Graduate School of Information Sciences, Tohoku University, Sendai, Miyagi, Japan; Department of Bioscience, Nagahama Institute of Bio-Science and Technology, Nagahama, Shiga, Japan.
Graduate School of Information Sciences, Tohoku University, Sendai, Miyagi, Japan; Medical Research Institute, Tokyo Medical and Dental University, Bunkyo-ku, Tokyo, Japan.
Biophys J. 2024 Jun 18;123(12):1751-1762. doi: 10.1016/j.bpj.2024.05.018. Epub 2024 May 21.
The anion exchanger solute carrier family 26 (SLC26)A9, 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, STAS domain removal (ΔSTAS), and C-terminus removal (ΔC), to investigate their conformational dynamics and ion-binding properties. Stable binding of ions to the binding sites was exclusively observed in the ΔC model in these simulations. Comparing the full-length and ΔC simulations, the ΔC model displayed enhanced motion of the STAS domain. Furthermore, comparing the ΔSTAS and ΔC simulations, the ΔSTAS 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 the 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's alternate-access mechanism. 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.
阴离子交换溶质载体家族 26（SLC26）A9 由跨膜（TM）结构域和细胞质 STAS 结构域组成，在调节跨细胞膜的氯离子转运中发挥着重要作用。最近的研究表明，C 端螺旋阻止了假定的离子转运途径的入口。然而，STAS 结构域和 C 端螺旋的精确功能以及控制转运过程的潜在分子机制仍知之甚少。在这项研究中，我们对三种不同的人类 SLC26A9 模型（全长、STAS 结构域缺失（ΔSTAS）和 C 端缺失（ΔC））进行了分子动力学模拟，以研究它们的构象动力学和离子结合特性。在这些模拟中，仅在 ΔC 模型中观察到离子稳定结合到结合位点上。将全长和 ΔC 模拟进行比较，ΔC 模型中 STAS 结构域的运动增强。此外，将 ΔSTAS 和 ΔC 模拟进行比较，尽管在两个系统中 C 端都不阻断离子传输途径，但 ΔSTAS 模拟未能稳定地将离子结合到结合位点上。这些结果表明，C 端的缺失不仅使离子能够进入渗透途径，而且还触发了 STAS 结构域的运动，使 TM 结构域打开，促进离子进入其结合位点。进一步分析表明，STAS 结构域的不对称运动导致 TM 结构域内离子渗透途径的扩张，导致靠近离子结合位点的 TM12 螺旋的柔性增加。TM12 螺旋的这种结构变化稳定了氯离子的结合，这对于 SLC26A9 的交替进入机制是必不可少的。总的来说，我们的研究为 SLC26A9 转运的分子机制提供了新的见解，并为开发与离子转运失调相关疾病的新治疗方法铺平了道路。
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