Charged residues in the cytoplasmic loop of MotA are required for stator assembly into the bacterial flagellar motor

MotA and MotB form a transmembrane proton channel that acts as the stator of the bacterial flagellar motor to couple proton flow with torque generation. The C-terminal periplasmic domain of MotB plays a role in anchoring the stators to the motor. However, it remains unclear where their initial binding sites are. Here, we constructed Salmonella strains expressing GFP-MotB and MotA-mCherry and investigated their subcellular localization by fluorescence microscopy. Neither the D33N and D33A mutations in MotB, which abolish the proton flow, nor depletion of proton motive force affected the assembly of GFP-MotB into the motor, indicating that the proton translocation activity is not required for stator assembly. Overexpression of MotA markedly inhibited wild-type motility, and it was due to the reduction in the number of functional stators. Consistently, MotA-mCherry was observed to colocalize with GFP-FliG even in the absence of MotB. These results suggest that MotA alone can be installed into the motor. The R90E and E98K mutations in the cytoplasmic loop of MotA (MotA_C), which has been shown to abolish the interaction with FliG, significantly affected stator assembly, suggesting that the electrostatic interaction of MotA_C with FliG is required for the efficient assembly of the stators around the rotor.