Curli biogenesis and function

Abstract

Curli are the major proteinaceous component of a complex extracellular matrix produced by many Enterobacteriaceae. Curli were first discovered in the late 1980s on Escherichia coli strains that caused bovine mastitis, and have since been implicated in many physiological and pathogenic processes of E. coli and Salmonella spp. Curli fibers are involved in adhesion to surfaces, cell aggregation, and biofilm formation. Curli also mediate host cell adhesion and invasion, and they are potent inducers of the host inflammatory response. The structure and biogenesis of curli are unique among bacterial fibers that have been described to date. Structurally and biochemically, curli belong to a growing class of fibers known as amyloids. Amyloid fiber formation is responsible for several human diseases including Alzheimer's, Huntington's, and prion diseases, although the process of in vivo amyloid formation is not well understood. Curli provide a unique system to study macromolecular assembly in bacteria and in vivo amyloid fiber formation. Here, we review curli biogenesis, regulation, role in biofilm formation, and role in pathogenesis.

Figure 1. Model of curli assembly

A schematic diagram of the two curli gene operons is shown (bottom). CsgD is a positive transcriptional regulator of the csgBA operon. All the proteins encoded by the csg operons, except for CsgD, contain sec signal sequences for translocation into the periplasm. CsgG is an outer membrane protein required for the secretion of the two curli structural subunits CsgA and CsgB. CsgA is secreted outside of the cell where CsgB nucleates it into a fiber. CsgE and CsgF both interact with CsgG and are required for efficient curli assembly.

Figure 2. Phenotype of csg mutant strains after growth on CR-indicator plates

(a) A CR-indicator plate demonstrating interbacterial complementation. csgA⁻/csgB⁺ acceptor and csgA⁺/csgB⁻ donor strains were struck from top to bottom of a CR-indicator plate. csgA⁻ and csgB⁻ strains were struck from left to right on the same CR-indicator plate. When a csgA⁻ strain is cross-streaked with a csgB⁻ strain, CR binding occurs. A CR-indicator plate of wild-type (Wt), csgA⁻, csgE⁻, and csgF⁻ strains is shown after (b) 30 and (c) 48 h of growth at 26°C.

Figure 3. High-resolution EM of purified CsgG

Purified CsgG was visualized by high-resolution EM (left). At least two shapes of CsgG were observed: donut-like structures, which might indicate a top or bottom view of CsgG oligomers, and cylinders, which might represent a side view of the CsgG oligomeric complex (right).

Figure 4

Morphotypes of Salmonella typhimurium grown on CR-indicator plates for 48 h at 26°C. Figure was kindly provided by Ute Romling.

Figure 5. Electron micrographs of curli

Negative-stain EMs of (a)a bacterium-expressing curli and (b) bacteria not expressing curli are shown. (c) High-resolution EM of purified CsgA fibers.

Figure 6. Model of CsgA and CsgB structure

(a)A model of the predicted strand-loop-strand motif of CsgA (19). A similar prediction was made for CsgB (86). (b) CsgA and (c) CsgB are composed of five repeating units (R1–R5), and each repeating unit is the equivalent of one strand-loop-strand. Residues that are conserved within the repeating units are colored the same. The red residues in R5 of (c) CsgB indicate residues that differ from R1–R4 of CsgB.