Structural basis of FliG – FliM interaction in H elicobacter pylori

Summary FliG and FliM are switch proteins that regulate the rotation and switching of the flagellar motor. Several assembly models for FliG and FliM have recently been proposed; however, it remains unclear whether the assembly of the switch proteins is conserved among different bacterial species. We applied a combination of pull‐down, thermodynamic and structural analyses to characterize the FliM – FliG association from the mesophilic bacterium H elicobacter pylori.   FliM binds to FliG with micromolar binding affinity, and their interaction is mediated through the middle domain of FliG ( FliG M ), which contains the EHPQR motif. Crystal structures of the middle domain of H . pylori   FliM ( FliM M ) and FliG M – FliM M complex revealed that FliG binding triggered a conformational change of the FliM α3‐α1′ loop, especially Asp 130 and Arg 144. We furthermore showed that various highly conserved residues in this region are required for FliM – FliG complex formation. Although the FliM – FliG complex structure displayed a conserved binding mode when compared with T hermotoga maritima , variable residues were identified that may contribute to differential binding affinities across bacterial species. Comparison of the thermodynamic parameters of FliG – FliM interactions between H . pylori and E scherichia coli suggests that molecular basis and binding properties of FliM to FliG is likely different between these two species.