Phosphatidylinositol-4,5-bisphosphate ionization and domain formation in the presence of lipids with hydrogen bond donor capabilities
Phosphatidylinositol-4,5-bisphosphate ionization and domain formation in the presence of lipids with hydrogen bond donor capabilities
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DOI：
10.1016/j.chemphyslip.2012.07.003
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发表时间：
2012-09-01
影响因子：
3.4
通讯作者：
Kooijman, Edgar E.
中科院分区：
文献类型：
--
作者：
Graber, Zachary T.;Jiang, Zhiping;Kooijman, Edgar E.
磷脂酰肌醇-4，5-二磷酸(PI(4，5)P-2)是一种重要的脂类信号分子，参与多种细胞过程。它与蛋白质的相互作用和横向分布受单酯基团的电离状态和形成分子内和分子间氢键的能力控制。在本研究中，我们研究了PI(4，5)P-2在含有磷脂酰乙醇胺(PE)、磷脂酰丝氨酸(PS)或磷脂酰肌醇(PI)的三元脂泡体系中的电离状态。在PE存在下，PI(4，5)P-2的电离度增加，这可以归因于PE和PI(4，5)P-2单酯基团之间形成氢键而增加了去质子化。然而，PE对PI(4，5)P-2电离的影响明显小于先前在PE存在下发现的磷脂酸(Kooijman等人，2005年)。PE对PI(4，5)P-2电离的影响减弱可归因于磷酸单酯基团与相邻羟基之间竞争形成分子内氢键。值得注意的是，PE的存在比4-磷酸对5-磷酸基团的电离影响更大，这表明PE与5-磷酸的相互作用更强。在PI(4，5)P-2/PS/PC脂泡中，PS的存在可能会增加PI(4，5)P-2单酯基团的质子化程度，这是由于界面负电荷增加导致界面pH降低所致。然而，PS对PI(4，5)P-2电离的影响很小，这可能表明PS和PI(4，5)P-2是分离的。PI(4，5)P-2/PI/PC囊泡体系具有令人惊讶的混合行为，具有潜在的深远影响：生理浓度下PI(4，5)P-2/PI/PC组成的巨大单膜囊泡的荧光显微镜测量表明，PI和PI(4，5)P-2形成宏观的流体相区，与富含PC的流体相接触(液/液分离)。尽管PI和PI(4，5)P-2是共定域的，但PI对PI(4，5)P-2电离行为的影响只有在pH 7以上才显著。显然，两种相反的效应导致了观察到的行为：由于阴离子PI的存在，界面pH下降，这可能导致PI(4，5)P-2磷酸单酯基团的质子化增强。反过来，PI和PI(4，5)P-2之间的氢键形成会导致相反的结果，即磷酸单酯基团的去质子化程度增加。当pH值小于7左右时，这两种作用相互补偿，而对于较高的pH值，PI存在时界面pH的升高比PI/PI(4，5)P-2氢键的形成有更大的影响。PI/PI(4，5)P-2混合域的协同形成对肌醇磷脂介导的信号事件的空间组织具有潜在的重要意义。(C)2012爱思唯尔爱尔兰有限公司。保留所有权利。
Phosphatidylinositol-4,5-bisphosphate (PI(4,5)P-2) is an important lipidic signaling molecule that is involved in a broad range of cellular processes. Its interaction with proteins and its lateral distribution are governed by the ionization state of the phosphomonoester groups and its ability to form intra- and intermolecular hydrogen bonds. In this study we have investigated the ionization state of PI(4,5)P-2 in ternary lipid vesicle systems that contain in addition to PI(4,5)P-2 and phosphatidylcholine (PC) either phosphatidylethanolamine (PE), phosphatidylserine (PS) or phosphatidylinositol (PI). In the presence of PE we find an increased ionization of PI(4,5)P-2, which can be attributed to increased deprotonation due to hydrogen bond formation between PE and the PI(4,5)P-2 phosphomonoester groups. However, the effect of PE on PI(4,5)P-2 ionization is significantly smaller than it had been found previously for phosphatidic acid in the presence of PE (Kooijman et al., 2005). The reduced impact of PE on PI(4,5)P-2 ionization can be attributed to competing intramolecular hydrogen bond formation between the phosphomonoester groups and neighboring hydroxyl groups. It is noteworthy that the presence of PE affects more strongly the ionization of the 5-phosphate group than that of the 4-phosphate, suggesting that the interaction of PE with the 5-phosphate is stronger. In PI(4,5)P-2/PS/PC lipid vesicles, the presence of PS was expected to yield an increased protonation of the PI(4,5)P-2 phosphomonoester groups due to a decreased interfacial pH as a result of the increased negative interfacial charge. However, the effect of PS on PI(4,5)P-2 ionization is only minor, potentially suggesting that PS and PI(4,5)P-2 are demixed. The PI(4,5)P-2/PI/PC vesicle system was characterized by a surprising mixing behavior that has potentially far reaching consequences: fluorescence microscopy measurements of giant unilammellar vesicles composed of PI(4,5)P-2/PI/PC at physiological concentrations show that PI and PI(4,5)P-2 form macroscopic, fluid phase domains in contact with a fluid PC rich phase (fluid/fluid demixing). Despite the fact that PI and PI(4,5)P-2 co-localize, the effect of PI on PI(4,5)P-2 ionization behavior is only noticeable above pH 7. Apparently two opposing effects lead to the observed behavior: Due to the presence of the anionic PI, the interfacial pH drops, which is expected to lead to an enhanced protonation of the PI(4,5)P-2 phosphomonoester groups. In turn, hydrogen bond formation between PI and PI(4,5)P-2 would lead to the opposite, i.e. increased deprotonation of the phosphomonoester group. Apparently these two effects compensate each other for pH values smaller than about 7, while for higher pH values the increased interfacial pH in the presence of PI has a stronger impact than PI/PI(4,5)P-2 hydrogen bond formation. The cooperative formation of PI/PI(4,5)P-2 mixed domains has potentially important ramifications for the spatial organization of phosphoinositide mediated signaling events. (C) 2012 Elsevier Ireland Ltd. All rights reserved.