| The FliG, FliM, and FliN proteins of Escherichia coli are essential for flagellar assembly and function. A combination of physiological, molecular genetic, and biochemical approaches was used here to study the arrangement of these proteins and their functions in the flagellar motor. The results provide evidence that FliG, FliM, and FliN function together in a complex on the rotor of the flagellar motor, which is distinct from the MotA/MotB torque generators. Specifically, FliG, FliM, and FliN bind one another in all possible pairwise combinations, and each also self-associates. FliG and FliM also interact weakly with the stator component MotA; these interactions might be important for torque generation. A set of mot-mutations previously reported in fliM and fliN was also examined in this study. The results suggest that FliM and FliN are less closely involved in torque generation than previously believed. The site of torque generation is likely, therefore, to be on FliG. Finally, the domain organization of the FliM protein was analyzed. The data suggest that approximately 90 residues at the C-terminus of FliM form an independently folded domain that is the primary binding site for FliN, and that the FliM/FliN interaction is essential for flagellar assembly. |
