AbstractA periplasmic flagellar chaperone protein, FlgA, is required for P-ring assembly in bacterial flagella of taxa such as Salmonella enterica or Escherichia coli. The mechanism of chaperone-mediated P-ring formation is poorly understood. Here we present the open and closed crystal structures of FlgA from Salmonella enterica serovar Typhimurium, grown under different crystallization conditions. An intramolecular disulfide cross-linked form of FlgA caused a dominant negative effect on motility of the wild-type strain. Pull-down experiments support a specific protein-protein interaction between FlgI, the P-ring component protein, and the C-terminal domain of FlgA. Surface plasmon resonance and limited-proteolysis indicate that flexibility of the domain is reduced in the covalently closed form. These results show that the structural flexibility of the C-terminal domain of FlgA, which is related to the structural difference between the two crystal forms, is intrinsically associated with its molecular chaperone function in P-ring assembly.