Drug efflux by a small multidrug resistance protein is inhibited by a transmembrane peptide

Drug-resistant bacteria use several families of membrane-embedded transporters to remove antibiotics from the cell. One such family is the small multidrug resistance proteins (SMRs) that, because of their relatively small size (ca. 110 residues with four transmembrane [TM] helices), must form (at least) dimers to efflux drugs. Here, we use a Lys-tagged synthetic peptide with exactly the same sequence as TM4 of the full-length SMR Hsmr from Halobacterium salinarum [TM4 sequence: AcA(Sar)(3)-VAGVVGLALIVAGVVVLNVAS-KKK (Sar = N-methylglycine)] to compete with and disrupt the native TM4-TM4 interactions believed to constitute the locus of Hsmr dimerization. Using a cellular efflux assay of the fluorescent SMR substrate ethidium bromide, we determined that bacterial cells containing Hsmr are able to remove cellular ethidium via first-order exponential decay with a rate constant (k) of 10.1 × 10(-3) ± 0.7 × 10(-3) s(-1). Upon treatment of the cells with the TM4 peptide, we observed a saturable ~60% decrease in the efflux rate constant to 3.7 × 10(-3) ± 0.2 × 10(-3) s(-1). In corresponding experiments with control peptides, including scrambled sequences and a sequence with d-chirality, a decrease in ethidium efflux either was not observed or was marginal, likely from nonspecific effects. The designed peptides did not evoke bacterial lysis, indicating that they act via the α-helicity and membrane insertion propensities of the native TM4 helix. Our overall results suggest that this approach could conceivably be used to design hydrophobic peptides for disruption of key TM-TM interactions of membrane proteins and represent a valuable route to the discovery of new therapeutics.