Folding, assembly, and stability of the major light-harvesting complex of higher plants, LHCII, in the presence of native lipids

The influence of thylakoid lipids on the association kinetics and thermal s tability of the major light-harvesting complex of photosytem II (LHCII) has been studied in vitro. The apoprotein, light-harvesting chlorophyll nib-bi nding protein (Lhcb1), can be refolded and complexed with pigments in deter gent solution even in the absence of lipids. Two thylakoid lipids, phosphat idyl glycerol and digalactosyl diacylglycerol, are known to interact specif ically with LHCII in vivo. Here we show that both of these lipids, as well as monogalactosyl diacylglycerol, stabilize reconstituted LHCII toward ther mal denaturation. Two slow kinetic phases are connected with the establishm ent of energy transfer between chlorophyll b and chlorophyll a and, thus, a re thought to reflect the formation of the pigment-protein complex with tig htly coupled chlorophylls. The lipids studied here all have the same effect on the rate of complex assembly in vitro and slow these two kinetic phases by the same degree. Both kinetic phases also slow when reactant concentrat ions are decreased, suggesting that the corresponding reaction step(s) invo lve(s) pigment binding.