Single molecule studies of the bacterial curli protein CsgA reveal a structurally dynamic monomeric structure
E. coli curli, a critical biofilm scaffold and antimicrobial target, is assembled mainly from the protein CsgA guided by many other chaperones in a regulated functional amyloid assembly pathway. CsgA is highly aggregation prone, confounding high-resolution studies of its structure and amyloid formation mechanism. Ensemble studies show CsgA is intrinsically disordered, while structure predictions show a well-folded {beta}-solenoid. Here, single-molecule force spectroscopy with optical tweezers is used to define the conformational dynamics in single CsgA molecules. CsgA monomer shows unfolding/refolding consistent with fully folded, partially folded and collapsed yet disordered conformations, indicating that CsgA exists in metastable folded states in equilibrium with disordered states. These folded states are likely highly aggregation prone and thus have avoided characterization at the ensemble level for the past 20 years. These studies help us better understand curli amyloid formation as a novel drug target and to improve engineering amyloid.