Structural characteristics of thylakoid membranes of Arabidopsis mutants deficient in lipid fatty acid desaturation

Composition, phosphorylation and dynamic organization of photosynthetic protein complexes in plant thylakoid membrane

The photosystems (PS), catalyzing the photosynthetic reactions of higher plants, are unevenly distributed in the thylakoid membrane: PSII, together with its light harvesting complex (LHC)II, is enriched in the appressed grana stacks, while PSI-LHCI resides in the non-appressed stroma thylakoids, which wind around the grana stacks. The two photosystems interact in a third membrane domain, the grana margins, which connect the grana and stroma thylakoids and allow the loosely bound LHCII to serve as an additional antenna for PSI. The light harvesting is balanced by reversible phosphorylation of LHCII proteins. Nevertheless, light energy also damages PSII and the repair process is regulated by reversible phosphorylation of PSII core proteins. Here, we discuss the detailed composition and organization of PSII-LHCII and PSI-LHCI (super)complexes in the thylakoid membrane of angiosperm chloroplasts and address the role of thylakoid protein phosphorylation in dynamics of the entire protein complex network of the photosynthetic membrane. Finally, we scrutinize the phosphorylation-dependent dynamics of the protein complexes in context of thylakoid ultrastructure and present a model on the reorganization of the entire thylakoid network in response to changes in thylakoid protein phosphorylation.

Functional Implications of Photosystem II Crystal Formation in Photosynthetic Membranes

The structural organization of proteins in biological membranes can affect their function. Photosynthetic thylakoid membranes in chloroplasts have the remarkable ability to change their supramolecular organization between disordered and semicrystalline states. Although the change to the semicrystalline state is known to be triggered by abiotic factors, the functional significance of this protein organization has not yet been understood. Taking advantage of an Arabidopsis thaliana fatty acid desaturase mutant (fad5) that constitutively forms semicrystalline arrays, we systematically test the functional implications of protein crystals in photosynthetic membranes. Here, we show that the change into an ordered state facilitates molecular diffusion of photosynthetic components in crowded thylakoid membranes. The increased mobility of small lipophilic molecules like plastoquinone and xanthophylls has implications for diffusion-dependent electron transport and photoprotective energy-dependent quenching. The mobility of the large photosystem II supercomplexes, however, is impaired, leading to retarded repair of damaged proteins. Our results demonstrate that supramolecular changes into more ordered states have differing impacts on photosynthesis that favor either diffusion-dependent electron transport and photoprotection or protein repair processes, thus fine-tuning the photosynthetic energy conversion.

Chloroplast ultrastructure regeneration with protection of photosystem II is responsible for the functional 'stay-green' trait in wheat

CN17 is a functional stay-green wheat variety that exhibits delayed leaf senescence and enhanced photosynthetic competence. To better understand these valuable traits, levels of chlorophyll a and b, soluble proteins, unsaturated fatty acids, and other components of CN17 were assayed. In addition, chloroplast ultrastructure, chloroplast number, and differences in gene expression between CN17 and a control variety, MY11, were examined. By 21 d post-anthesis (DPA), CN17 leaves exhibited a significantly higher maximal photochemical efficiency for photosystem II (PSII) (F(v) /F(m) ) and a significantly higher efficiency of excitation capture by open PSII reaction centres (F(v) '/F(m) '). In addition, chlorophyll degradation in CN17 was delayed by approximately 14 d, and was not blocked as observed in cosmetic stay-green phenotypes. The soluble protein content (Ps) of CN17 was higher than MY11 at all timepoints assayed, and the ratio of unsaturated to saturated fatty acids was significantly higher. CN17 also exhibited isolated granal lamellae associated with vesicles and diminished peroxidation, and between 35 and 42 DPA, a sharp decrease in chloroplast number was detected. Taken together, these results strongly support the hypothesis that chloroplast ultrastructure regeneration is responsible for the functional stay-green trait of CN17, and gene expression data provide insight into the mechanistic details.

Molecular crowding and order in photosynthetic membranes

The integrity and maintenance of the photosynthetic apparatus in thylakoid membranes of higher plants requires lateral mobility of their components between stacked grana thylakoids and unstacked stroma lamellae. Computer simulations based on realistic protein densities suggest serious problems for lateral protein and plastoquinone diffusion especially in grana membranes, owing to strong retardation by protein complexes. It has been suggested that three structural features of grana thylakoids ensure efficient lateral transport: the organization of protein complexes into supercomplexes; the arrangement of supercomplexes into structured assemblies, which facilitates diffusion process in crowded membranes; the limitation of the diameter of grana discs to less than approximately 500 nm, which keeps diffusion times short enough to support regulation of light harvesting and repair of photodamaged photosystem II.

