Graduate School of Frontier Biosciences, Osaka University

Japanese

Energy transduction mechanism used in bacterial flagellar type III protein export

Journal Nat Commun 2, 475 (2011)
Authors Tohru Minamino, Yusuke V. Morimoto, Noritaka Hara and Keiichi Namba
Title An energy transduction mechanism used in bacterial flagellar type III protein export
PubMed 21934659
Laboratory JEOL YOKOGUSHI Research Alliance Laboratories 〈Prof. Namba〉
Abstract Proton motive force (PMF), the electrochemical potential difference of protons across a biological membrane, is utilized as the energy source of many biological activities, including protein export, ion transport, ATP synthesis, and flagellar motor rotation. PMF consists of two components: the electric potential difference (Δψ) and the proton concentration difference (ΔpH). Δψ and ΔpH are equivalent driving forces for proton movement in physics.

Salmonella enterica can swim by rotating multiple flagella that arise randomly over the cell surface (Fig. 1a). The bacterial flagellum consists of at least three substructures: the basal body, the hook and the filament (Fig. 1b and 1c). Flagellar assembly begins with the basal body, followed by the hook and finally the filament. For construction of the flagellum, many of the flagellar component proteins are transported to the distal end of the flagellar structure by a specific export apparatus. The export apparatus consists of three soluble proteins, FliH, FliI, and FliJ, and six integral membrane proteins, FlhA, FlhB, FliO, FliP, FliQ, and FliR. FliI forms a complex with FliH and FliJ and escorts export substrates from the cytoplasm to the export gate complex, which is made of the six membrane proteins. The export gate utilizes PMF for protein translocation, but the mechanism remains unknown.

In this report, we have shown that Δψ alone is sufficient for flagellar protein export in wild-type cells, but both Δψ and ΔpH become essential in the absence of FliH and FliI. These results indicate that the two components of PMF play distinct roles in the flagellar protein export process in the absence of FliH and FliI. Therefore, we suggest that the export gate by itself is a proton-protein antiporter that uses Δψ and ΔpH for different steps of the protein export process. We have also shown that FliJ requires the support of FliH and FliI to efficiently and properly interact with FlhA for the full activation of the export gate to be an efficient, Δψ–driven protein export apparatus. This suggests that a specific interaction of FliJ with FlhA brought about by the FliH-FliI complex turns the export gate, which by itself is a low-efficient proton-protein antiporter, into a highly efficient, Δψ-driven protein export apparatus (Fig. 2).

Figure 1
Schematic drawing and electron micrograph of the bacterial flagellum. (a) Swimming bacteria with a bundle of helical flagellar filaments. (b) Electron micrograph of the flagellum isolated from Salmonella wild-type cells. (c) Schematic diagrams of the flagellum, which consists of three parts: the basal body, which acts as a reversible rotary motor; the hook, which functions as a universal joint; and the filament, which acts as a helical screw. OM, outer membrane; PG, peptidoglycan layer; CM, cytoplasmic membrane.

Fig1_Minamino.jpg

Figure 2
Schematic diagrams of the flagellar protein export apparatus. The flagellar protein export apparatus consists of six integral membrane proteins, FlhA, FlhB, FliO, FliP, FliQ, and FliR, and three soluble proteins, FliH, FliI, and FliJ. The export gate, which is made up of the six integral membrane proteins, utilizes PMF as the energy source for flagellar protein export. The export gate is intrinsically a proton-protein antiporter that requires both Δψ and ΔpH as the energy source. In contrast, in the presence of FliH, FliI and FliJ, a specific binding of FliJ with FlhA is brought about by the FliH-FliI complex and turns the export gate into a highly efficient, Δψ-driven protein export apparatus. OM, outer membrane; PG, peptidoglycan layer; CM, cytoplasmic membrane; H+, proton.

Fig.2_Minamino.jpg