神經(jīng)元細(xì)胞擁有不同的轉(zhuǎn)運(yùn)蛋白,，但這些轉(zhuǎn)運(yùn)蛋白如何工作迄今還是一個(gè)謎。據(jù)美國(guó)物理學(xué)家組織網(wǎng)4月24日?qǐng)?bào)道,，美國(guó)科學(xué)家最近終于弄清楚了轉(zhuǎn)運(yùn)蛋白分子的工作機(jī)制,，研究發(fā)表在24日出版的《自然》雜志上?？茖W(xué)家表示，新研究有望改進(jìn)對(duì)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(yùn)蛋白是內(nèi)嵌于神經(jīng)元細(xì)胞膜內(nèi)的分子機(jī)器，其作用是調(diào)節(jié)神經(jīng)細(xì)胞之間的信號(hào)傳導(dǎo)并循環(huán)利用神經(jīng)遞質(zhì),。在大腦中,，神經(jīng)元之間通過(guò)向突觸(兩個(gè)神經(jīng)元的相接處)釋放神經(jīng)遞質(zhì)來(lái)“通話”。為了讓信號(hào)傳遞停止,，需要專(zhuān)門(mén)的轉(zhuǎn)運(yùn)蛋白將突觸處的神經(jīng)遞質(zhì)運(yùn)回原細(xì)胞內(nèi),。然而，讓神經(jīng)遞質(zhì)集結(jié)在突觸處對(duì)很多疾病的治療大有裨益,?？挂钟羲幬锞褪峭ㄟ^(guò)干預(yù)特定轉(zhuǎn)運(yùn)蛋白，使神經(jīng)遞質(zhì)集結(jié)在突觸處來(lái)起作用,，可卡因和安非他明等興奮劑也如此,。
最新實(shí)驗(yàn)中，威爾康乃爾醫(yī)學(xué)院生理學(xué)和生物物理學(xué)副教授斯科特·布蘭查德領(lǐng)導(dǎo)的科研團(tuán)隊(duì)使用單分子熒光共振能量轉(zhuǎn)移(smFRET)技術(shù),，對(duì)2005年從原核生物中發(fā)現(xiàn)的一種亮氨酸轉(zhuǎn)運(yùn)蛋白分子(LeuT,，與哺乳動(dòng)物神經(jīng)遞質(zhì)鈉轉(zhuǎn)運(yùn)體在結(jié)構(gòu)和功能上非常相似)進(jìn)行了成像，監(jiān)測(cè)出LeuT在組成和動(dòng)力學(xué)方面的變化,，闡釋了LeuT內(nèi)的分子活動(dòng),。他們將熒光染料貼在蛋白的運(yùn)動(dòng)部分，當(dāng)染料間的距離變化時(shí),，熒光染料會(huì)釋放出不同數(shù)量的光,。在整個(gè)過(guò)程中，轉(zhuǎn)運(yùn)蛋白的移動(dòng),、熒光團(tuán)之間的距離依時(shí)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(yùn)機(jī)制的動(dòng)力學(xué)過(guò)程。
新實(shí)驗(yàn)證明,，依附于LeuT的丙氨酸會(huì)增加轉(zhuǎn)運(yùn)蛋白在兩個(gè)形態(tài)之間變換的速率：一個(gè)形態(tài)是面朝外,，好像轉(zhuǎn)運(yùn)蛋白準(zhǔn)備接受從細(xì)胞外傳來(lái)的基質(zhì)(朝內(nèi)關(guān)閉)。另一個(gè)形態(tài)是面朝內(nèi),，好像轉(zhuǎn)運(yùn)蛋白朝細(xì)胞釋放其所包含的物質(zhì)(朝內(nèi)開(kāi)啟),。另外,，鈉對(duì)丙氨酸增強(qiáng)這種動(dòng)力機(jī)制來(lái)說(shuō)是必需的。
但只有鈉離子而沒(méi)有丙氨酸時(shí),，轉(zhuǎn)運(yùn)蛋白開(kāi)啟和關(guān)閉狀態(tài)之間的轉(zhuǎn)化速度會(huì)減少,。抗抑郁的氯米帕明就是阻擋丙氨酸的這種效果并將該轉(zhuǎn)運(yùn)蛋白限制在其朝內(nèi)關(guān)閉的狀態(tài),，以抑制轉(zhuǎn)運(yùn)過(guò)程,。威爾康乃爾醫(yī)學(xué)院計(jì)算生物醫(yī)學(xué)研究所所長(zhǎng)阿雷爾·溫斯坦表示，只有理解了這種動(dòng)力學(xué),，我們才能真正理解藥物分子的工作原理,。
溫斯坦表示，因?yàn)榧?xì)菌和哺乳動(dòng)物的轉(zhuǎn)運(yùn)蛋白幾乎是一樣的,，該研究結(jié)果很有可能適用于哺乳動(dòng)物,，包括人體神經(jīng)細(xì)胞中的轉(zhuǎn)運(yùn)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.
