LIPID-PROTEIN INTERACTIONS IN ADP-ATP CARRIER EGG PHOSPHATIDYLCHOLINE RECOMBINANTS STUDIED BY SPIN-LABEL ESR SPECTROSCOPY

The stoichiometry and specificity of lipid–protein interaction, as well as the lipid exchange rates at the protein interface, have been determined from the electron spin resonance spectra of spin-labeled lipids in reconstituted complexes of the mitochondrial ADP–ATP carrier with egg phosphatidylcholine. With the exception of cardiolipin and phosphatidic acid, the lipids studied are found to compete for approximately 50 sites at the intramembranous surface of the protein dimer. This number of first-shell lipid sites is unusually large for a protein of this size. The specificity for the protein is in the order stearic acid ≈ phosphatidic acid ≈ cardiolipin ˃ phosphatidylserine ˃ phosphatidylglycerol ≈ phosphatidylcholine, with the maximum association constant relative to phosphatidylcholine being approximately 4. The selectivity for anionic lipids was partially screened with increasing ionic strength, but to a lesser extent for cardiolipin and phosphatidic acid than for stearic acid. Only in the case of phosphatidylserine was the selectivity reduced at high ionic strength to a level close to that for phosphatidylcholine. The off rates for lipid exchange at the protein surface were independent of lipid/protein ratio and correlated in a reciprocal fashion with the different lipid selectivities, varying from 5 × 106 s−1 for stearic acid at low ionic strength to 2 × 107 s−1 for phosphatidylcholine and phosphatidylglycerol. The off rates for cardiolipin were unusually low in comparison with the observed selectivity, and indicated the existence of a special population of sites (ca. 30% of the total) for cardiolipin, at which the exchange rate was very low. These results are discussed with reference to earlier observations on this hydrophobic membrane protein. © 1990, American Chemical Society. All rights reserved.