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 is known to form a proton channel.
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.