Phosphatidylinositol-4,5-bisphosphate ionization and domain formation in the presence of lipids with hydrogen bond donor capabilities
Phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) 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)P2 in ternary lipid vesicle systems that contain in addition to PI(4,5)P2 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)P2, which can be attributed to increased deprotonation due to hydrogen bond formation between PE and the PI(4,5)P2 phosphomonoester groups. However, the effect of PE on PI(4,5)P2 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)P2 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)P2/PS/PC lipid vesicles, the presence of PS was expected to yield an increased protonation of the PI(4,5)P2 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)P2 ionization is only minor, potentially suggesting that PS and PI(4,5)P2 are demixed. The PI(4,5)P2/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)P2/PI/PC at physiological concentrations show that PI and PI(4,5)P2 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)P2 co-localize, the effect of PI on PI(4,5)P2 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)P2 phosphomonoester groups. In turn, hydrogen bond formation between PI and PI(4,5)P2 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)P2 hydrogen bond formation. The cooperative formation of PI/PI(4,5)P2 mixed domains has potentially important ramifications for the spatial organization of phosphoinositide mediated signaling events.