Characterization of the Inward- and Outward-Facing Substrate Binding Sites of the Prokaryotic Aspartate Transporter, GltPh.

Crystal structures of the prokaryotic aspartate transporter, GltPh, have provided important insights into the mechanism of amino acid transport by GltPh and related eukaryotic members of the glutamate transporter family (SLC1A family). Identification of inhibitors of GltPh can provide valuable tools for understanding the molecular basis for substrate and inhibitor specificity and selectivity of SLC1A members, but at present, few inhibitors of GltPh have been identified. We have screened a collection of commercially available aspartate analogues and identified new transportable and nontransportable GltPh inhibitors. We have explored the inhibition profile of GltPh by utilizing a thiol modification assay that isolates sided populations of the transporters reconstituted in liposomes to determine if any aspartate analogues display a preference for either the inwardly or outwardly directed binding sites. Here, we have characterized several new inhibitors of GltPh and identified three β-carbon-substituted molecules that display a strong preference for the outwardly directed binding site of GltPh.