On the microstructure and phase diagram of dimyristoylphosphatidylcholine-glycophorin bilayers. The role of defects and the hydrophilic lipid-protein interaction

Glycophorin-dimyristoylphosphatidylcholine bilayers were studied at the limit of low protein concentrations by a number of different techniques. These included freeze-fracture electron microscopy, calorimetry, electron paramagnetic resonance spectroscopy, the photobleaching technique and energy transfer measurements. The liquidus and solidus lines of the dimyristoylphosphatidylcholine-glycophorin system were determined by differential thermal analysis. Structural information was obtained from changes in the texture of the freeze-fracture micrographs. Electron microscopy, together with diffusion measurements, established the important role of linear defects in the solid lipid phases for the incorporation of small amounts of glycophorin and for the fast transport of the protein. Particles were observed above 1 mol‰ of protein content, below 10°C, which form due to the immiscibility of glycophorin in the crystalline lipid phase. Lipid-protein aggregates are expelled from the bilayer into the aqueous phase. The concentration dependence of the energy transfer between fluorescein- and eosin-carrying protein headgroups is interpreted in terms of a two-conformation model: At low concentrations (x_p <0.8 mol‰) the carbohydrate-carrying headgroup spreads at the lipid-water interface forming a two-dimensional (pancake-like) structure, while at higher concentrations the headgroup starts to assume a three-dimensional conformation protruding into the aqueous phase. The concentration dependence of the heat of transition led to the conclusion that one protein molecule interacts with about 300 lipid molecules in the first conformation and with about 100 lipids in the second one. The high number of protein-bound lipids is explained by hydrophilic lipid-protein interaction. All experiments demonstrated that the protein concentration of 0.8 mol‰ plays a critical role. (1) The solidus line starts to exhibit a horizontal deflection between 0.4 and 3.2 mol‰ demonstrating solid state phase separation. (2) The spin label order parameter in the fluid lipid phase exhibits a minimum, indicating a maximum in the lipid packing density. (3) The regular ripple phase completely vanishes at 0.4 ‰ while it reappears at higher concentrations. This is related to a headgroup conformational change. The results are summarized in a tentative phase diagram of the dimyristoylphosphatidylcholine-glycophorin system.