神經(jīng)元細胞擁有不同的轉(zhuǎn)運蛋白,，但這些轉(zhuǎn)運蛋白如何工作迄今還是一個謎,。據(jù)美國物理學家組織網(wǎng)4月24日報道,，美國科學家最近終于弄清楚了轉(zhuǎn)運蛋白分子的工作機制，研究發(fā)表在24日出版的《自然》雜志上,?？茖W家表示，新研究有望改進對精神疾病治療的效果,，加深理解可卡因等神經(jīng)藥物的作用原理,。
轉(zhuǎn)運蛋白是內(nèi)嵌于神經(jīng)元細胞膜內(nèi)的分子機器，其作用是調(diào)節(jié)神經(jīng)細胞之間的信號傳導并循環(huán)利用神經(jīng)遞質(zhì),。在大腦中,，神經(jīng)元之間通過向突觸(兩個神經(jīng)元的相接處)釋放神經(jīng)遞質(zhì)來“通話”。為了讓信號傳遞停止,，需要專門的轉(zhuǎn)運蛋白將突觸處的神經(jīng)遞質(zhì)運回原細胞內(nèi),。然而，讓神經(jīng)遞質(zhì)集結(jié)在突觸處對很多疾病的治療大有裨益,?？挂钟羲幬锞褪峭ㄟ^干預特定轉(zhuǎn)運蛋白，使神經(jīng)遞質(zhì)集結(jié)在突觸處來起作用,，可卡因和安非他明等興奮劑也如此,。
最新實驗中，威爾康乃爾醫(yī)學院生理學和生物物理學副教授斯科特·布蘭查德領導的科研團隊使用單分子熒光共振能量轉(zhuǎn)移(smFRET)技術(shù),，對2005年從原核生物中發(fā)現(xiàn)的一種亮氨酸轉(zhuǎn)運蛋白分子(LeuT,，與哺乳動物神經(jīng)遞質(zhì)鈉轉(zhuǎn)運體在結(jié)構(gòu)和功能上非常相似)進行了成像，監(jiān)測出LeuT在組成和動力學方面的變化,，闡釋了LeuT內(nèi)的分子活動。他們將熒光染料貼在蛋白的運動部分,，當染料間的距離變化時,，熒光染料會釋放出不同數(shù)量的光。在整個過程中,，轉(zhuǎn)運蛋白的移動,、熒光團之間的距離依時間而產(chǎn)生的變化都被直接成像，從而首次定量地洞悉了轉(zhuǎn)運機制的動力學過程,。
新實驗證明,，依附于LeuT的丙氨酸會增加轉(zhuǎn)運蛋白在兩個形態(tài)之間變換的速率：一個形態(tài)是面朝外，好像轉(zhuǎn)運蛋白準備接受從細胞外傳來的基質(zhì)(朝內(nèi)關閉)。另一個形態(tài)是面朝內(nèi),，好像轉(zhuǎn)運蛋白朝細胞釋放其所包含的物質(zhì)(朝內(nèi)開啟),。另外，鈉對丙氨酸增強這種動力機制來說是必需的,。
但只有鈉離子而沒有丙氨酸時,，轉(zhuǎn)運蛋白開啟和關閉狀態(tài)之間的轉(zhuǎn)化速度會減少?？挂钟舻穆让着撩骶褪亲钃醣彼岬倪@種效果并將該轉(zhuǎn)運蛋白限制在其朝內(nèi)關閉的狀態(tài),，以抑制轉(zhuǎn)運過程。威爾康乃爾醫(yī)學院計算生物醫(yī)學研究所所長阿雷爾·溫斯坦表示,，只有理解了這種動力學,，我們才能真正理解藥物分子的工作原理。
溫斯坦表示,，因為細菌和哺乳動物的轉(zhuǎn)運蛋白幾乎是一樣的,，該研究結(jié)果很有可能適用于哺乳動物，包括人體神經(jīng)細胞中的轉(zhuǎn)運蛋白,。（生物谷Bioon.com）
生物谷推薦原文出處：
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.
