Structural plasticity of the Salmonella FliS flagellar export chaperone

The Salmonella FliS flagellar export chaperone is a highly α-helical protein. Proteolytic experiments suggest that FliS has a compact core. However, the calorimetric melting profile of FliS does not show any melting transition in the 25-110 °C temperature range. Circular dichroism measurements reveal that FliS is losing its helical structure over a broad temperature range upon heating. These observations indicate that FliS unfolds in a noncooperative way and its native state shows features reminiscent of the molten globule state of proteins possessing substantial structural plasticity. As FliS has several binding partners within the cell, conformational adaptability seems to be an essential requirement to fulfill its multiple roles.