Proton-coupled monocarboxylate transporters MCT1-4 catalyze the transmembrane movement of metabolically essential monocarboxylates and have been targeted for cancer treatment because of their enhanced expression in various tumors. Here, we report five cryo-EM structures, at resolutions of 3.0-3.3 Å, of human MCT1 bound to lactate or inhibitors in the presence of Basigin-2, a single transmembrane segment (TM)-containing chaperon. MCT1 exhibits similar outward-open conformations when complexed with lactate or the inhibitors BAY-8002 and AZD3965. In the presence of the inhibitor 7ACC2 or with the neutralization of the proton-coupling residue Asp309 by Asn, similar inward-open structures were captured. Complemented by structural-guided biochemical analyses, our studies reveal the substrate binding and transport mechanism of MCTs, elucidate the mode of action of three anti-cancer drug candidates, and identify the determinants for subtype-specific sensitivities to AZD3965 by MCT1 and MCT4. These findings lay out an important framework for structure-guided drug discovery targeting MCTs.
Keywords: 7ACC2; AZD3965; BAY-8002; Basigin/CD147; MCT; SLC transporters; alternating access; cryo-EM; lactate transport; monocarboxylate transporter; proton coupling.
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