The folding-limited nucleation of curli hints at an evolved safety mechanism for functional amyloid production

It is nearly two decades ago that the thin aggregative fimbriae which had been shown to enhance the biofilm formation of Salmonella enteriditis and Escherichia coli were identified as amyloid fibers. The realization that natural proteins can develop amyloidogenic traits as part of their functional repertoire instigated a search for similar proteins across all kingdoms of life. That pursuit has since unearthed dozens of candidates which now constitute the family of proteins referred to as functional amyloids (FA). FAs are promising candidates for future synthetic biology applications in that they marry the structural benefits of the amyloid fold (self-assembly and stability) while steering clear of the cytotoxicity issues that are typically linked to amyloid associated human pathologies. Unfortunately, the extreme aggregation propensity of FAs and the associated operational difficulties are restricting their adoption in real-world applications, underscoring the need for additional processes to control the amyloid reaction. Here we untangle the molecular mechanism of amyloid formation of the canonical functional amyloid curli using NMR, native mass spectrometry and cryo-electron microscopy. Our results are consistent with folding-limited one-step amyloid nucleation that has emerged as an evolutionary balance between efficient extracellular polymerization, while steering clear of pre-emptive nucleation in the periplasm. Sequence analysis of the amyloid curlin kernel suggests a finetuning of the rate of monomer folding via modulation of the secondary structure propensity of the pre-amyloid species, opening new potential avenues towards control of the amyloid reaction.