Lipid-protein interactions in crystals of plant light-harvesting complex

Two different thylakoid lipids are specifically associated with the light-harvesting complex of photosystem II (LHC-II). Digalactosyl diacyl glycerol (DGDG) binds to the isolated complex but can be removed by mild detergent treatment and anion-exchange chromatography. Removal of this lipid renders the complex unable to form two-dimensional or three-dimensional crystals. The ability to crystallize is completely restored by addition of pure DGDG, at a ratio of about four molecules per polypeptide for three-dimensional crystals, suggesting several binding sites at the periphery of the trimeric complex. Two-dimensional crystals of purified protein grown in the presence of DGDG are more highly ordered than those obtained from the unfractionated complex. The other lipid, phosphatidyl glycerol (PG), binds more firmly and cannot be removed with non-ionic detergent. Complete delipidation of LHC-II can be achieved either with phospholipase or by proteolytic cleavage of 49 amino acid residues at the N terminus. Both treatments dissociate the native, trimeric complex into monomers. This indicates that PG is directly involved in the formation of trimers, which are a prerequisite for two-dimensional and three-dimensional crystallization. Both lipids are therefore present in two-dimensional and three-dimensional crystals and have distinct roles in the structure of the complex.