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Think positively : the structural basis of cation-binding and coupling of the multidrug and toxic-compound Extrusion (MATE) transporter family

Abstract

Transporters of the multidrug and toxic compound extrusion (MATE) family play vital roles in plant metabolite transport as well as bacterial and mammalian multidrug resistance (MDR). As secondary transporters, MATEs couple substrate transport to electrochemical gradients and are the last remaining class of MDR transporters whose structure has not been determined. The X-ray structure of the MATE transporter NorM from Vibrio cholera (VC) determined to 3.65 Å reveals an outward-facing conformation with two portals open to the outer leaflet of the lipid bilayer and has a unique topology distinct from other known MDR transporters. A discrete cation-binding site in close proximity to residues critical for transport as reported previously is identified at the C-terminal half of the transporter. This conformation likely represents a stage of the transport cycle with high- affinity to monovalent cations and low-affinity to substrates. Homology modeling of other distantly related multidrug MATE transporters indicates the conservation of this cation-binding site, suggesting a similar functional attribute. A comparison between the two prevalent secondary transporter families, MATE and MFS, revealed the similarities and differences between the structure and function of the two transporter families. The MATE and MFS families have distinct protein-folds and topologies, and use residues from different transmembrane (TM) helices for cation binding and, likely, substrate recognition. These findings suggest that the cation binding and the coupling mechanism to substrate efflux in MATE and MFS families likely arose independently. With the most conserved residues still facing the internal cavity in the low- substrate-affinity conformation, the MATEs likely transport substrates/cations using a rocker-switch model similar to the MFS. In this alternating access model, the access and binding of substrates/cations from the two sides of the lipid membrane are mediated by rigid-body movements between the N- and C-terminal halves of the transporter. The substrate and cation binding affinity of each conformational state depends on the spatial rearrangement of a similar set of residues within the internal cavity of the transporter

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