Abstract
We studied the interaction of spectrin dimers from human erythrocytes with (bilayer and monolayer) model membranes of mixtures of dimyristoylphosphatidylethanolamine, dimyristoyl-phosphatidylcholine, and dimyristoylphosphatidylserine (DMPS) by (1) densitometric evaluation of phase transitions and phase boundaries, (2) film balance experiments, and (3) microfluorescence. We demonstrate that spectrin readily adsorbs to mixed bilayers and monolayers even in the presence of small DMPS concentrations (30 mol %) whereas no appreciable interaction with lamellae containing zwitterionic lipids alone is observed. The selectivity of the DMPS/spectrin interaction is established by quantitative evaluation of the shifts of the phase boundaries (liquidus and solidus line) caused by the lipid/protein interaction as a function of the composition of the binary lipid mixtures. Quantitative information about the free energy of the lipid/protein interaction is obtained by computer simulation of the phase diagram of the lipid mixture in the absence or in the presence of a very small molar fraction of the protein and comparison of calculated and measured shifts. A binding energy of about 10-17J per spectrin molecule is found. The present perturbation method can be generalized to study selective lipid/protein interaction mechanisms in ternary or higher component mixtures. The present results provide evidence that in addition to the binding to band III, spectrin may also couple directly to the lipid moiety of the inner monolayer of erythrocytes. The spectrin/phosphatidylserine interaction energy is, however, not large enough to account solely for the asymmetric distribution of this lipid in erythrocytes. Since spectrin is flexible, it is expected to be highly folded and can thus act as an entropy spring, the stiffness of which may be controlled by the lipid/protein interaction.
Original language | English |
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Pages (from-to) | 2983-2990 |
Number of pages | 8 |
Journal | Biochemistry |
Volume | 26 |
Issue number | 11 |
DOIs | |
State | Published - 1987 |