Identification of key binding site residues of MCT1 for AR-C155858 reveals the molecular basis of its isoform selectivity

The proton-linked monocarboxylate transporters (MCTs) are required for lactic acid transport into and out of all mammalian cells. Thus, they play an essential role in tumour cells that are usually highly glycolytic and are promising targets for anti-cancer drugs. AR-C155858 is a potent MCT1 inhibitor (K-i similar to 2 nM) that also inhibits MCT2 when associated with basigin but not MCT4. Previous work [Ovens, M.J. et al. (2010) Biochem. J. 425, 523-530] revealed that AR-C155858 binding to MCT1 occurs from the intracellular side and involves transmembrane helices (TMs) 7-10. In the present paper, we generate a molecular model of MCT4 based on our previous models of MCT1 and identify residues in the intracellular substrate-binding cavity that differ significantly between MCT4 and MCT1/MCT2 and so might account for differences in inhibitor binding. We tested their involvement using site-directed mutagenesis (SDM) of MCT1 to change residues individually or in combination with their MCT4 equivalent and determined inhibitor sensitivity following expression in Xenopus oocytes. Phe(360) and Ser(364) were identified as important for AR-C155858 binding with the (FY)-Y-360/S(364)G mutant exhibiting >100-fold reduction in inhibitor sensitivity. To refine the binding site further, we used molecular dynamics (MD) simulations and additional SDM. This approach implicated six more residues whose involvement was confirmed by both transport studies and [H-3]-AR-C155858 binding to oocyte membranes. Taken together, our data imply that Asn(147), Arg(306) and Ser(364) are important for directing AR-C155858 to its final binding site which involves interaction of the inhibitor with Lys(38), Asp(302) and Phe(360) (residues that also play key roles in the translocation cycle) and also Leu(274) and Ser(278).