Domain Morphologies of Complex Phosphoinositide/Lipid Langmuir Films in the Presence of Bivalent Cations

Abstract

Although phosphatidylinositol (PI) and phosphoinositides (PIPs) only comprise a small percentage of the inner leaflet of the plasma membrane, they mediate a large variety of signaling events. In previous studies, we have observed the absence of macroscopically discernible domains in mixtures of PI/PC and PI(4,5)P2/PC. The addition of cholesterol to these mixtures results in condensation of the monolayer and hence domain formation. To better mimic the ionic conditions and hydrogen bonding properties of the inner leaflet plasma membrane, we investigated in this study the effect of common inner leaflet plasma membrane lipids like phosphatidylethanolamine (PE), phosphatidylserine (PS) and PI, on phosphoinositide domain behavior in the presence of cholesterol and/or bivalent cations. We find that the addition of varying concentrations of Ca2+ to PI(4,5)P2/cholesterol monolayer leads to a size reduction of the domains. We hypothesize 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. We find for PE/PI(4,5)P2 and PI/PI(4,5)P2 homogeneous mixing of the monolayer. The addition of cholesterol to PE/PI(4,5)P2 leads to the formation of small domains at low surface pressures that disappear at higher pressures. In addition to cholesterol, we also investigate in this study the effect of bivalent cations on these lipid mixtures.