Membrane biophysics, Photosynthesis

In chloroplast thylakoid membranes, the functional state of which is a bilayer, the non-bilayer lipid monogalatosyl-diacylglycerol (MGDG) accounts for about half of the lipid content. MGDG is known to play key role in the operation of violaxanthin de-epoxidase and is present in most pigment-protein complexes, but this does not explain its high abundance in all oxygenic photosynthetic organisms. It has been hypothesized that non-bilayer lipids, via their segregation capability, regulate the protein-to-lipid ratio of the membranes, and contribute to the structural flexibility of thylakoid membranes. Earlier, using 31P-NMR and steady state and time-resolved fluorescence measurements, using merocyanine 540 (MC540) on spinach thylakoid membranes we have shown the coexistence of a non-bilayer lipid phase and the bilayer phase of thylakoid membranes; and experiments on dgd1 mutant of Arabidopsis revealed the dependence of membrane stability on the concentration of MGDG. Here, in a series of experiments, using circular dichroism spectroscopy and time-resolved fluorescence on MC540-stained thylakoid membranes and by applying co-solute treatments of thylakoid membranes, we have further substantiated these conclusions. Our recent 31P-NMR experiments revealed the presence of two isotropic phases and one inverted hexagonal phase, in addition to the bilayer phase and showed that the phase behavior of isolated thylakoid membranes (i) is sensitive to the osmolarity and ionic strength of the medium, (ii) can be modulated by rigidifying the membranes, and (iii) exhibit a marked increase of one of the isotropic phases upon lowering the pH of the medium. We propose a membrane model in which fusion channels at the granum-stroma junctions and lipocalin:lipid assemblies play important roles and in which non-bilayer lipid phases, by adding a new dimension to the structural flexibility of thylakoid membranes, determine the dynamic features of membranes.