Biogenesis: Bacterial Amyloid Assembly by the Type VIII Secretion Pathway, p 163-171. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/ecosalplus.ESP-0037-2018
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Figure 1

Transmission electron micrographs of individual cells producing curli fibers and curli-like fibers grown from purified CsgA . Scale bars: 500 nm and 200 nm, respectively. Schematic organization of the and curli gene clusters and architecture of the curli subunits CsgA (blue) and CsgB (dark blue). Subunits comprise an N-terminal signal sequence (SEC) that is cleaved upon export into the periplasm. The mature proteins contain curlin pseudorepeat regions (N22, R1 to R5) that guide substrate specificity in the secretion pathway and form the amyloidogenic core of the curli subunits. Repeats that efficiently self-polymerize are underscored. Theoretical model of CsgA predicted based on amino acid covariation analysis ( ). The predictions point to a right- or left-handed β-helix made up from stacked curlin repeats (labeled R1 to R5). Representation of typical CsgA polymerization profiles in the absence (red) or presence (blue) of preformed fibers or the CsgB nucleator. In the presence of CsgE (1:1 ratio) or CsgC (1:500 ratio), no CsgA polymerization is observed (black curve).

Citation: Bhoite S, van Gerven N, Chapman M, Remaut H. 2019. Curli Biogenesis: Bacterial Amyloid Assembly by the Type VIII Secretion Pathway, p 163-171. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/ecosalplus.ESP-0037-2018
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Figure 2

Curli subunits enter the periplasm via the SecYEG translocon, from where they progress to the cell surface as unfolded polypeptides via the curli transporter CsgG. Premature folding and polymerization of CsgA in the periplasm (right dotted line) are inhibited by CsgE and CsgC. CsgE binds and targets subunits to the secretion channel, while CsgC provides a safeguard against runaway polymerization, likely by the binding and neutralization of early assembly intermediates and/or nascent fibers. CsgG forms a nonameric complex that acts as a peptide diffusion channel and cooperates with the periplasmic factor CsgE, which binds the channel and forms a capping structure to the secretion complex. Recruitment and (partial) enclosure of CsgA in the secretion complex are proposed to create an entropy gradient over the channel that favors CsgA’s outward diffusion as an unfolded, soluble polypeptide. Once secreted, curli fiber formation and elongation are templated by CsgB, in a CsgF-dependent manner. CsgF is likely to be in contact or close proximity to the CsgG channel. The exact role of CsgF and whether fibers extend from the proximal or distal end (dashed arrows) are presently unknown. Abbreviations: IM, inner membrane; OM, outer membrane.

Citation: Bhoite S, van Gerven N, Chapman M, Remaut H. 2019. Curli Biogenesis: Bacterial Amyloid Assembly by the Type VIII Secretion Pathway, p 163-171. In Sandkvist M, Cascales E, Christie P (ed), Protein Secretion in Bacteria. ASM Press, Washington, DC. doi: 10.1128/ecosalplus.ESP-0037-2018
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