Induction of Ordered Lipid Raft Domain Formation by Loss of Lipid Asymmetry

How lipid asymmetry impacts ordered lipid domain (raft) formation may yield important clues to how ordered domain formation is regulated in vivo. Under some conditions, a sphingomyelin (SM) and cholesterol-rich ordered domain in one leaflet induces ordered domain formation in the corresponding region of an opposite leaflet composed of unsaturated phosphatidylcholine (PC) and cholesterol. In other conditions, the formation of ordered domains in a SM and cholesterol-rich leaflet can be suppressed by an opposite leaflet containing unsaturated PC and cholesterol. To explore how PC unsaturation influences the balance between these behaviors, domain formation was studied in asymmetric and symmetric lipid vesicles composed of egg SM, cholesterol, and either unsaturated dioleoyl PC (DOPC) or 1-palmitoyl-2-oleoyl PC (POPC). The temperature dependence of ordered domain formation was measured using Forster resonance energy transfer, which detects nanodomains as well as large domains. In cholesterol-containing asymmetric SM+PC outside/PC inside vesicles, the PC-containing inner leaflet tended to destabilize ordered domain formation in the SM+PC-containing outer leaflet relative to ordered domain stability in cholesterol-containing symmetric SM/PC vesicles. Residual ordered domain formation was detected in cholesterol-containing asymmetric SM+DOPC outside/DOPC inside vesicles, but ordered domain formation was completely or almost completely suppressed by asymmetry in cholesterol-containing SM+POPC outside/POPC inside vesicles over the entire temperature range measured. Suppression of ordered domain formation in the latter vesicles was confirmed by fluorescence anisotropy measurements. Because mixtures of SM, POPC, and cholesterol form domains in symmetric vesicles, and this lipid composition mimics plasma membranes to a significant degree, it is possible that under some conditions in vivo the loss of lipid asymmetry could trigger ordered domain formation.