Structures of bacterial flagellar motors from two FliF-FliG gene fusion mutants

Flagella purified from Salmonella enterica serovar Typhimurium contain FliG , FliM, and FliN, cytoplasmic proteins that are important in torque generat ion and switching, and FliF, a transmembrane structural protein. The motor portion of the flagellum (the basal body complex) has a cytoplasmic C ring and a transmembrane M ring. Incubation of purified basal bodies at pH 4.5 r emoved FliM and FliN but not FliG or FliF. These basal bodies lacked C ring s but had intact M rings, suggesting that FliM and FliN are part of the C r ing but not a detectable part of the M ring., Incubation of basal bodies at pH 2.5 removed FliG, FliM, and FliN but not FliF. These basal bodies lacke d the C ring, and the cytoplasmic face of the M ring was altered, suggestin g that FliG makes, up at least part of the cytoplasmic face of the M ring. Further insights into FliG were obtained from cells expressing a fusion pro tein of FliF and FliG. Flagella from these mutants still rotated but cells were not chemotactic. One mutant is a full-length fusion of FliF and FliG; the second mutant has a deletion lacking the last 56 residues, of FliF and the first 94 residues of FliG. In the former, C rings appeared complete, bu t a portion of the M ring was shifted to higher radius. The C-ring-M-ring i nteraction appeared to be altered. In basal bodies with the fusion-deletion protein, the C ring was smaller in diameter, and one of its domains occupi ed space vacated by missing portions of FliF and FliG.