Multidrug Resistance Transporter Dynamics
Multidrug resistance promotes cell survival in the presence of toxic compounds such as many antibacterial or cancer chemotherapeutic agents. Often, multidrug resistance results from expression of transport proteins that eject the toxicant from the cell in exchange for Na+ or H+ ions. Researchers believe that most of these transporters work by transitioning between a number of distinct conformations that move the substrate and ions from one side of the membrane to the other. Thus, the inability to capture more than one conformation in crystal structures of the multidrug and toxic compound extrusion (MATE) family transporter NorM-Vc from Vibrio cholerae posed a serious challenge to understanding the protein’s function. To address this problem, Vanderbilt Basic Sciences investigator Hassane Mchaourab and his lab applied the technique of double electron-electron resonance (DEER) to evaluate conformational changes occurring in NorM-Vc in response to substrate and ion binding. Deer provides a way to measure the distance between different residues in a protein under various conditions likely to result in conformational changes. The researchers’ experiments demonstrated that the conformation of the protein captured in reported crystal structures is likely not favored in solution. They also identified a group of highly conserved residues (Asp-36, Asn-174, Asn-178, and Thr-200) in NorM-Vc’s N-terminal domain (NTD) that likely serve as a site of ion-mediated conformational change. Furthermore, they showed that two conserved carboxyl-containing residues (Glu-255 and Asp-371) in the C-terminal domain (CTD) are involved in protonation/deprotonation-mediated substrate binding and release rather than serving as a Na+ binding site as previously thought. The results suggest that ion entry into the NTD induces conformational transitions that promote substrate release, which is further facilitated by protonation of Glu-255 and Asp-371 in the CTD. The findings provide a solid foundation for further work directed toward understanding the functional dynamics of MATE family multidrug resistance transporters. The work is published in the journal Proceedings of the National Academy of Sciences U.S.A. [D. P. Claxton, et al., Proc. Natl. Acad. Sci. U.S.A., (2018) published online June 18, DOI:10.1073/pnas.1802417115].