Phospholipid Headgroup Charge Modifies Condensing Effect of Gangliosides on Lipid Films

Abstract

In model membrane mixtures that mimic lipid raft compositions, the more ordered domains are enriched in the ganglioside, GM1, a glycolipid with a headgroup containing four neutral sugars and a negatively charged sialic acid. To understand the organization and partitioning of GM1 in cell membranes, the outer leaflet of the cell membrane was modeled using Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC), a phospholipid with a zwitterionic headgroup, and varying concentrations of GM1. At low biologically relevant concentrations, GM1 condenses the DPPC monolayer while at higher concentrations, it fluidizes, with a switch-over point between the two behaviors at a ratio of 3:1 DPPC:GM1.To examine the role of electrostatics, lipids with negatively charged phosphatidylglycerol (PG) and positively charged trimethylammonium-propane (TAP) headgroups were combined with various ratios of GM1. Fluidity of the monolayer was systematically altered by changing the hydrocarbon tail length. Additivity plots constructed for all mixtures show negative deviations from ideal mixing or condensation of the monolayer regardless of headgroup charge. For the zwitterionic and negatively charged lipids, the greatest condensation effect compared to the pure lipid was seen at low GM1 concentrations. In binary mixtures containing positively charged lipids, a similar magnitude of condensation occurred at all GM1 ratios. For less fluid lipid nears their triple point temperature, the addition of GM1 caused minimal condensation suggesting the effect is specific to lipids that can be easily ordered.