Clusters of charged residues at the C terminus of MotA and N terminus of MotB are important for function of the Escherichia coli flagellar motor.

MotA contains a conserved C-terminal cluster of negatively charged residues, and MotB contains a conserved N-terminal cluster of positively charged residues. Charge-altering mutations affecting these residues impair motility but do not diminish Mot protein levels. The motility defects are reversed by second-site mutations targeting the same or partner protein.