1.A.104 The Proposed Flagellar Biosynthesis Na⁺ Channel, FlhA (FlhA) Family 

The flagellar type III export apparatus utilizes ATP and the proton motive force (PMF) to transport flagellar proteins to the distal end of the growing flagellar structure for self-assembly. The transmembrane export gate complex is a H⁺-protein antiporter. Activity is greatly augmented by an associated cytoplasmic ATPase complex. Minamino et al. 2016 reported that the export gate complex can use the sodium motive force (SMF) in addition to the PMF to drive protein export. Protein export was considerably reduced in the absence of the ATPase complex and a pH gradient across the membrane, but Na⁺ increased it dramatically. Phenamil, a blocker of Na⁺ translocation, inhibited protein export. Overexpression of FlhA increased the intracellular Na⁺ concentration in the presence of 100 mM external NaCl but not in its absence, suggesting that FlhA acts as a Na⁺ channel. In wild-type cells, however, neither Na⁺ nor phenamil affected protein export, indicating that the Na⁺ channel activity of FlhA is suppressed by the ATPase complex. Minamino et al. 2016 proposed that the export gate by itself is a dual fuel engine that uses both the PMF and the SMF for protein export, and that the ATPase complex switches this dual fuel engine into a PMF-driven export machinery to become much more robust against environmental changes in external pH and Na⁺ concentration. Δpsi is required for efficient interaction between FliJ and FlhA to open the FlhA ion channel to conduct protons which drives flagellar protein export in a Δpsi-dependent manner (Minamino et al. 2021).

A conserved GYXLI motif in FlhA is involved in dynamic domain motions of FlhA and is required for flagellar protein export (Minamino et al. 2022). The C-terminal cytoplasmic domain of FlhA (FlhAC) serves as a docking platform for export of substrates and flagellar chaperones, and it plays an important role in hierarchical protein targeting and export. FlhAC consists of domains D1, D2, D3, and D4 and adopts open and closed conformations. Gly-368 of Salmonella FlhA is located within the highly conserved GYXLI motif and is critical for the dynamic domain motions of FlhAC. Periodic conformational changes of the GYXLI motif induce a remodeling of hydrophobic side chain interaction networks in FlhAC and promote the cyclic open-close domain motions of FlhAC (Minamino et al. 2022).