The role of chloride ions in serotonin transport.

Abstract

The human serotonin (5-HT⁺) transporter SERT facilitates 5-HT⁺ transport into cells by coupling it to Na⁺ symport and K⁺ antiport. Although extracellular Cl^- is also essential for transport, Cl^- cotransport has been disputed, raising questions about the role of Cl^- ions and why they are required. We show that Cl^- gradients do not impact 5-HT⁺ accumulation, indicating that Cl^- does not provide a driving force for uptake and arguing against stoichiometric Cl^- symport. The presence of Cl^- had only a small effect on Na⁺-mediated cytoplasmic pathway closure but markedly reduced the accessibility of residues in the extracellular pathway, consistent with modulation of the outward-facing states. Simulations illustrate that Cl^- interacts strongly with a bound Na⁺ ion and stabilizes helix packing on the extracellular side. We propose that Cl^- acts as an essential architectural cofactor by enhancing Na⁺ affinity and interactions between helices, thereby facilitating transport-related conformational transitions.