The Effect of Divalent Cations on Complex Phosphoinositide/Lipid Monolayers at the Air/Water Interface

Abstract

ObjectiveInvestigate the effect of common inner leaflet plasma membrane lipids such as phosphatidylethanolamine (PE), phosphatidylserine (PS) and phosphatidylinositol (PI), on phosphoinositide domain behavior in the presence of cholesterol and/or divalent cations.BackgroundPI and phosphoinositides (PIPs) may only comprise a small percentage of the inner leaflet of the plasma membrane, but they have major implications in a variety of signaling events. Previously, we have observed the absence of macroscopically discernible domains in mixtures of PI/PE and PI(4,5)P2/PE. Upon the addition of cholesterol to these mixtures, we observe condensation of the monolayer and domain formation.MethodsWe use epifluorescence microscopy to image domain formation in lipid monolayers at the air/water interfaceResultsWe find that the addition of varying concentrations of Ca2+ to PE/PI(4,5)P2/Cholesterol monolayers result in a decrease in the size of the domains. Our hypothesis is that this is due to a penetration of the Ca2+ ions into the PI(4,5)P2 headgroup region, leading to a disruption of the hydrogen bond network formed by the PI(4,5)P2 headgroup and cholesterol. For both PE/PI(4,5)P2 and PI/PI(4,5)P2, we find homogeneous mixing of the monolayer. Alternatively, when cholesterol is added to PE/PI(4,5)P2 we observe the formation of small domains at low pressures. In addition to cholesterol, we also investigate the effect of divalent cations on these lipid mixtures.