Are “Power-Packs” Associated with the Switch-Motor Complex of Bacterial Flagella?

Abstract

Bacterial flagella are rotated by a motor embedded in the cytoplasmic membrane, the direction of rotation being dictated by a switch at the base of the motor. In the case of bacteria such as E.coli and Salmonella, the motor is driven by the proton-motive force (PMF) across the membrane. The PMF is also used, inter alia, to synthesize ATP by means of the reversible membrane enzyme F0F1ATPase, whose mechanism involves the rotary action of a spindle. Recently, the switch-motor complex of bacterial flagella was found to be associated with a number of non-flagellar proteins, which, in spite of not belonging to the chemotaxis system, affect the function of the flagella. The observation that one of these proteins, fumarate reductase, is involved in electron transport under anaerobic conditions raised the question of whether other energy-linked enzymes are associated with the switch-motor complex as well. Here, we identified two additional such enzymes in E.coli. Employing Forster resonance energy transfer in vivo and pull-down assays in vitro, we provided evidence for the interaction of F0F1ATP synthase via its β subunit with the flagellar switch protein FliG and for the interaction of NADH-ubiquinone oxidoreductase with FliG, FliM, and possibly FliN. Furthermore, we measured higher rates of ATP synthesis, ATP hydrolysis, and electron transport from NADH to oxygen in membrane areas adjacent to the flagellar motor than in other membrane areas. All these observations suggest the association of energy complexes with the flagellar switch-motor complex. Finding that deletion of the β subunit in vivo affected the direction of flagellar rotation and switching frequency further implied that the interaction of F0F1ATP synthase with FliG is important for the function of the switch of bacterial flagella.