Spin label EPR study of lipid solvation of supramolecular photosynthetic protein complexes in thylakoids

Lipid-protein association in the chloroplast membrane and its various thylakoid fractions from higher plants, namely pea and maize, rich in Photosystem I (PSI) and Photosystem II (PSII), respectively, were studied using EPR spectroscopy of spin-labelled lipid molecules. All the EPR spectra consisted of two spectral components corresponding to bulk fluid lipids and solvation lipids motionally restricted at the hydrophobic surface of membrane proteins. Spin-labelled stearic acid and phosphatidylglycerol exhibited marked selectivity towards the supramolecular protein complexes of both PSI and PSII although to different extent. In addition, lipid-protein titration experiments are described for partially delipidated PSII-enriched membrane fractions of pea chloroplasts, incorporating unlabelled egg phosphatidylcholine prior to or after the incorporation of spin-labelled lipids. Two sets of solvation sites were resolved by timed labelling experiments and a significant result of these studies was that a well-defined population of solvation sites (approx. 100 mol lipids/820 kDa protein) was rapidly exchanged by laterally diffusing membrane lipids, while other solvation sites (approx. 50 mol lipids/820 kDa protein) were exchanged much slower or not exchanged at all.