Elevator motion of chloride ion transporter SLC26A9 induced by STAS domain

The anion exchanger SLC26A9, consisting of the transmembrane (TM) domain and the cytoplasmic STAS domain, plays an essential role in regulating chloride transport across cell membranes. Recent studies have indicated that C-terminal helices block the entrance of the putative ion transport pathway. However, the precise functions of the STAS domain and C-terminal helix, as well as the underlying molecular mechanisms governing the transport process, remain poorly understood. In this study, we performed molecular dynamics simulations of three distinct models of human SLC26A9: full-length (FL), STAS domain removal ({Delta}STAS), and C-terminus removal ({Delta}C), to investigate their conformational dynamics and ion binding properties. Stable binding of ions to the binding sites was exclusively observed in the {Delta}C model in these simulations. Comparing the FL and {Delta}C simulations, the {Delta}C model displayed enhanced motion of the STAS domain. Furthermore, comparing the {Delta}STAS and {Delta}C simulations, the {Delta}STAS simulation failed to exhibit stable ion bindings to the sites despite the absence of the C-terminus blocking the ion transmission pathway in both systems. These results suggest that the removal of the C-terminus not only unblocks the access of ions to the permeation pathway but also triggers STAS domain motion, gating the TM domain to promote ions entry into their binding site. Further analysis revealed that the asymmetric motion of STAS domain leads to the expansion of the ion permeation pathway within the TM domain, resulting in the stiffening of the flexible TM12 helix near the ion binding site. This structural change in the TM12 helix stabilizes chloride ion binding, which is essential for SLC26A9 elevator motion. Overall, our study provides new insights into the molecular mechanisms of SLC26A9 transport and may pave the way for the development of novel treatments for diseases associated with dysregulated ion transport. SIGNIFICANCEWe explored the mechanism by which the human protein SLC26A9 transports chloride in the cell. SLC26A9 is a potential therapeutic target for patients with cystic fibrosis, as by targeting drugs to it, it may be possible to restore chloride ion transport in epithelial cells. To design therapeutic drugs, it is essential to understand how the protein works. Our findings support an elevator-type mechanism, in which chloride ions bind to SLC26A9 inside the cell and are then released by the protein to the extracellular environment. We find that the STAS domain of SLC26A9 has critical roles in binding chloride and induces conformational changes in the transmembrane domain that facilitate chloride transport.