Microscopic Characterization of the Chloride Permeation Pathway in the Human Excitatory Amino Acid Transporter 1 (EAAT1).

ABSTRACT: Excitatory amino acid transporters (EAATs) are glutamate transporters that belong to the solute carrier 1A (SLC1A) family. They couple glutamate transport to the cotransport of three sodium (Na⁺) ions and one proton (H⁺) and the counter-transport of one potassium (K⁺) ion. In addition to this coupled transport, binding of cotransported species to EAATs activates a thermodynamically uncoupled chloride (Cl^-) conductance. Structures of SLC1A family members have revealed that these transporters use a twisting elevator mechanism of transport, where a mobile transport domain carries substrate and coupled ions across the membrane, while a static scaffold domain anchors the transporter in the membrane. We recently demonstrated that the uncoupled Cl^- conductance is activated by the formation of an aqueous pore at the domain interface during the transport cycle in archaeal Glt_Ph. However, a pathway for the uncoupled Cl^- conductance has not been reported for the EAATs, and it is unclear if such a pathway is conserved. Here, we employ all-atom molecular dynamics (MD) simulations combined with enhanced sampling, free-energy calculations, and experimental mutagenesis to approximate large-scale conformational changes during the transport process and identified a Cl^--conducting conformation in human EAAT1 (hEAAT1). Sampling the large-scale structural transitions in hEAAT1 allowed us to capture an intermediate conformation formed during the transport cycle with a continuous aqueous pore at the domain interface. The free-energy calculations performed for the conduction of Cl^- and Na⁺ ions through the captured conformation highlight the presence of two hydrophobic gates that control low-barrier movement of Cl^- through the aqueous pathway. Overall, our findings provide insights into the mechanism by which a human neurotransmitter transporter supports functional duality of active transport and passive Cl^- permeation and confirm the commonality of this mechanism in different members of the SLC1A family.

SUBMITTER: Pant S 

PROVIDER: S-EPMC9725111 | biostudies-literature | 2022 Mar

REPOSITORIES: biostudies-literature