Polymorphic phase behaviour of phosphatidylglycerine in spinach thylakoid membranes

Our data show that the phospholipids of chloroplast thylakoid membranes participate in non-lamellar phases and polymorphic changes. Although 31P NMR is sensitive solely to phospholipids, it seems plausible to assume that the transitions involve the entire lipid mixture, the non-lamellar propensity of which is due to its major lipid, the non-bilayer forming lipid monogalactosyl diacylglycerol. We found that the phase transitions of the phosphatidylglycerol (PG) in spinach thylakoid membranes occur at temperatures below 25 oC. Even at temperature as low as 7 oC PG is involved in at least two phases (lamellar and hexagonal). With the increase of the temperature or illumination with strong light PG gradually leaves the lamellar phase and contributes to hexagonal phase and a phase characterised by isotropic motion of the lipid molecules, which can be due to cubic organisation of the molecules. The fact that the illumination can trigger the phase transitions, similarly to the increase of the temperature, strongly suggests a thermo-optic origin. According to the thermo-optic mechanism, the excess excitation energy, which can not be used in photosynthesis and thus dissipated as heat, triggers structural rearrangements of the proteins in the thylakoid membrane (Cseh Z. et al., 2000, Biochemistry, 39: 15250; Dobrikova A. et al., 2003, Biochemistry 42: 11272). Although earlier data have clearly shown that lipids play a crucial role in the structural flexibility of light harvesting complex II-containing macroassemblies (Simidjiev I. et al., 1998, Biochemistry 37: 4139), this is the first direct evidence showing that these reorganizations in the protein matrix are accompanied, or perhaps even triggered, by lipid phase transitions. The mechanistic details of the light- and heat-induced phase transitions are currently investigated. It can, however, be concluded that the polymorphic phase behavior of lipids contributes significantly to the structural flexibility of the thylakoid membranes.