LAUSR

Glutamate transporters are cation-coupled secondary active membrane transporters that clear the neurotransmitter L-glutamate from the synaptic cleft. These transporters are homotrimers, with each protomer functioning independently by an elevator-type mechanism, in which a mobile transport domain alternates between inward- and outward-oriented states. Using single-particle cryo-EM we have determined five structures of the glutamate transporter homologue Glt Tk , a Na + - L-aspartate symporter, embedded in lipid nanodiscs. Dependent on the substrate concentrations used, the protomers of the trimer adopt a variety of asymmetrical conformations, consistent with the independent movement. Six of the 15 resolved protomers are in a hitherto elusive state of the transport cycle in which the inward-facing transporters are loaded with Na + ions. These structures explain how substrate-leakage is prevented – a strict requirement for coupled transport. The belt protein of the lipid nanodiscs bends around the inward oriented protomers, suggesting that membrane deformations occur during transport. Glutamate transporters are membrane transporters that clear the neurotransmitter L-glutamate from the synaptic cleft via a so-called elevator mechanism. Here the authors present five cryo-EM structures of the transporter homologue Glt Tk , which explain how substrate leakage is prevented.