1.A.30 The H+- or Na+-translocating Bacterial Flagellar Motor/ExbBD Outer Membrane Transport Energizer (Mot-Exb) Superfamily
The Mot-Exb Superfamily consists of six distant families, each probably with a distinct physiological function, although all may function as H+/Na+ channels, driving an energy-requiring process. The MotAB family energizes bacterial flagellar rotation while the ExbBD family energizes accumulation of large molecules (i.e. iron-siderophores, vitamin B12, DNA from phage, and colicins) from the external medium across the outer Gram-negative bacterial membrane into the periplasm. The AglRS system powers gliding motility while the SilAB systems energize gian adhesin export. The function of a 5th family (TC# 1.A.30.5) is not known, but the ZorAB systems have been reported to function as parts of antiphage defense systems. The pmf (or smf) is probably the driving force in all cases. MotAB and PomAB are homologous to ExbBD and TolQR. MotAB of E. coli, the stator, is known to form a proton channel. This stator is composed of MotA and MotB proteins, which form a hetero-hexameric complex with a stoichiometry of four MotA and two MotB molecules. MotA can form a tetramer in the absence of MotB (Takekawa et al. 2016).
About 10 stators (MotA/MotB complexes) are docked around a rotor, and the stator recruitment depends on the load, ion motive force, and coupling ion flux. The MotA(M206I) mutation slows motor rotation and decreases the number of docked stators in Salmonella. Suzuki et al. 2019 showed that lowering the external pH improves the assembly of the mutant stators. Neither the collapse of the ion motive force nor a mutation mimicking the proton-binding state inhibited stator localization to the motor. Thus, MotA-Met206 is involved in torque generation and proton translocation, and stator assembly is stabilized by protonation of the stator.
Yonekura et al. (2011) presented the first three-dimensional structure of the PomAB torque-generating stator unit analyzed by electron microscopy. The structure of PomAB revealed two arm domains, which contain the PG-binding site, connected to a large base made of the transmembrane and cytoplasmic domains. The arms lean downward to the membrane surface, likely representing a 'plugged' conformation, which would prevent ions leaking through the channel. They propose a model for how PomAB units are placed around the flagellar basal body to function as torque generators.
Leu46 of MotB acts as the gate for hydronium ion permeation, which induces the formation of a water wire that may mediate the proton transfer to Asp32 on MotB. The free energy barrier for H3O+ permeation is consistent with the proton transfer rate deduced from the flagellar rotational speed and the number of protons per rotation, suggesting that gating is the rate-limiting step (Nishihara and Kitao 2015). Structure and dynamics of MotA/B with nonprotonated and protonated Asp32 suggested size-dependent ion selectivity. In MotA/B with the nonprotonated Asp32, the A3 segment in MotA maintains a kink whereas protonation induces a straighter shape. Assuming that the cytoplasmic domain not included in the atomic model moves as a rigid body, the protonation/deprotonation of Asp32 is inferred to induce a ratchet motion of the cytoplasmic domain, correlated with the motion of the flagellar rotor (Nishihara and Kitao 2015).
ExbBD forms both hexameric and pentameric complexes that coexist, with the proportion of the hexamers increasing with pH. Channel current measurements and 2D crystallography thus support the existence of and transition between the two oligomeric states in membranes. The hexameric complex consists of six ExbB subunits and three ExbD transmembrane helices enclosed within the central channel (Maki-Yonekura et al. 2018).