Structural and biochemical characterization of the human neutral amino acid transporter ASCT2

Abstract

The membrane transporter ASCT2 is a neutral amino acid exchanger that is in particular important to maintain glutamine homeostasis in human cells. ASCT2 has increasingly gained attention as a promising target for drug design in anti-cancer and anti-retroviral therapy. We used biochemical methods in combination with single-particle cryo-EM to functionally and structurally characterize human ASCT2. Structures of ASCT2 in inward- and outward-facing conformations, each in glutamine-free and -bound states, have revealed a one-gate elevator mechanism of transport, where the access to the binding side is controlled by the same flexible helical hairpin loop (the HP2 loop) on both sides of the membrane. Interestingly, ASCT2 in detergent micelle environment appears to favour inward-facing conformations, but reconstitution of ASCT2 in lipid nanodiscs promotes outward-facing states of the transporter, demonstrating a major effect of the bilayer environment on the energy landscape. Moreover, ASCT2 is an example, where an integrated approach using computer-aided compound design, functional testing, and structure determination with cryo-EM, was used to rationally design novel inhibitors for ASCT2. Our results provide a basis for future functional and structural characterisation of ASCT2