An energy transduction mechanism used in bacterial flagellar type III protein export

Abstract

Flagellar proteins of bacteria are exported by a specific export apparatus. FliI ATPase forms a complex with FliH and FliJ and escorts export substrates from the cytoplasm to the export gate complex, which is made up of six membrane proteins. The export gate complex utilizes proton motive force across the cytoplasmic membrane for protein translocation, but the mechanism remains unknown. Here we show that the export gate complex by itself is a proton–protein antiporter that uses the two components of proton motive force, Δψ and ΔpH, for different steps of the protein export process. However, in the presence of FliH, FliI and FliJ, a specific binding of FliJ with an export gate membrane protein, FlhA, is brought about by the FliH–FliI complex, which turns the export gate into a highly efficient, Δψ-driven protein export apparatus.

Protein Export According to Schedule: Architecture, Assembly, and Regulation of Type III Secretion Systems from Plant- and Animal-Pathogenic Bacteria

“…However, as mentioned above, flagellar T3S can also occur in the absence of the ATPase and is then driven by the PMF, which consists of ⌬⌿ and ⌬pH. A recent publication revealed a contribution of FliJ to the ⌬⌿-driven export of flagellar T3S substrates that probably depends on the interaction of FliJ with the linker region of the cytoplasmic domain of the IM protein FlhA (FlhA C ) (384). In agreement with this hypothesis, the binding sites of FliJ in FlhA were shown to be required for the functioning of both proteins, and vice versa.…”

“…PMF was also shown to contribute to flagellar T3S in Salmonella spp. in the absence of the ATPase FliI and its regulator FliH (376,384,435). It was therefore proposed that the PMF drives protein transport across both membranes, whereas the ATPase is required for the efficient initial docking of T3S substrates to the secretion channel.…”

“…In agreement with this hypothesis, the binding sites of FliJ in FlhA were shown to be required for the functioning of both proteins, and vice versa. The authors therefore proposed that FliJ alters the conformation of FlhA C to activate ⌬⌿-driven protein export (384).…”

Insight into the flagella type III export revealed by the complex structure of the type III ATPase and its regulator

“…Interestingly, FliJ can partially act as a rotor within the central pore of the A 3 B 3 complex of the Thermus thermophilus A-type ATPase (Tt A-ATPase) (15). It has been shown that infrequent ATP hydrolysis is sufficient for processive protein transport during flagellar assembly, suggesting that ATP hydrolysis by the FliI 6 FliJ complex activates the export gate through an interaction between FliJ and FlhA (16)(17)(18)(19).…”

Crystallization and preliminary X-ray analysis of the periplasmic domain of FliP, an integral membrane component of the bacterial flagellar type III protein-export apparatus

“…Both FlhA C and FlhB C act as an export switch to coordinate flagellar protein export with assembly (Hirano et al, 2009;Kinoshita et al, 2013). A specific interaction of FliJ with FlhA brought about by FliH and FliI allows the export gate to utilize only the electric potential difference part (Á ) of the proton motive force across the cytoplasmic membrane to drive protein translocation Paul et al, 2008;Minamino et al, 2011;Ibuki et al, 2013). Although crystal structures of FlhA C and FlhB C have been solved by X-ray crystallography (Bange et al, 2010;Meshcheryakov et al, 2013;Moore & Jia, 2010;Saijo-Hamano et al, 2010), it remains unknown how the export gate recognizes export substrates according to flagellar assembly and transports them in a Á -dependent manner.…”