Coupling of Proton Conduction to Rotation in the Bacterial Flagellar Motor

摘要

Rotation of the flagella of Escherichia coli is driven by an inward H(+) current maintained by the transmembrane proton motive force. The first objective of the work was to determine, by genetic suppression analysis, interactions between the two proteins (MotA and MotB) that make up the stator of the flagellar motor. The second objective was to determine how the stator interacts with the rotor. We demonstrated that amino acid substitutions affecting portions of MotB thought to contact or lie adjacent to the peptidoglycan layer of the cell wall and that cause severe motility defects can be suppressed by mutations altering the external face or transmembrane regions of MotA. Some motB mutations can also be suppressed by mutations in the fliG gene, which encodes a component of the rotor. Our overall conclusion is that suppression is achieved by events that affect distant sites within the motor. Thus, a mutation in motB that misaligns the stator with respect to the rotor can be corrected by a compensating change in the stator (a motA mutation) or in the rotor (a fliG mutation). Similar experiments with missense mutations in motA produced different results in that suppressors fell chiefly into a defined region encoding residues 136-138 of MotB. In contrast to the suppressors of motB mutations, we propose that the residues of MotB identified in this suppression analysis may directly contact MotA and help determine correct alignment or tight binding.