Functional insights of two MATE transporters from Vibrio fluvialis

Functional characterization of H- and D-MATE (Multidrug and Toxin Extrusion) transporters from clinical isolates of Vibrio fluvialis revealed H-type conferred resistance to fluoroquinolones, ethidium bromide and safranin whereas D-type exhibited marginal resistance towards ethidium bromide only. Both H-/D-type transporters were inhibited by reserpine resulting in increased intracellular norfloxacin concentration. The efflux was facilitated by both Na+/K+ ions, suggesting that these efflux pumps were ion-dependent. In presence of various classes of EPIs, there was decrease in MIC exhibited by H-/D-type efflux pumps towards norfloxacin which didnt translate into transport inhibition. But reserpine presented a conclusive pattern with decrease in MIC towards norfloxacin and increased norfloxacin accumulation inferring maximum inhibition. Substrate binding and electrostatic charge distribution of both the transporters was similar to other known MATE transporters. The H-type exhibited 10 transmembranes and D-type exhibited 11 TMs which was different from other MATE transporters known to have 12 TMs (Transmembranes). Data derived from molecular docking and ion binding studies revealed that Aspartic Acid residue in 1st TM acts as ion binding site with transport mechanism similar to NorM. Electrostatic potential map of both the transporters revealed that there is a cavity formation within the transporters surrounded by charged electronegative amino acid residues. Interestingly, surface models of both transporters revealed that 1st TM forms covalent bond with 7th TM towards extracellular space. Docking studies also revealed that reserpine covalently binds to central pocket of both transporters and serves as excellent EPI against these transporters as evidenced by MIC and drug accumulation assays.