Thermal stability of bivalent cation/phosphoinositide domains in model membranes.

As key mediators in a wide array of signaling events, phosphoinositides (PIPs) orchestrate the recruitment of proteins to specific cellular locations at precise moments. This intricate spatiotemporal regulation of protein activity often necessitates the localized enrichment of the corresponding PIP. We investigate the extent and thermal stabilities of phosphatidylinositol-4-phosphate (PI(4)P), phosphatidylinositol-4,5-bisphosphate (PI(4,5)P and phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P) clusters with calcium and magnesium ions. We observe negligible or minimal clustering of all examined PIPs in the presence of Mg ions. While PI(4)P shows in the presence of Ca no clustering, PI(4,5)P forms with Ca strong clusters that exhibit stablity up to at least 80°C. The extent of cluster formation for the interaction of PI(3,4,5)P with Ca is less than what was observed for PI(4,5)P, yet we still observe some clustering up to 80°C. Given that cholesterol has been demonstrated to enhance PIP clustering, we examined whether bivalent cations and cholesterol synergistically promote PIP clustering. We found that the interaction of Mg or Ca with PI(4)P remains extraordinarily weak, even in the presence of cholesterol. In contrast, we observe synergistic interaction of cholesterol and Ca with PI(4,5)P. Also, in the presence of cholesterol, the interaction of Mg with PI(4,5)P remains weak. PI(3,4,5)P does not show strong clustering with cholesterol for the experimental conditions of our study and the interaction with Ca and Mg was not influenced by the presence of cholesterol.