伊利諾斯大學香檳（Urbana-Champaign）分校和亞利桑那大學的研究人員將計算機模擬手段和實驗室技術(shù)聯(lián)合起來，發(fā)現(xiàn)aquaporins（水通道蛋白）發(fā)揮門控機制（gating mechanism）的一種關(guān)鍵亞基——loop D，不僅控制水分子的出入而且與離子傳導有關(guān),，發(fā)揮哪種功能主要取決于刺激細胞的信號途徑,。研究結(jié)果發(fā)表于《Structure》9月刊,。
Aquaporin是一系列能夠控制水分子在細胞和細胞外環(huán)境間傳遞的膜蛋白組成的家族。伊利諾斯大學香檳（Urbana-Champaign）分校的研究人員和亞利桑那大學的研究人員發(fā)現(xiàn)此家族的成員之一,，aquaporin-1也具有離子通道（ion channels）的功能。
伊利諾斯大學生化教授,、Beckman 尖端科技研究所研究員Emad Tajkhorshid說：“弄清門控通道的分子機制,，有助于蛋白工程學研究,。”
Tajkhorshid等利用已知的aquaporin-1晶體結(jié)構(gòu)和已掌握的分子動力學模擬（molecular dynamics simulations）技術(shù)，發(fā)現(xiàn)aquaporin-1的中心孔道（central pore,，生物通編者譯）能夠傳遞,，其門控功能受到細胞內(nèi)環(huán)磷酸鳥苷（cGMP或cyclic GMP或3'-5'-cyclic guanosine monophosphate）信號控制。cGMP誘導aquaporin的中心孔道的 loop D構(gòu)象改變,。
“loop D環(huán)非常柔韌,，每排中有四個帶正電荷的精氨酸殘基，并且有的精氨酸殘基會延伸到中心孔道,，”Tajkhorshid說,，“我們發(fā)現(xiàn)cGMP與loop D結(jié)合，促進了精氨酸殘基向細胞外的運動,，刺激了門道的打開,。”
研究人員高度強調(diào)了將模擬和實驗結(jié)合使用的優(yōu)勢。利用模擬技術(shù),，研究人員設(shè)計了一種突變模型,，模型中兩個精氨酸被兩個丙胺酸(Alanines)取代。在Arizona實驗室進行的實驗證明,，這種突變通道的導電功能幾乎消失了,，但是輸水功能不受影響。
“弄清這種機制有助于我們更好地控制中心孔道的開啟和關(guān)閉” Tajkhorshid說,，“通過修飾孔道中的殘基或者改變控制門控孔道的loop D的長度,，我們可以關(guān)閉傳導離子的功能，或者設(shè)計出新的開啟更為方便,、導電率更高的水通道蛋白,。”
研究受到美國國立衛(wèi)生研究院資助,。合作者包括：伊利諾斯大學Jin Yu（于金，音譯）,、Klaus Schulten和亞利桑那大學Andrea J. Yool,。
英文原文：
One protein, two channels: Scientists explain mechanism in aquaporins
Using computer simulations and experimental results, researchers at the University of Illinois at Urbana-Champaign and the University of Arizona have identified a key component of the gating mechanism in aquaporins that controls both the passage of water and the conduction of ions.
Aquaporins are a class of proteins that form membrane channels in cell walls and allow for water movement between a cell and its surroundings. A number of aquaporins, including aquaporin-1, have been found to function as ion channels, as well.
“Understanding the molecular mechanism behind gating in membrane channels could lead to more effective protein engineering,” said Emad Tajkhorshid, a professor of biochemistry at Illinois and a researcher at the Beckman Institute for Advanced Science and Technology.
In work funded by the National Institutes of Health, Tajkhorshid and co-workers show that the same protein can be used as a water channel or an ion channel depending on the signaling pathway activated in the cell. The scientists report their findings in the September issue of the journal Structure.
Taking advantage of the known crystal structure of aquaporin-1 and the power of molecular dynamics simulations, the researchers explored the central pore as a candidate pathway for conducting ions. Gating of the central pore is controlled by cyclic guanosine monophosphate, a signaling nucleotide inside the cell, which induces a conformational change in one of the aquaporin loops (loop D).
“This loop is very flexible, has four positively charged arginine residues in a row, and extends into the central pore,” Tajkhorshid said. “We believe the cGMP interacts with loop D, facilitating its outward motion, which triggers the opening of the gate.”
The work highlights a close interaction between simulation and experiment. Based on their simulation results, the researchers designed a mutant in which two arginines in loop D were replaced by two alanines. In laboratory experiments performed at Arizona, the substitution caused an almost complete removal of ion conduction, but had no appreciable effect on water passage.
“Knowing the mechanism gives us a new handle to control the opening or closing of the central pore,” Tajkhorshid said. “By modifying the pore-lining residue, or altering the length of loop D that gates the pore, we can shut down the ion conductivity completely, or engineer new aquaporins that can be opened more easily or have a higher ion conduction rate once open.”
With Tajkhorshid, co-authors of the paper are Illinois graduate student and lead author Jin Yu, University of Arizona experimentalist Andrea J. Yool, and Illinois physicist Klaus Schulten.
