Structural basis of excitatory amino acid transporter 3 substrate recognition

• 成果类型: Article
• 署名作者: Qiu, Biao; Boudker, Olga
• 署名单位: Cornell University; Weill Cornell Medicine; Cornell University; Weill Cornell Medicine; Howard Hughes Medical Institute
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-10359
• DOI: 10.1073/pnas.2501627122
• 发表日期: 2025-04-22
• 关键词: neuronal glutamate transporter; cryo-em structure; functional-characterization; cysteine uptake; aspartate; stoichiometry; inhibition; expression; cloning; brain

摘要

Excitatory amino acid transporters (EAATs) reside on cell surfaces and uptake substrates, including L-glutamate, L-aspartate, and D-aspartate, using ion gradients. Among five EAATs, EAAT3 is the only isoform that can efficiently transport L-cysteine, a substrate for glutathione synthesis. Recent studies suggest that EAAT3 also transports the oncometabolite R-2-hydroxyglutarate (R-2HG). Here, we examined the structural basis of substrate recognition by determining the cryogenic electron microscopy (cryo-EM) structures of EAAT3 bound to different substrates. We found that L-cysteine binds to EAAT3 in thiolate form, and EAAT3 recognizes different substrates by fine-tuning local conformations of the coordinating residues. However, using purified human EAAT3, we could not observe R-2HG binding or transport. Imaging of EAAT3 bound to L-cysteine revealed several conformational states, including an outward-facing state with a semi-open gate and a disrupted sodium-binding site. These structures demonstrate that the full gate closure, coupled with the binding of the last sodium ion, occurs after substrate binding. Furthermore, we observed that different substrates affect how the transporter distributes between a fully outward-facing conformation and intermediate occluded states on a path to the inward-facing conformation, suggesting that translocation rates are substrate-dependent.