Agonist binding in ligand-gated ion channels is coupled to structural rearrangements around the binding site, followed by the opening of the channel pore. In this process, agonist efficacy describes the equilibrium between open and closed conformations in a fully ligand-bound state. Calcium-activated chloride channels in the TMEM16 family are important sensors of intracellular calcium signals and are targets for pharmacological modulators, yet a mechanistic understanding of agonist efficacy has remained elusive. Using a combination of cryo-electron microscopy, electrophysiology, and autocorrelation analysis, we now show that agonist efficacy in the ligand-gated channel TMEM16A is dictated by the conformation of the pore-lining helix α6 around the Ca²⁺-binding site. The closure of the binding site, which involves the formation of a π-helix below a hinge region in α6, appears to be coupled to the opening of the inner pore gate, thereby governing the channel's open probability and conductance. Our results provide a mechanism for agonist binding and efficacy and a structural basis for the design of potentiators and partial agonists in the TMEM16 family.

中文翻译：

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钙激活氯离子通道 TMEM16A 配体功效的机制基础

配体门控离子通道中的激动剂结合与结合位点周围的结构重排偶联，随后通道孔打开。在此过程中，激动剂功效描述了完全配体结合状态下开放构象和闭合构象之间的平衡。TMEM16 家族中的钙激活氯离子通道是细胞内钙信号的重要传感器，也是药理调节剂的靶标，但对激动剂功效的机制理解仍然难以捉摸。通过结合冷冻电子显微镜、电生理学和自相关分析，我们现在表明配体门控通道 TMEM16A 中的激动剂功效由 Ca ²⁺结合位点周围的孔内衬螺旋 α6 的构象决定。结合位点的关闭涉及在α6铰链区下方形成π螺旋，似乎与内孔门的打开耦合，从而控制通道的开放概率和电导。我们的结果提供了激动剂结合和功效的机制，以及 TMEM16 家族中增强剂和部分激动剂设计的结构基础。