The effect of an integral membrane protein on lipid polymorphism in the cardiolipin-Ca2+ system

The addition of Ca2+ to aqueous dispersions of cardiolipin triggers complete hexagonal (HII) phase formation at Ca2+/cardiolipin molar ratios greater than or equal to 1.0 as detected by 31PNMR and freeze-fracture electron microscopy. Incorporation of the integral membrane protein glycophorin prevents the bilayer leads to hexagonal (HII) phase transition at Ca2+/cardiolipin ratios as high as 15:1. Removal of the outwardly oriented, negatively charged sialic-acid-containing sugar groups of glycophorin with trypsin had little effect on the bilayer-stabilizing capacity of the protein. As the Ca2+ binding was found to be similar in both the cardiolipin and the cardiolipin-glycophorin systems, it can be concluded that the protein exerts a bilayer-stabilizing effect on the cardiolipin. In addition, the possibility that glycophorin may prevent vesicle fusion is also discussed.