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. 2019 Dec 20;9:19479. doi: 10.1038/s41598-019-55722-6

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Schematics of Na⁺-binding effects on LeuT-fold transporters. In a functional transport cycle (light green region), the transporter (two helical bundles shown in purple and green rectangles) traverses OF apo (2nd column, top), OF Na⁺-bound (3rd column, top), OF Na⁺/substrate-bound (4th column, top), IF Na⁺/substrate-bound (4th column, bottom), IF substrate-bound (3rd column, bottom), and IF apo (2nd column, bottom) states, schematically indicated by the relative angle between two helical bundles and the loading state of Na⁺ (yellow sphere) and substrate. Compared to the OF apo state, Na⁺ binding stabilizes the substrate-binding site (the excavated shapes in the helical bundles) in a local conformation favorable for substrate binding (upper middle). Meanwhile, Na⁺ binding to the OF apo and IF apo states introduces an extended ion-leaking cycle (red line circled region), in which the OF ↔ IF transition is disfavored to avoid energy dissipation (1st column). Such OF ↔ IF transition is allowed in the apo form (2nd column) and in the Na⁺/substrate-bound form (4th column) in the functional transport cycle.