Biogenesis of the Flagellar Switch Complex in Escherichia coli: Formation of Sub-Complexes Independently of the Basal-Body MS-Ring.

Direction switching in the flagellar motor of Escherichia coli is under the control of a complex on the rotor formed from the proteins FliG, FliM, and FliN. FliG lies at the top of the switch complex (i.e., nearest the membrane) and is arranged with its C-terminal domain (FliGC) resting on the middle domain (FliGM) of the neighboring subunit. This organization requires the protein to adopt an open conformation that exposes the surfaces engaging in intersubunit FliGC/FliGM contacts. In a recent study, Baker and coworkers [13] obtained evidence that FliG in the cytosol is monomeric and takes on a more compact conformation, with FliGC making intramolecular contact with FliGM of the same subunit. In the present work, we examine the conformational preferences and interactions of FliG through in vivo crosslinking experiments in cells that lack either all other flagellar proteins or just the MS-ring protein FliF. The results indicate that FliG has a significant tendency to form multimers independently of other flagellar components. The multimerization of FliG is promoted by FliF and also by FliM. FliM does not multimerize efficiently by itself but does so in the presence of FliG. Thus, pre-assemblies of the switch-complex proteins can form in the cytosol and might function as intermediates in assembly.