Coupling of Type III Secretion to the Membrane Potential

The bacterial flagellum and the virulence-associated bacterial needle are intricate nanomachines, which possess a type III secretion system for export of external components. The type III secretion system contains five highly conserved, essential membrane proteins. In the flagellum, these proteins are named: FlhA, FlhB, FliP, FliQ, and FliR. To understand how the secretion system functions, it will be necessary to characterize how these proteins interact and are regulated. This study reveals that modulating the level of the electron-carrier ubiquinone (Coenzyme Q) in the membrane can control protein export through flagellar secretion system. It is also shown that the N-terminal transmembrane regions of FlhA and FlhB are functionally associated. In the Salmonella flagellum, wild-type FlhB was replaced with a homologous FlhB chimera, made of the N-terminal transmembrane region of Aquifex aeolicus FlhB fused to the C-terminal cytoplasmic domain of Salmonella FlhB. Cells expressing the FlhB chimera were mostly non-motile and only rare flagella were made. Flagellar biogenesis was recovered in motile suppressor mutants isolated from soft-typtone agar, expressing the FlhB chimera and bearing spontaneous suppressor mutations. The identified suppressor mutations belonged to two categories: the first category of mutation were found in non-flagellar genes, affecting enzymes for the biosynthesis of ubiquinone; and the second category of mutation were encoded by the flhA gene. The mutations in the genes for the biosynthesis of ubiquinone decreased levels of ubiquinone and this reduced the membrane potential. Consequently, efficient flagellar assembly requires coupling between export through the type III secretion system and the energized state of the membrane.