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 stud ied using EPR spectroscopy of spin-labelled lipid molecules. AU the EP R spectra consisted of two spectral components corresponding to bulk f luid lipids and solvation lipids motionally restricted at the hydropho bic surface of membrane proteins. Spin-labelled stearic acid and phosp hatidylglycerol exhibited marked selectivity towards the supramolecula r protein complexes of both PSI and PSII although to different extent. In addition, lipid-protein titration experiments are described for pa rtially delipidated PSII-enriched membrane fractions of pea chloroplas ts, incorporating unlabelled egg phosphatidylcholine prior to or after the incorporation of spin-labelled lipids. Two sets of solvation site s were resolved by timed labelling experiments and a significant resul t of these studies was that a well-defined population of solvation sit es (approx. 100 mol lipids/820 kDa protein) was rapidly exchanged by l aterally diffusing membrane lipids, while other solvation sites (appro x. 50 mol lipids/820 kDa protein) were exchanged much slower or not ex changed at all.