An overall scheme for 5-HT transport by SERT. In step A, the transporter, with Na1 and Cl sites occupied, binds an extracellular Na⁺ ion at the Na2 site, which stabilizes the open-out conformation. Subsequent binding of a 5-HT molecule in step B allows a conformational change that closes the extracellular pathway (step C) and allows the concerted opening of the cytoplasmic pathway and dissociation of the ion bound at Na2 (step D) along with the peak current (Fig. 9 B). Dissociation of 5-HT (step E) and intracellular K⁺ binding (step F) might not occur in an ordered sequence, but the presence of bound 5-HT prevents formation of the conductive state responsible for the steady current (Fig. 9, A and E; this current may be a property of the intermediate shown between steps F and G). Binding of K⁺ reverses the charge movement that occurs in step D, possibly because it induces a conformational change that closes the cytoplasmic pathway. With K⁺ bound, SERT undergoes a conformational transition to an outward-open state (step G), followed by K⁺ dissociation to the extracellular medium (step H). For charge movements to be balanced, the entry of Na⁺ and 5-HT⁺ must be offset by efflux of a cation (shown as exit of a proton in step G) or by the transport of 5-HT in its neutral rather than charged form.