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

Abstract

E. coli curli, an archetypical bacterial amyloid, is a critical biofilm scaffold comprised mainly of the CsgA protein. CsgA is highly aggregation prone, confounding high-resolution studies of its structure and amyloid formation mechanism. Ensemble studies show that monomeric 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 molten globule 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. Steered molecular dynamics simulations reveal unfolding patterns consistent with the experimental data. These studies help us better understand curli amyloid formation that is important both as a novel antimicrobial drug target and engineered biomaterial.