Distinct regions of bacterial flagellar switch protein FliM interact with FliG, FliN and CheY

The FliG, FliM, and FliN proteins of the bacterial flagellar motor are believed to interact with one another to form the switch complex, which in turn is thought to interact with one of the chemotaxis proteins, CheY. In particular, FliM appears to be an intermediary between CheY and FliG: the current model suggests that CheY, when phosphorylated (CheY-P), binds to FliM and produces a conformational change in FliM that is propagated to FliG. The result of these interactions is to induce clockwise rotation of the flagellar motors and tumbling of the cell. Various genetic and biochemical studies have provided evidence that the switch proteins associate with each other and that CheY-P binds to FliM. Here, we have used affinity blotting to obtain direct evidence of interaction between Salmonella typhimurium FliM and FliN, FliM and FliG, and FliM and CheY-P. We have also examined the ability of various FliM deletion and truncation mutant proteins to bind to FliN, FliG, and CheY-P. From these data, we conclude that distinct regions of the FliM protein bind to each of these other proteins. We propose a model in which the N-terminal region of FliM binds to CheY-P, the middle region of FliM binds to FliG, and the C-terminal region binds to FliN.