The substrate import mechanism of the human serotonin transporter
The substrate import mechanism of the human serotonin transporter
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DOI：
10.1016/j.bpj.2022.01.024
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发表时间：
2022-03-01
影响因子：
3.4
通讯作者：
Shukla，Diwakar
中科院分区：
文献类型：
--
作者：
Chan，Matthew C.;Selvam，Balaji;Shukla，Diwakar
5-羟色胺转运体(SERT)启动细胞外5-羟色胺在突触中的重新摄取，从而终止神经传递。结合在ibogaine上的SERT和生理底物5-羟色胺在不同状态下的低温电子显微镜结构提供了原子分辨率下的功能构象的一瞥。然而，构象动力学和结构向中间态的转变还没有完全被理解。此外，5-羟色胺如何被识别和运输的分子基础仍不清楚。在这项研究中，我们对人类SERT进行了无偏向微秒长的模拟，以研究各种中间状态的结构动力学，并阐明了完整的底物输入途径。利用马尔可夫态模型，我们描述了一系列构象驱动的离子耦合底物结合和输运事件的顺序，并计算了与输入机制相关的构象转变的自由能垒。我们发现，从被阻挡态到内向态的转变是衬底输入的速率限制步骤，并且衬底降低了自由能垒以达到内向态。我们的研究为SERT动力学驱动的离子底物识别和转运的分子基础提供了见解，可以作为其他密切相关的神经递质转运蛋白的模型。
The serotonin transporter (SERT) initiates the reuptake of extracellular serotonin in the synapse to terminate neurotransmission. The cryogenic electron microscopy structures of SERT bound to ibogaine and the physiological substrate serotonin resolved in different states have provided a glimpse of the functional conformations at atomistic resolution. However, the conformational dynamics and structural transitions to intermediate states are not fully understood. Furthermore, the molecular basis of how serotonin is recognized and transported remains unclear. In this study, we performed unbiased microsecond-long simulations of the human SERT to investigate the structural dynamics to various intermediate states and elucidated the complete substrate import pathway. Using Markov state models, we characterized a sequential order of conformational-driven ion-coupled substrate binding and transport events and calculated the free energy barriers of conformation transitions associated with the import mechanism. We find that the transition from the occluded to inward-facing state is the rate-limiting step for substrate import and that the substrate decreases the free energy barriers to achieve the inward-facing state. Our study provides insights on the molecular basis of dynamics-driven ion-substrate recognition and transport of SERT that can serve as a model for other closely related neurotransmitter transporters.