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 two distant families, each with a distinct function. The Mot family energizes bacterial flagellar rotation while the Exb 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 pmf (or smf) is the driving force in both cases. MotAB and (PomAB) are homologous to ExbBD and TolQR. MotAB, 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).
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 TM 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 number of protons per rotation, suggesting that gating is the rate-limiting step (Nishihara and Kitao 2015). Structure and dynamics of the MotA/B with nonprotonated and protonated Asp32 suggested a size-dependent ion selectivity. In MotA/B with the nonprotonated Asp32, the A3 segment in MotA maintained a kink whereas protonation induced 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).