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 generation 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 removed FliM and FliN but not FliG or FliF. These basal bodies lacked C rings but had intact M rings, suggesting that FliM and FliN are part of the C ring 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 lacked the C ring, and the cytoplasmic face of the M ring was altered, suggesting 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 protein 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, but a portion of the M ring was shifted to higher radius. The C-ring-M-ring interaction appeared to be altered. In basal bodies with the fusion-deletion protein, the C ring was smaller in diameter, and one of its domains occupied space vacated by missing portions of FliF and FliG.
Thomas
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W. M. Keck Institute for Cellular Visualization, Rosenstiel Basic Medical Sciences Research Center, and Department of Biology, Brandeis University, Waltham, Massachusetts 02454.
Morgan
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DeRosier
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D019677:Artificial Gene Fusion; D001426:Bacterial Proteins / Q000032:analysis* / Q000235:genetics* / Q000502:physiology; D005407:Flagella / Q000737:chemistry* / Q000648:ultrastructure*; D007091:Image Processing, Computer-Assisted; D008565:Membrane Proteins; D020409:Molecular Motor Proteins / Q000737:chemistry / Q000648:ultrastructure; D009154:Mutation; D011487:Protein Conformation; D011993:Recombinant Fusion Proteins / Q000032:analysis; D012486:Salmonella typhimurium / Q000737:chemistry / Q000648:ultrastructure; D017384:Sequence Deletion
D001426:Bacterial Proteins; C075661:Flif protein, Bacteria; C075660:Flig protein, Bacteria; D008565:Membrane Proteins; D020409:Molecular Motor Proteins; D011993:Recombinant Fusion Proteins