Mechanisms of RND multidrug efflux pumps

Abstract

SummaryRND (Resistance-Nodulation-Division) family transporters are widespread especially among Gramnegative bacteria, and catalyze the active efflux of many antibiotics and chemotherapeutic agents. They have very large periplasmic domains, and form tripartite complexes with outer membrane channels and periplasmic adaptor proteins. AcrAB-TolC complex of Escherichia coli, which pumps out a very wide range of drugs, has been studied most intensively. Early studies showed that the transporter captures even those substrates that cannot permeate across the cytoplasmic membrane, such as dianionic β-lactams, suggesting that the capture can occur from the periplasm. It was also suggested that the capture occurs from the cytoplasmic membrane/periplasm interface, because most substrates contain a sizable hydrophobic domain; however, this may simply be a reflection of the nature of the binding site within AcrB. Genetic studies of chimeric transporters showed that much of the substrate specificity is determined by their periplasmic domains. Biochemical studies with intact cells recently led to the determination of the kinetic constants of AcrB for some β-lactams, and the result confirms the old prediction that AcrB is a rather slow pump. Reconstitution of purified AcrB and its relatives showed that the pump is a drug/proton antiporter, that AcrA strongly stimulates the activity of the pump, and that AcrB seems to have a highest affinity for conjugated bile salts. Structural study with mutants of the network of charged residues in the transmembrane domain showed that protonation here produced a far-reaching conformational change, which was found to be present in one of the protomers in the asymmetric crystal structure of the wild-type AcrB. The functional rotatory hypothesis then predicts that the drug bound in the periplasmic domain is extruded through this conformational change initiated by the protonation of one of the residues in the aforementioned network, an idea that was recently supported by disulfide cross-linking as well as by the behavior of linked AcrB protomers.

Quinazoline derivatives are efficient chemosensitizers of antibiotic activity in Enterobacter aerogenes, Klebsiella pneumoniae and Pseudomonas aeruginosa resistant strains

“…Moreover, several recent data on the E. coli AcrB efflux mechanism indicate that some amino acid residues are involved in the transport process to a varying extent during specific steps such as binding, transport, etc. [14][15][16]. This can explain the divergence observed in the competitive capacity of the compounds with drugs of different classes (chloramphenicol, quinolones, ethidium bromide) for binding sites within the pump type (AcrB, MexB) as well as the level of expressed efflux pump activity in the tested strains.…”

Ionic liquid-tolerant microorganisms and microbial communities for lignocellulose conversion to bioproducts

“…Efflux pumps are included in a class of membrane transporters that export toxins from the cell using the proton motive force (Dunlop et al 2011;Nikaido and Takatsuka 2009;Putman et al 2000) and have been shown to facilitate microbial resistance to toxic compounds including hydrocarbons (Delcour 2009;Dunlop et al 2011). …”

Fe(II) Oxidation Is an Innate Capability of Nitrate-Reducing Bacteria That Involves Abiotic and Biotic Reactions

“…The expression of heavy metal efflux pumps in response to anaerobic Fe(II) stress is a novel finding and suggests that these proteins may be involved in the export of metals other than those commonly considered to be substrates (40,41). Furthermore, because the RND family efflux pumps identified in A. ebreus are proton/solute antiporters, the energy cost is putatively less than that of an ATP-consuming transporter and RND family efflux pumps are able to load substrate from either the periplasm or the cytoplasm (42).…”