The Bacterial Curli System Possesses a Potent and Selective Inhibitor of Amyloid Formation

Abstract

Summary Curli are extracellular functional amyloids that are assembled by enteric bacteria during biofilm formation and host colonization. An efficient secretion system and chaperone network ensures that the major curli fiber subunit, CsgA, does not form intracellular amyloid aggregates. We discovered that the periplasmic protein CsgC was a highly effective inhibitor of CsgA amyloid formation. In the absence of CsgC, CsgA formed toxic intracellular aggregates. In vitro, CsgC inhibited CsgA amyloid formation at substoichiometric concentrations and maintained CsgA in a non-β-sheet rich conformation. Interestingly, CsgC inhibited amyloid assembly of human α-synuclein, but not Aβ42, in vitro. We identified a common D-Q-Φ-X0,1-G-K-N-ζ-E motif in CsgC client proteins that is not found in Aβ42. CsgC is therefore both an efficient and selective amyloid inhibitor. Dedicated functional amyloid inhibitors may be a key feature that distinguishes functional amyloids from disease-associated amyloids.

Specific Chaperones and Regulatory Domains in Control of Amyloid Formation

“…Its production is correlated with the other components of the Csg system (56), and it likely functions as a specific anti-amyloid chaperone in the production of curli fibers. Interestingly, CsgC was found to inhibit amyloid formation not only of CsgA, but also the Parkinson-associated ␣-synuclein, while not affecting the aggregation of the highly amyloidogenic A␤ peptide (57). In contrast, for the BRICHOS domain, it appears to be the other way around; it efficiently reduces A␤ aggregation and toxicity (see above), whereas it only marginally inhibits ␣-synuclein aggregation.…”

“…Instead, the preferred target sequence for CsgC was identified as a DQWXGKNSE motif located at the end of repeat 3 of CsgA. However, the glutamine-and asparagine-rich segments in other repeats as well as in CsgB may also be recognized (57). CsgC contains a high number of glutamine residues evenly distributed on its surface (Fig.…”

Amyloid Structures as Biofilm Matrix Scaffolds

“…Associated with this pore are the CsgE and CsgF proteins, which modulate subunit stability and proper assembly of curli fibers (18,19). The periplasmic protein CsgC may be involved in regulating the export of curli subunits through the regulation of the redox state of CsgG (20), and it was shown to inhibit CsgA amyloid formation in vitro (21). CsgD is the master regulator that controls at the transcriptional level the expression of the csgBAC operon and also adrA, which is involved in posttranscriptional regulation of cellulose synthesis and encodes another common component of the biofilm matrix (22)(23)(24)(25).…”

“…This gives us the clue that compounds able to inhibit human aberrant amyloids could be active also against bacterial functional amyloids and vice versa. In this regard, it has been recently reported that the periplasmic protein CsgC is highly effective in inhibiting human ␣-synuclein and CsgA amyloid formation, while it does not affect polymerization of A␤ 42 (21).…”

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

“…Although CsgA is prone to aggregation and forms amyloid fibers in vitro (2,27), the formation of curli fibers in vivo is nucleated by CsgB (28)(29)(30). CsgC exists in periplasm and is an effective inhibitor of CsgA polymerization, suppressing fibrilization at substoichiometric ratios as low as 1:500 (CsgC:CsgA) (31), and perhaps preventing premature periplasmic amyloid formation and toxicity to the bacterium.…”