Lipid polymorphism in chloroplast thylakoid membranes – as revealed by 31P-NMR and time-resolved merocyanine fluorescence spectroscopy

Abstract

Chloroplast thylakoid membranes contain virtually all components of the energy-converting photosynthetic machinery. Their energized state, driving ATP synthesis, is enabled by the bilayer organization of the membrane. However, their most abundant lipid species is a non-bilayer-forming lipid, monogalactosyl-diacylglycerol; the role of lipid polymorphism in these membranes is poorly understood. Earlier 31P-NMR experiments revealed the coexistence of a bilayer and a non-bilayer, isotropic lipid phase in spinach thylakoids. Packing of lipid molecules, tested by fluorescence spectroscopy of the lipophilic dye, merocyanine-540 (MC540), also displayed heterogeneity. Now, our 31P-NMR experiments on spinach thylakoids uncover the presence of a bilayer and three non-bilayer lipid phases; time-resolved fluorescence spectroscopy of MC540 also reveals the presence of multiple lipidic environments. It is also shown by 31P-NMR that: (i) some lipid phases are sensitive to the osmolarity and ionic strength of the medium, (ii) a lipid phase can be modulated by catalytic hydrogenation of fatty acids and (iii) a marked increase of one of the non-bilayer phases upon lowering the pH of the medium is observed. These data provide additional experimental evidence for the polymorphism of lipid phases in thylakoids and suggest that non-bilayer phases play an active role in the structural dynamics of thylakoid membranes.

Lipid‐polymorphism of plant thylakoid membranes. Enhanced non‐bilayer lipid phases associated with increased membrane permeability

“…OJIP transients also showed very characteristic changes, with a clear trend, increasing most prominently the initial rise, the O‐J phase, with increasing time of thylakoid membrane storage. In agreement with our earlier measurements (Garab et al ), the F v / F m ratio decreased only slightly and gradually, from 0.765 ± 0.025 (0 h) to 0.754 ± 0.027 (6 h) and to 0.740 ± 0.031 (8 h); also, the dependence of the F 0 as a function of the storage time showed no significant variation (data not shown). It is also noteworthy that storage exerted no discernible effect on the rise kinetics recorded in the presence of DCMU (Fig.…”

“…Instability of the lipid‐phase behaviour of thylakoid membranes has already been indicated by the strong temperature dependence of the 31 P‐NMR spectra of thylakoids (Krumova et al ): with the gradual increase of the temperature, between 7 and 28°C, the intensity of the bilayer‐phase resonances gradually diminished and disappeared, while the isotropic peak increased. In our earlier study (Garab et al ), we have observed a similar instability of the bilayer phase and the increase of the isotropic phase(s) during the storage of membranes in the dark at 5°C. These variations could not be correlated with similar changes in the basic structural and functional parameters of the thylakoid membranes, tested by CD and 77 K fluorescence spectroscopy and Chl a fluorescence of PSII, the F v / F m , ratio.…”

“…As shown in Fig. A, the 31 P‐NMR spectra of spinach thylakoid membranes displayed the four characteristic resonances observed earlier (Garab et al ), around −10 ppm, at 2.5 and 3.6 ppm and around 25 ppm, assigned to the bilayer, the two isotropic and the inverted hexagonal (H II ) phases, respectively. An inspection of the integrated intensities revealed that during storage, the intensity of the two peaks originating from the isotropic phases increased gradually by about 55% and almost 200%, on the low‐ and high‐field sides, respectively; at the same time, the intensity (area) of the bilayer phase dropped to about half of its original value, while that of the H II phase revealed a less pronounced trend of changes during time (Fig.…”

“…We have found that the changes in PSII stability, and in the chiral macro‐organization of the complexes, as well as the associated changes in the lipid phases, detected by time‐resolved MC540 fluorescence spectroscopy, are almost fully reversible, suggesting that most of the extruded lipids remain closely associated with the bilayer membrane, in agreement with the DEM (Kotakis et al ). These experiments along with those published earlier (Krumova et al ,b, Garab et al ) have (1) provided irrevocable evidence that plant thylakoid membranes cannot be portrayed as containing only one lipid phase, the bilayer, but rather a complex structure with characteristic lipid polymorphism; (2) revealed that the lipid phases are not static but exhibit remarkable structural plasticity, sensitively reacting to changes in the physico‐chemical environment, temperature, ionic strength and osmotic pressure as well as the pH of the medium, suggesting that lipid polymorphism of thylakoid membranes plays an important role in determining the structural and functional flexibility of the photosynthetic machinery. However, in general, the nature and significance of these newly observed dynamic features are not understood.…”

“…Earlier, we have shown that the 31 P‐NMR signature of freshly isolated, fully functional thylakoid membranes exhibited a surprisingly high instability during a few hours storage of the concentrated membrane suspension at 5°C in the dark (Garab et al ). These changes could not be correlated with the basic structural and functional parameters of thylakoid membranes tested under similar experimental conditions: circular dichroism (CD) spectra recorded at different intervals between 0 and 8 h were essentially indistinguishable from each other, showing the robustness of the macro‐organization of the protein complexes; the 77 K fluorescence emission spectra exhibited no systematic variations, revealing no significant changes in the distribution of excitation energy in the photosynthetic apparatus; also, variations in the F v /F m , the ratio of variable to maximal chlorophyll (Chl) a fluorescence intensities, were very small, about 1–3%, depending on the sample, showing essentially no changes in PSII activity (Garab et al ).…”