La3+ stabilizes the hexagonal II (HII) phase in phosphatidylethanolamine membranes

AbstractThe mechanism of the effects of the lanthanum ion (La3+) and the gadolinium ion (Gd3+), which are lanthanides, on the function of membrane proteins and the stability of the membrane structure is not well understood. We investigated the effects of La3+ on the stability of the hexagonal II (HII) phase of the phosphatidylethanolamine (PE) membrane at 20°C by small-angle X-ray scattering. As PE membrane we used DPOPE (dipalmitoleoylphosphatidylethanolamine) membrane, which was in the Lα phase in 10 mM PIPES buffer (pH 7.4) at 20°C. An Lα to HII phase transition occurred in the DPOPE membrane at 1.4 mM La3+ in 0 M KCl, and at 0.4 mM La3+ in 0.5 M KCl and above the critical concentrations the membranes were in the HII phase, indicating that La3+ stabilizes the HII phase rather than the Lα phase. The basis vector length, d, of DPOPE and DOPE (dioleoylphosphatidylethanolamine) membranes containing 16 wt% tetradecane in excess water condition did not change with an increase in La3+ concentration, suggesting that La3+ did not change the spontaneous curvature of these PE monolayer membranes. The chain-melting transition temperature of the dielaidoylphosphatidylethanolamine membrane increased with an increase in La3+ concentration, indicating that the lateral compression pressure increased. To elucidate the effects of a small percentage of ‘guest’ lipids with longer acyl chains than the average length of ‘host’ lipids on the stability of the HII phase, we investigated the effects of the concentration of a guest lipid (DOPE) in a host lipid (DPOPE) membrane on their phase behavior and structure. 12 mol% DOPE induced an Lα to HII phase transition in DOPE/DPOPE membrane, without changing the spontaneous curvature of the monolayer membrane. We found that Ca2+ also induced an Lα to HII phase transition in the DPOPE membrane, and compared the effects of Ca2+ on PE membranes with those of La3+. Based on these results, we have proposed a new model for the mechanism of the Lα to HII phase transition and the stabilization of the HII phase by La3+