Chimeric analysis of the multicomponent multidrug efflux transporters from gram-negative bacteria

Many multidrug transporters from gram-negative bacteria belong to the resistance-nodulation-cell division (RND) superfamily of transporters. RND-type multidrug transporters have an extremely broad substrate specificity and protect bacterial cells from the actions of antibiotics on both sides of the cytoplasmic membrane. They usually function as three-component assemblies spanning the outer and cytoplasmic membranes and the periplasmic space of gram-negative bacteria. The structural determinants of RND transporters responsible for multidrug recognition and complex assembly remain unknown. We constructed chimeric RND transporters composed of N-terminal residues of AcrB and C-terminal residues of MexB, the major RND-type transporters from Escherichia coli and Pseudomonas aeruginosa, respectively. The assembly of complexes and multidrug efflux activities of chimeric transporters were determined by coexpression of hybrid genes either with AcrA, the periplasmic component of the AcrAB transporter from E. coli, or with MexA and OprM, the accessory proteins of the MexAB-OprM pump from P. aeruginosa. We found that the specificity of interaction with the corresponding periplasmic component is encoded in the T60-V612 region of transporters. Our results also suggest that the large periplasmic loops of RND-type transporters are involved in multidrug recognition and efflux.