3.A.1.106.1
Phospholipid, LPS, lipid A and drug exporter, MsbA, which flips the substrate from the inner leaflet of the cytoplasmic membrane to the outer leaflet (Eckford and Sharom, 2010). MsbA also confers drug resistance to azidopine, daunomycin, vinblastine, Hoechst 33342 and ethidium (Reuter et al., 2003). Four x-ray structures, trapped in different conformations, two with and two without nucleotide, have been solved (Ward et al., 2007). They suggest an alternating accessibility mode of transport with major conformational changes.  The mechanism and conformational transitions have been discussed (Moradi and Tajkhorshid 2013).  MsbA is energized both by ATP hydrolysis and the H⁺ electrochemical gradient (Singh et al. 2016). Mi et al. 2017 used single-particle cryo-electron microscopy to elucidate the structures of lipid-nanodisc-embedded MsbA in three functional states. The 4.2 Å-resolution structure of the transmembrane domains of nucleotide-free MsbA revealed that LPS binds deeply inside MsbA at the height of the periplasmic leaflet. Two sub-nanometre-resolution structures of MsbA with ADP-vanadate and ADP revealed a closed and an inward-facing conformation, respectively. A  2.9 A resolution structure of MsbA in complex with G907, a selective small-molecule antagonist with bactericidal activity, revealed an unanticipated mechanism of ABC transporter inhibition. G907 traps MsbA in an inward-facing, lipopolysaccharide-bound conformation by wedging into an architecturally conserved transmembrane pocket. A second allosteric mechanism of antagonism occurs through structural and functional uncoupling of the nucleotide-binding domains (Ho et al. 2018). Coupled ATPase-adenylate kinase activity in ABC transporters including MsbA has been demonstrated (Kaur et al. 2016). Close-proximity effects and structural coupling of the transmembrane domains with the NBDs has been suggested (Josts et al. 2019). Two first-generation inhibitors of MsbA, TBT1 and G247, induce opposite effects on ATP hydrolysis. Using single-particle cryo-electron microscopy and functional assays, TBT1 and G247 were found to bind adjacent yet separate pockets in the MsbA transmembrane domains (Thélot et al. 2021). MsbA adopts the wide inward-open conformation in E. coli cells (Galazzo et al. 2022).