MexY-Promoted Aminoglycoside Resistance in Pseudomonas aeruginosa: Involvement of a Putative Proximal Binding Pocket in Aminoglycoside Recognition

The resistance-nodulation-division (RND) family multidrug efflux system MexXY-OprM is a major determinant of aminoglycoside resistance in Pseudomonas aeruginosa, although the details of aminoglycoside recognition and export by MexY, the substrate-binding RND component of this efflux system, have not been elucidated. To identify regions/residues of MexY important for aminoglycoside resistance, plasmid-borne mexY was mutagenized and mutations that impaired MexY-promoted aminoglycoside (streptomycin) resistance were identified in a Delta mexY strain of P. aeruginosa. Sixty-one streptomycin-sensitive mexY mutants were recovered; among these, 7 unique mutations that yielded wild-type levels of MexY expression were identified. These mutations compromised resistance to additional aminoglycosides and to other antimicrobials and occurred in both the transmembrane and periplasmic regions of the protein. Mapping of the mutated residues onto a 3-dimensional structure of MexY modeled on Escherichia coli AcrB revealed that these tended to occur in regions implicated in general pump operation (transmembrane domain) and MexY trimer assembly (docking domain) and, thus, did not provide insights into aminoglycoside recognition. A region corresponding to a proximal binding pocket connected to a periplasm-linked cleft, part of a drug export pathway of AcrB, was identified in MexY and proposed to play a role in aminoglycoside recognition. To test this, selected residues (K79, D133, and Y613) within this pocket were mutagenized and the impact on aminoglycoside resistance was assessed. Mutations of D133 and Y613 compromised aminoglycoside resistance, while, surprisingly, the K79 mutation enhanced aminoglycoside resistance, confirming a role for this putative proximal binding pocket in aminoglycoside recognition and export. IMPORTANCE Bacterial RND pumps do not typically accommodate highly hydrophilic agents such as aminoglycosides, and it is unclear how those, such as MexY, which accommodate these unique substrates, do so. The results presented here indicate that aminoglycosides are likely not captured and exported by this RND pump component in a unique manner but rather utilize a previously defined export pathway that involves a proximal drug-binding pocket that is also implicated in the export of nonaminoglycosides. The observation, too, that a mutation in this pocket enhances MexY-mediated aminoglycoside resistance (K79A), an indication that it is not optimally designed to accommodate these agents, lends further support to earlier proposals that antimicrobials are not the intended pump substrates.