Abstract
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.