Isolation, characterization and structure of bacterial flagellar motors containing the switch complex

J Mol Biol. 1994 Jan 28;235(4):1261-70. doi: 10.1006/jmbi.1994.1079.


A putative complex of the three switch proteins, FliG, FliM and FliN appears to be directly involved in torque generation and control of direction of rotation. We have developed a preparative procedure for flagellar motors that retains these proteins as evidenced by Western blots using anti-FliG, anti-FliM and anti-FliN antibodies. Immunogold labeling with these three antibodies shows that the three switch proteins are localized to the motor. Electron micrographs of frozen-hydrated preparations reveal a large, new component we have termed the "C ring complex" attached to the cytoplasmic face of the M ring. In a three-dimensional reconstruction of the cylindrically averaged structure, the M-S ring complex appears thicker and wider by the addition of extra material to the cytoplasmic surface of the M ring. In addition, extending into the cytoplasm from the thickened M ring is the C ring complex, a thin-walled cylinder having a length of 170 A and an outer diameter of 450 A compared to the 290 A diameter of the M ring. We provide evidence that the thickened M ring contains FliG and that the C ring complex may contain FliM and FliN. The large diameter of the C ring complex may permit interaction with the M ring and with the circlet of studs thought to be the MotA/MotB complex.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Bacterial Proteins / isolation & purification*
  • Bacterial Proteins / ultrastructure
  • Flagella / chemistry*
  • Flagella / ultrastructure
  • Image Processing, Computer-Assisted
  • Immunoblotting
  • Salmonella typhimurium / chemistry
  • Salmonella typhimurium / ultrastructure*


  • Bacterial Proteins
  • FliN protein, Bacteria
  • Flig protein, Bacteria
  • FliM protein, Bacteria