Changes in the hydrophobic network of the FliG(MC) domain induce rotational switching of the flagellar motor

The FliG protein plays a pivotal role in switching the rotational direction of the flagellar motor between clockwise and counterclockwise. Although we previously showed that mutations in the Gly-Gly linker of FliG induce a defect in switching rotational direction, the detailed molecular mechanism was not elucidated. Here, we studied the structural changes in the FliG fragment containing the middle and C-terminal regions, named FliG(MC), and the switch-defective FliG(MC)-G215A, using nuclear magnetic resonance (NMR) and molecular dynamics simulations. NMR analysis revealed multiple conformations of FliG(MC), and the exchange process between these conformations was suppressed by the G215A residue substitution. Furthermore, changes in the intradomain orientation of FliG were induced by changes in hydrophobic interaction networks throughout FliG. Our finding applies to FliG in a ring complex in the flagellar basal body, and clarifies the switching mechanism of the flagellar motor.