The ion-coupling mechanism of human excitatory amino acid transporters.

ABSTRACT: Excitatory amino acid transporters (EAATs) maintain glutamate gradients in the brain essential for neurotransmission and to prevent neuronal death. They use ionic gradients as energy source and co-transport transmitter into the cytoplasm with Na⁺ and H⁺ , while counter-transporting K⁺ to re-initiate the transport cycle. However, the molecular mechanisms underlying ion-coupled transport remain incompletely understood. Here, we present 3D X-ray crystallographic and cryo-EM structures, as well as thermodynamic analysis of human EAAT1 in different ion bound conformations, including elusive counter-transport ion bound states. Binding energies of Na⁺ and H⁺ , and unexpectedly Ca²⁺ , are coupled to neurotransmitter binding. Ca²⁺ competes for a conserved Na⁺ site, suggesting a regulatory role for Ca²⁺ in glutamate transport at the synapse, while H⁺ binds to a conserved glutamate residue stabilizing substrate occlusion. The counter-transported ion binding site overlaps with that of glutamate, revealing the K⁺ -based mechanism to exclude the transmitter during the transport cycle and to prevent its neurotoxic release on the extracellular side.

SUBMITTER: Canul-Tec JC 

PROVIDER: S-EPMC8724772 | biostudies-literature | 2022 Jan

REPOSITORIES: biostudies-literature