Single molecule studies of the bacterial curli protein CsgA reveal a structurally dynamic monomeric structure

Bacterial biofilm constructed with functional amyloid are a substantial health concern. The major subunit of curli fibrils, CsgA, has recently had its amyloid structure explored by cryoEM, but the folding of the subunit had yet to be carefully examined at the level of the single molecule. We describe the first in vitro single molecule folding trajectories of CsgA, demonstrating a dynamic mixture of metastable states or a molten globule, but also unfolding of the R domains. In order to content with the aggregation propensity of this protein, we developed an assay to characterise the observation of individual folding trajectories of a single molecule of CsgA using force spectroscopy with optical tweezers. This work was made possible by an engineered disulfide trap and the controlled reduction of its cysteines. When folding, CsgA does not get locked into a β-solenoid, but may be found in a molten globule or metastable state in about a quarter of the times it is unfolded.