The three functional regions of bacterial flagellar switch protein FliM.

摘要

The flagellar switch of Salmonella typhimurium and Escherichia coli controls the direction of rotation of flagellar motors in response to signals from the environment. During counterclockwise rotation, the helical flagellar filaments form a bundle that propels the cell forward in a smooth swimming motion; during clockwise rotation, the bundle flies apart and the smooth swimming is interrupted by erratic tumbling that randomly reorients the cell. The switch is a complex of three proteins, FliG, FliM, and FliN, that is believed to interact with a chemotaxis protein, CheY. Each of the three switch proteins is involved in several aspects of flagellar motor function: assembly of the flagellum, motor rotation, and switching. To probe the detailed workings of the switch complex, I have carried out functional and biochemical analyses of FliM. I have systematically mutagenized the protein and observed the effects of these mutations on its function. I have also used affinity blotting to examine the interactions of FliM with FliG, FliN, and CheY. These experiments suggest that the FliM polypeptide is composed of three distinct functional regions that correlate roughly with division of the protein into thirds: the N-terminal third binds CheY and is primarily involved in switching; the middle third binds FliG and is primarily involved in motor rotation; the C-terminal third binds FliN and is primarily involved in assembly of FliM into flagellar structure.