Adelman Joshua L, Sheng Ying, Choe Seungho, Abramson Jeff, Wright Ernest M, Rosenberg John M, Grabe Michael
Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania.
School of Basic Science, College of Convergence, Daegu Gyeongbuk Institute of Science & Technology, Daegu, Korea.
Biophys J. 2014 Mar 18;106(6):1280-9. doi: 10.1016/j.bpj.2014.01.006.
Sodium-glucose transporters (SGLTs) facilitate the movement of water across the cell membrane, playing a central role in cellular homeostasis. Here, we present a detailed analysis of the mechanism of water permeation through the inward-facing state of vSGLT based on nearly 10 μs of molecular dynamics simulations. These simulations reveal the transient formation of a continuous water channel through the transporter that permits water to permeate the protein. Trajectories in which spontaneous release of galactose is observed, as well as those in which galactose remains in the binding site, show that the permeation rate, although modulated by substrate occupancy, is not tightly coupled to substrate release. Using a, to our knowledge, novel channel-detection algorithm, we identify the key residues that control water flow through the transporter and show that solvent gating is regulated by side-chain motions in a small number of residues on the extracellular face. A sequence alignment reveals the presence of two insertion sites in mammalian SGLTs that flank these outer-gate residues. We hypothesize that the absence of these sites in vSGLT may account for the high water permeability values for vSGLT determined via simulation compared to the lower experimental estimates for mammalian SGLT1.
钠-葡萄糖转运蛋白（SGLTs）促进水跨细胞膜的移动，在细胞内稳态中发挥核心作用。在此，我们基于近10微秒的分子动力学模拟，对通过向内开放状态的vSGLT的水渗透机制进行了详细分析。这些模拟揭示了通过转运蛋白形成的连续水通道的瞬时形成，该通道允许水透过蛋白质。观察到半乳糖自发释放的轨迹以及半乳糖保留在结合位点的轨迹表明，尽管渗透速率受底物占据情况的调节，但与底物释放并非紧密耦合。据我们所知，使用一种新型的通道检测算法，我们确定了控制水通过转运蛋白流动的关键残基，并表明溶剂门控受细胞外表面少数残基的侧链运动调节。序列比对揭示了哺乳动物SGLTs中存在两个位于这些外部门控残基两侧的插入位点。我们推测，vSGLT中不存在这些位点可能解释了与哺乳动物SGLT1较低的实验估计值相比，通过模拟确定的vSGLT的高水渗透值。
