La~(3+) stabilizes the hexagonal II (H_(II)) phase in phosphatidylethanolamine membranes

摘要

The mechanism of the effects of the lanthanum ion (La~(3+)) and the gadolinium ion (Gd~(3+)), 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 La~(3+) on the stability of the hexagonal II (H_(II)) phase of the phosphatidylethanolamine (PE) membrane at 20 ℃ by small-angle X-ray scattering. As PE membrane we used DPOPE (dipalmitoleoylphosphatidylethanolamine) membrane, which was in the L_α phase in 10 mM PIPES buffer (pH 7.4) at 20 ℃. An L_α to H_(II) phase transition occurred in the DPOPE membrane at 1.4 mM La~(3+) in 0 M KCl, and at 0.4 mM La~(3+) in 0.5 M KCl and above the critical concentrations the membranes were in the H_(II) phase, indicating that La~(3+) stabilizes the H_(II) phase rather than the L_α phase. The basis vector length, d, of DPOPE and DPOE (dioleoylphosphatidylethanolamine) membranes containing 16 wt% tetradecane in excess water condition did not change with an increase in La~(3+) concentration, suggesting that La~(3+) 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 La~(3+) 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 H_(II) 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 L_α to H_(II) phase transition in DOPE/DPOPE membrane, without changing the spontaneous curvature of the monolayer membrane. We found that Ca~(2+) also induced an L_α to H_(II) phase transition in the DPOPE membrane, and compared the effects of Ca~(2+) on PE membranes with those of La~(3+). Based on these results, we have proposed a new model for the mechanism of the L_α to H_(II) phase transition and the stabilization of the H_(II) phase by La~(3+).