Differential Effects of Liquid Ordered and Liquid Disordered Phases on Membrane Permeability

Phospholipid membranes segregate into lateral domains of liquid ordered (lo) and liquid disordered (ld) phases when cholesterol and mixed species of lipids with saturated and unsaturated acyl chains are present. To examine membrane permeability and detergent solubility in pure lo, pure ld, and mixed lo/ld phases, LUVs were prepared based on the ternary phase diagram of POPC, sphingomyelin, and cholesterol. These LUVs were loaded with 2mM carboxyfluorescein (CF) and formed by extrusion at 70C. using a stopped-flow fluorometer, changes in CF fluorescence were measured when LUVs were exposed to sudden osmotic gradients, pH gradients, or 0.1% Triton. Acyl chain and phospholipid headgroup packing were assessed in all compositions with time-resolved measurements of DPH fluorescence lifetime and anisotropy decay. Water permeability was highest in the pure ld phase, and a factor of more than 100 lower in the pure lo phase. In the lo/ld coexistence region water permeability decreased approximately exponentially with increasing percent lo phase. Proton permeability was lowest in the pure ld phase, increasing linearly with increasing percent lo up to the “percolation point” (connected phase switches from lo to ld) at which point it remained approximately the same with increased lo. The rate of membrane solubilization was highest in the pure ld phase and did not decrease substantially until the percolation point was reached, and decreased by a factor of 40 in the pure lo phase. Water permeability was found to correlate approximately exponentially with acyl chain packing, decreasing with increased membrane order. Proton permeability increased linearly with increasing membrane order.