MOTILITY PROTEIN INTERACTIONS IN THE BACTERIAL FLAGELLAR MOTOR

Five proteins (MotA, MotB, FliG, FliM, and FliN) have been implicated in energizing flagellar rotation in Escherichia coli and Salmonella ty phimurium. One model for flagellar function envisions that MotA and Mo tB comprise the stator of a rotary motor and that FliG, FliM, and FliN are part of the rotor. MotA probably functions as a transmembrane pro ton channel, and MotB has been proposed to anchor MotA to the peptidog lycan of the cell wall. To study interactions between the Mot proteins themselves and between them and other components of the flagellar mot or, we attempted to isolate extragenic suppressors of 13 dominant or p artially dominant motB missense mutations. Four of these yielded suppr essors, which exhibited widely varying efficiencies of suppression. Th e pattern of suppression was partially allele-specific, but no suppres sor seriously impaired motility in a motB(+) strain. Of 20 suppressors from the original selection, 15 were characterized by DNA sequencing. Fourteen of these cause single amino acid changes in MotA. Thirteen a fter residues in, or directly adjacent to, the putative periplasmic lo ops of MotA, and the remaining one alters a residue in the middle of t he fourth predicted transmembrane helix of MotA. We conclude that the MotA and MotB proteins form a complex and that their interaction direc tly involves or is strongly influenced by the periplasmic loops of Mot A. The 15th suppressor from the original selection and 2 motB suppress ors identified during a subsequent search cause single amino acid subs titutions in FliG. This finding suggests that the postulated Mot-prote in complex may be in close proximity to FliG at the stator-rotor inter face of the flagellar motor.