Regulation of membrane fatty acid composition by temperature in mutants of Arabidopsis with alterations in membrane lipid composition

BACKGROUND

A wide range of cellular responses occur when plants are exposed to elevated temperature, including adjustments in the unsaturation level of membrane fatty acids. Although membrane bound desaturase enzymes mediate these adjustments, it is unknown how they are regulated to achieve these specific membrane compositions. Furthermore, the precise roles that different membrane fatty acid compositions play in photosynthesis are only beginning to be understood. To explore the regulation of the membrane composition and photosynthetic function in response to temperature, we examined the effect of temperature in a collection of mutants with altered membrane lipid fatty acid composition.

RESULTS

In agreement with previous studies in other species, the level of unsaturation of membrane fatty acids in Arabidopsis was inversely correlated with growth temperature. The time required for the membrane fatty acids to attain the composition observed at elevated temperature was consistent with the timing required for the synthesis of new fatty acids. Comparisons of temperature-induced fatty acid alterations in membranes were made among several Arabidopsis lines including wild-type Columbia, and the compositional mutants, fad5, fad6, act1 and double mutants, fad7 fad8 and act1 fad6. The results revealed key changes that occur in response to elevated temperature regardless of the specific mutations in the glycerolipid pathway, including marked decreases in trienoic fatty acids and consistent increases in unsaturated 16:0 and in dienoic 18:2 levels. Fluorescence measurements of various mutants indicated that photosynthetic stability as well as whole plant growth at elevated temperature is influenced by certain membrane fatty acid compositions.

CONCLUSIONS

The results of this study support the premise that defined proportions of saturated and unsaturated fatty acids in membrane lipids are required for photosynthetic thermostability and acclimation to elevated temperature. The results also suggest that changes in the membrane fatty acid composition brought about in response to temperature are regulated in such a way so as to achieve highly similar unsaturation levels despite mutations that alter the membrane composition prior to a high-temperature exposure. The results from examination of the mutant lines also suggest that interorganellar transfer of fatty acids are involved in mediating temperature-induced membrane alterations, and reveal steps in the fatty acid unsaturation pathway that appear to have key roles in the acclimatization of membranes to high temperature.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Effect of modification of light-harvesting complex II on fluorescence properties of thylakoid membranes of Arabidopsis thaliana

The 77 K chlorophyll fluorescence spectra of Arabidopsis thaliana mutants deficient in lipid fatty acid desaturation have been used in order to further explore the influence of the modification of LHC II after mutation and proteolitic treatment on the energy transfer between the chlorophyll-protein complexes, as well as on the structure-function relationship in the supramolecular complex of Photosystem II. The gaussian decomposition and analysis of the fluorescence bands associated with PS II complex show the controversial action of the trypsin in the investigated thylakoid membranes. This reveals that the organization of PS II complexes is different in the wild type and both mutants indicating altered connection between the LHC II and the RC core complexes of PS II in both mutants. The results obtained demonstrate that different amounts of oligomer and monomer forms of LHC II in the mutants (LK3 and JB67), arising from lipid modification, are responsible for different proteolytic action in their thylakoid membranes.

Multiple crystal types reveal photosystem II to be a dimer

Three types of photosystem II (PS II) crystals have been produced using a variety of detergents. Intermediate stages of crystal formation were examined and it was determined that each crystal probably originates from a single grana membrane. Each crystal type was examined by electron microscopy and image processing, providing three different projection maps. The highest resolution results came from type 1 and type 2 crystals. Projection maps from these crystals were examined for two-fold symmetry via difference maps between the unsymmetrized averages and their 180 degrees rotation. A comparison of the final maps shows a high degree of two-fold symmetry, with only slight differences noted in the low density regions of the two halves of the structure. The interpretation is that PS II is a dimer, with the further suggestion that the two reaction center cores may have slightly different complements of antennae polypeptides.

Surface electric properties of thylakoid membranes from Arabidopsis thaliana mutants

Electric light scattering measurements of thylakoid membranes from wild type and two mutant forms (JB67 and LK3) of Arabidopsis thaliana have shown that application of external electric pulses induces electric dipole moments of different origin. The asymmetric surface charge distribution and electric polarizability are significantly altered by the lipid modification. Mild trypsin treatment of Arabidopsis thylakoids leading to digestion of small polypeptides from the light-harvesting chlorophyll a/b protein complex of photosystem II (LHCP II) gives evidence for a lower content of LHCP II in the mutant forms. The results demonstrate the significance of the level of thylakoid lipid unsaturation in determining the surface charge distribution through changes either in the pigment-protein content and membrane appression induced by the lipid modification or in the exposure of charged polypeptides on the thylakoid membrane surface(s) arising from alteration of the lipid geometry.

Low unsaturation level of thylakoid membrane lipids limits turnover of the D1 protein of photosystem II at high irradiance

Turnover of the D1 protein of photosystem II (PSII) was studied in mutants of Synechocystis sp. PCC 6803 defective in the desaturation of thylakoid membrane lipids. The lack of polyunsaturated fatty acids from membranes made PSII extremely susceptible to photoinhibition and caused a significant reduction in the D1 protein content of the thylakoid membranes at high irradiances. These results may be attributed to an impaired function in the protein synthesis machinery, most probably at the translational or posttranslational level.