Non-raft submicron domain formation in cholesterol-containing lipid bilayers induced by polyunsaturated phosphatidylethanolamine.

Domain formation in "HLC" ternary lipid bilayers, comprising a high transition temperature (High-T) lipid, a Low-T lipid, and cholesterol (Chol), has been extensively studied as raft-resembling systems. Recently, we reported the formation of submicron domains in an "LLC" lipid bilayer, encompassing Low-T phosphatidylethanolamine (PE), Low-T phosphatidylcholine (PC), and Chol. We hypothesized that the formation of this unique domain is driven by polyunsaturated PE. In this study, we explored the effects of the degree of PE unsaturation and the double bond distribution at the sn-position on the mechanism of formation and the composition of submicron domains. Supported lipid bilayers (SLBs), comprising PE with various degrees of unsaturation, monounsaturated PC (POPC), and Chol, were investigated using fluorescence microscopy, atomic force microscopy, and the force-distance curve measurement. The area fraction of submicron domains in PE+POPC+Chol-SLB increased with the PE concentration and degree of unsaturation of the PE acyl chain. The results indicated that the submicron domains were enriched with polyunsaturated PE and were in the liquid-disordered-like state, whereas their surrounding regions were in the liquid-ordered-like state. Segregation of polyunsaturated PE from the Chol-containing region generated submicron domains in the LLC lipid bilayer. We propose a mechanism for the formation of these submicron domains based on molecular interactions involving the hydrophobic and hydrophilic parts of the bilayer membrane.