Selective Protein-Lipid Interactions at Membrane Surfaces: A Deuterium and Phosphorus Nuclear Magnetic Resonance Study of the Association of Myelin Basic Protein with the Bilayer Head Groups of Dimyristoylphosphatidylcholine and Dimyristoylphosphatidylglycerol
Sixl F., Watts A., Brophy PJ.
Basic protein from bovine spinal cord was reconstituted into bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine, 1,2-dimyristoyl-in-glycero-3-phospho-rac-glycerol, and equimolar mixtures of both phospholipids at various protein concentrations up to 58 wt %. The lipids were selectively deuterated at all positions in their polar head groups, the motional rate and amplitude of which were characterized by deuterium and phosphorus-31 nuclear magnetic resonanee (NMR). Basic protein, which binds peripherally to membranes, did not perturb any part of the head groups in phosphatidylcholine bilayers in such a way as to induce any significant changes in the NMR spectra. However, basic protein induced large effects on the phosphorus-31 and the deuterium NMR spectra of bilayers of dimyristoylphosphatidylglycerol. When compared to those of pure phosphatidylglycerol bilayers at 35 °C, the measured quadrupole splittings decreased by 36%, 28%, and 50% for the α-CD2, β-CD, and γ-CD2 glycerol segments, respectively, and the phosphorus-31 chemical shift anisotropy (CSA) was reduced by 34% from -39 to -25 ppm by the addition of 50 wt % of basic protein. Deuterium spin-lattice relaxation times T1 for the β-CD and the γ-CD2 segments decreased only slightly after the addition of basic protein. These results indicate that basic protein interacts with the acidic polar head group of phosphatidylglycerol, affecting its orientation or amplitude of motion, as well as its rate of high-frequency segmental motions. Basic protein was also reconstituted into mixed bilayers containing equimolar amounts of labeled and unlabeled phospholipid. When the head group of phosphatidylglycerol contained the label, the quadrupole splittings, spin-lattice relaxation times T1, and phosphorus-31 CSA values were sensitive to increasing amounts of protein up to 50 wt %, the magnitude of the induced changes being very similar to those measured for phosphatidylglycerol bilayers devoid of phosphatidylcholine. However, the major perturbation of the phosphatidylglycerol head group motion in these mixed bilayers was due not to the protein but to the phosphatidylcholine lipids. For mixed bilayers in which the phosphatidylcholine was labeled, the experimental parameters showed little change when up to 58 wt % of basic protein was added, the choline head group motion being considerably more sensitive only to the phosphatidylglycerol molecules. An analysis of the results showed that in all systems studied with protein concentrations of less than 33 wt %, fast exchange of all the lipids takes place between the bulk, protein-free bilayer phase and the protein-associating phase, with no evidence for lateral phase separation of lipidprotein domains with a lifetime of longer than approximately 1 ms. The number of acidic lipids interacting with the basic protein in these complexes is estimated to be between 15 and 28 molecules. © 1984, American Chemical Society. All rights reserved.