Abstract

The direction of rotation of the Escherichia coli flagellum is controlled by an assembly called the switch complex formed from multiple subunits of the proteins FliG, FliM, and FliN. Structurally, the switch complex corresponds to a drum-shaped feature at the bottom of the basal body, termed the C-ring. Stimulus-regulated reversals in flagellar motor rotation are the basis for directed movement such as chemotaxis. In E. coli, the motors turn counterclockwise (CCW) in their default state, allowing the several filaments on a cell to join together in a bundle and propel the cell smoothly forward. In response to the chemotaxis signaling molecule phospho-CheY (CheY^P), the motors can switch to clockwise (CW) rotation, causing dissociation of the filament bundle and reorientation of the cell. CheY^P has previously been shown to bind to a conserved segment near the N terminus of FliM. Here, we show that this interaction serves to capture CheY^P and that the switch to CW rotation involves the subsequent interaction of CheY^P with FliN. FliN is located at the bottom of the C-ring, in close association with the C-terminal domain of FliM (FliM_C), and the switch to CW rotation has been shown to involve relative movement of FliN and FliM_C. Using a recently developed structural model for the FliN/FliM_C array, and the CheY^P-binding site here identified on FliN, we propose a mechanism by which CheY^P binding could induce the conformational switch to CW rotation.