Excitatory amino acid transporters (EAATs) mediate glial and neuronal glutamate uptake to terminate synaptic transmission and to ensure low resting glutamate concentrations. Effective glutamate uptake is achieved by cotransport with 3 Na+ and 1 H+ , in exchange with 1 K+ . The underlying principles of this complex transport stoichiometry remain poorly understood. We use molecular dynamics simulations and electrophysiological experiments to elucidate how mammalian EAATs harness K+ gradients, unlike their K+ -independent prokaryotic homologues. Glutamate transport is achieved via elevator-like translocation of the transport domain. In EAATs, glutamate-free re-translocation is prevented by an external gate remaining open until K+ binding closes and locks the gate. Prokaryotic GltPh contains the same K+ -binding site, but the gate can close without K+ . Our study provides a comprehensive description of K+ -dependent glutamate transport and reveals a hitherto unknown allosteric coupling mechanism that permits adaptions of the transport stoichiometry without affecting ion or substrate binding.
Keywords: K+ binding; allosteric coupling; excitatory amino acid transporters; secondary active transport; transport stoichiometry.
© 2019 The Authors. Published under the terms of the CC BY 4.0 license.