Subunit organization and reversal-associated movements in the flagellar switch of Escherichia coli

Bacterial flagella contain a rotor-mounted protein complex termed the switch complex that functions in flagellar assembly, rotation, and clockwise/counterclockwise direction control. In Escherichia coli and Salmonella, the switch complex contains the proteins FliG, FliM, and FliN and corresponds structurally with the C-ring in the flagellar basal body. Certain features of subunit organization in the switch complex have been deduced previously, but details of subunit organization in the lower part of the C-ring and the molecular movements responsible for motor switching remain unclear. In this study, we use cross-linking, binding, and mutational experiments to examine subunit organization in the bottom of the C-ring and to probe movements that occur upon switching. The results show that FliN tetramers alternate with FliM C-terminal domains to form the bottom of the C-ring in an arrangement that closely reproduces the major features observed in electron microscopic reconstructions. When motors were switched to clockwise rotation by a repellent stimulus, cross-link yields were altered in a pattern indicating relative movement of FliN and FliM(C). These results are discussed in the framework of a structurally grounded hypothesis for the switching mechanism.