The curli nucleator protein, CsgB, contains an amyloidogenic domain that directs CsgA polymerization

The molecular basis of functional bacterial amyloid polymerization and nucleation

Amyloid fibers are filamentous proteinaceous structures commonly associated with mammalian neurodegenerative diseases. Nucleation is the rate-limiting step of amyloid propagation, and its nature remains poorly understood. Escherichia coli assembles functional amyloid fibers called curli on the cell surface using an evolved biogenesis machine. In vivo, amyloidogenesis of the major curli subunit protein, CsgA, is dependent on the minor curli subunit protein, CsgB. Here, we directly demonstrated that CsgB(+) cells efficiently nucleated purified soluble CsgA into amyloid fibers on the cell surface. CsgA contains five imperfect repeating units that fulfill specific roles in directing amyloid formation. Deletion analysis revealed that the N- and C-terminal most repeating units were required for in vivo amyloid formation. We found that CsgA nucleation specificity is encoded by the N- and C-terminal most repeating units using a blend of genetic, biochemical, and electron microscopic analyses. In addition, we found that the C-terminal most repeat was most aggregation-prone and dramatically contributed to CsgA polymerization in vitro. This work defines the elegant molecular signatures of bacterial amyloid nucleation and polymerization, thereby revealing how nature directs amyloid formation to occur at the correct time and location.

Sequence determinants of bacterial amyloid formation

Amyloids are proteinaceous fibers commonly associated with neurodegenerative diseases and prion-based encephalopathies. Many different polypeptides can form amyloid fibers, leading to the suggestion that amyloid is a primitive main chain-dominated structure. A growing body of evidence suggests that amino acid side chains dramatically influence amyloid formation. The specific role fulfilled by side chains in amyloid formation, especially in vivo, remains poorly understood. Here, we determined the role of internally conserved polar and aromatic residues in promoting amyloidogenesis of the functional amyloid protein CsgA, which is the major protein component of curli fibers assembled by enteric bacteria such as Escherichia coli and Salmonella spp. In vivo CsgA polymerization into an amyloid fiber requires the CsgB nucleator protein. The CsgA amyloid core region is composed of five repeating units, defined by regularly spaced Ser, Gln and Asn residues. The results of a comprehensive alanine scan mutagenesis screen showed that Gln and Asn residues at positions 49, 54, 139 and 144 were critical for curli assembly. Alanine substitution of Q49 or N144 impeded the ability of CsgA to respond to CsgB-mediated heteronucleation, and the ability of CsgA to self-polymerize in vitro. However, CsgA proteins harboring these mutations were still seeded by preformed wild-type CsgA fibers in vitro. This suggests that CsgA-fibril-mediated seeding and CsgB-mediated heteronucleation have distinguishable mechanisms. Remarkably, Gln residues at positions 49 and 139 could not be replaced by Asn residues without interfering with curli assembly, suggesting that the side chain requirements were especially stringent at these positions. This analysis demonstrates that bacterial amyloid formation is driven by specific side chain contacts, and provides a clear illustration of the essential roles of specific side chains in promoting amyloid formation.

Curli provide the template for understanding controlled amyloid propagation

The uncontrolled formation of amyloid fibers is the hallmark of more than twenty human diseases. In contrast to disease-associated amyloids, which are the products of protein misfolding, E. coli assembles functional amyloid fibers called curli on its surface using an elegant biogenesis machine. Composed of a major subunit, CsgA, and a minor subunit, CsgB, curli play important roles in host cell adhesion, long-term survival and other bacterial community behaviors. Assembly of curli fibers is a template-directed conversion process where membrane-tethered CsgB initiates CsgA polymerization. The CsgA amyloid core is composed of five imperfect repeating units. In a series of in vivo and in vitro experiments, we determined the sequence and structural determinants that guide the initiation and propagation of CsgA polymers. The CsgA N- and C-terminal repeating units govern its polymerization and responsiveness to CsgB. Specifically, conserved glutamine and asparagine residues present in the CsgA N- and C-terminal repeating units are required for CsgB-mediated nucleation and efficient self-assembly.

Polymerizing the fibre between bacteria and host cells: the biogenesis of functional amyloid fibres

Amyloid fibres are proteinaceous aggregates associated with several human diseases, including Alzheimer's, Huntington's and Creutzfeldt Jakob's. Disease-associated amyloid formation is the result of proteins that misfold and aggregate into beta sheet-rich fibre polymers. Cellular toxicity is readily associated with amyloidogenesis, although the molecular mechanism of toxicity remains unknown. Recently, a new class of 'functional' amyloid fibres was discovered that demonstrates that amyloids can be utilized as a productive part of cellular biology. These functional amyloids will provide unique insights into how amyloid formation can be controlled and made less cytotoxic. Bacteria produce some of the best-characterized functional amyloids, including a surface amyloid fibre called curli. Assembled by enteric bacteria, curli fibres mediate attachment to surfaces and host tissues. Some bacterial amyloids, like harpins and microcinE492, have exploited amyloid toxicity in a directed and functional manner. Here, we review and discuss the functional amyloids assembled by bacteria. Special emphasis will be paid to the biology of functional amyloid synthesis and the connections between bacterial physiology and pathology.