神经元细胞拥有不同的转运蛋白，但这些转运蛋白如何工作迄今还是一个谜。据美国物理学家组织网4月24日报道，美国科学家最近终于弄清楚了转运蛋白分子的工作机制，研究发表在24日出版的《自然》杂志上。科学家表示，新研究有望改进对精神疾病治疗的效果，加深理解可卡因等神经药物的作用原理。
转运蛋白是内嵌于神经元细胞膜内的分子机器，其作用是调节神经细胞之间的信号传导并循环利用神经递质。在大脑中，神经元之间通过向突触(两个神经元的相接处)释放神经递质来“通话”。为了让信号传递停止，需要专门的转运蛋白将突触处的神经递质运回原细胞内。然而，让神经递质集结在突触处对很多疾病的治疗大有裨益。抗抑郁药物就是通过干预特定转运蛋白，使神经递质集结在突触处来起作用，可卡因和安非他明等兴奋剂也如此。
最新实验中，威尔康乃尔医学院生理学和生物物理学副教授斯科特·布兰查德领导的科研团队使用单分子荧光共振能量转移(smFRET)技术，对2005年从原核生物中发现的一种亮氨酸转运蛋白分子(LeuT，与哺乳动物神经递质钠转运体在结构和功能上非常相似)进行了成像，监测出LeuT在组成和动力学方面的变化，阐释了LeuT内的分子活动。他们将荧光染料贴在蛋白的运动部分，当染料间的距离变化时，荧光染料会释放出不同数量的光。在整个过程中，转运蛋白的移动、荧光团之间的距离依时间而产生的变化都被直接成像，从而首次定量地洞悉了转运机制的动力学过程。
新实验证明，依附于LeuT的丙氨酸会增加转运蛋白在两个形态之间变换的速率：一个形态是面朝外，好像转运蛋白准备接受从细胞外传来的基质(朝内关闭)。另一个形态是面朝内，好像转运蛋白朝细胞释放其所包含的物质(朝内开启)。另外，钠对丙氨酸增强这种动力机制来说是必需的。
但只有钠离子而没有丙氨酸时，转运蛋白开启和关闭状态之间的转化速度会减少。抗抑郁的氯米帕明就是阻挡丙氨酸的这种效果并将该转运蛋白限制在其朝内关闭的状态，以抑制转运过程。威尔康乃尔医学院计算生物医学研究所所长阿雷尔·温斯坦表示，只有理解了这种动力学，我们才能真正理解药物分子的工作原理。
温斯坦表示，因为细菌和哺乳动物的转运蛋白几乎是一样的，该研究结果很有可能适用于哺乳动物，包括人体神经细胞中的转运蛋白。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09971
Substrate-modulated gating dynamics in a Na+-coupled neurotransmitter transporter homologue
Yongfang Zhao,1, 2, 4, 7 Daniel S. Terry,5, 7 Lei Shi,5, 6, 7 Matthias Quick,1, 2, 4 Harel Weinstein,5, 6 Scott C. Blanchard5 & Jonathan A. Javitch1, 2, 3, 4
Neurotransmitter/Na+ symporters (NSSs) terminate neuronal signalling by recapturing neurotransmitter released into the synapse in a co-transport (symport) mechanism driven by the Na+ electrochemical gradient1, 2, 3, 4, 5, 6. NSSs for dopamine, noradrenaline and serotonin are targeted by the psychostimulants cocaine and amphetamine1, as well as by antidepressants7. The crystal structure of LeuT, a prokaryotic NSS homologue, revealed an occluded conformation in which a leucine (Leu) and two Na+ are bound deep within the protein8. This structure has been the basis for extensive structural and computational exploration of the functional mechanisms of proteins with a LeuT-like fold9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22. Subsequently, an ‘outward-open’ conformation was determined in the presence of the inhibitor tryptophan23, and the Na+-dependent formation of a dynamic outward-facing intermediate was identified using electron paramagnetic resonance spectroscopy24. In addition, single-molecule fluorescence resonance energy transfer imaging has been used to reveal reversible transitions to an inward-open LeuT conformation, which involve the movement of transmembrane helix TM1a away from the transmembrane helical bundle22. We investigated how substrate binding is coupled to structural transitions in LeuT during Na+-coupled transport. Here we report a process whereby substrate binding from the extracellular side of LeuT facilitates intracellular gate opening and substrate release at the intracellular face of the protein. In the presence of alanine, a substrate that is transported ~10-fold faster than leucine15, 25, we observed alanine-induced dynamics in the intracellular gate region of LeuT that directly correlate with transport efficiency. Collectively, our data reveal functionally relevant and previously hidden aspects of the NSS transport mechanism that emphasize the functional importance of a second substrate (S2) binding site within the extracellular vestibule15, 20. Substrate binding in this S2 site appears to act cooperatively with the primary substrate (S1) binding site to control intracellular gating more than 30?? away, in a manner that allows the Na+ gradient to power the transport mechanism.
