The importance of non-planar bilayer regions in photosynthetic membranes and their stabilisation by galactolipids

Archaeal lipids

The major archaeal membrane glycerolipids are distinguished from those of bacteria and eukaryotes by the contrasting stereochemistry of their glycerol backbones, and by the use of ether-linked isoprenoid-based alkyl chains rather than ester-linked fatty acyl chains for their hydrophobic moieties. These fascinating compounds play important roles in the extremophile lifestyles of many species, but are also present in the growing numbers of recently discovered mesophilic archaea. The past decade has witnessed significant advances in our understanding of archaea in general and their lipids in particular. Much of the new information has come from the ability to screen large microbial populations via environmental metagenomics, which has revolutionised our understanding of the extent of archaeal biodiversity that is coupled with a strict conservation of their membrane lipid compositions. Significant additional progress has come from new culturing and analytical techniques that are gradually enabling archaeal physiology and biochemistry to be studied in real time. These studies are beginning to shed light on the much-discussed and still-controversial process of eukaryogenesis, which probably involved both bacterial and archaeal progenitors. Puzzlingly, although eukaryotes retain many attributes of their putative archaeal ancestors, their lipid compositions only reflect their bacterial progenitors. Finally, elucidation of archaeal lipids and their metabolic pathways have revealed potentially interesting applications that have opened up new frontiers for biotechnological exploitation of these organisms. This review is concerned with the analysis, structure, function, evolution and biotechnology of archaeal lipids and their associated metabolic pathways.

Omega-3 PUFA profoundly affect neural, physiological, and behavioural competences - implications for systemic changes in trophic interactions

In recent decades, much conceptual thinking in trophic ecology has been guided by theories of nutrient limitation and the flow of elements, such as carbon and nitrogen, within and among ecosystems. More recently, ecologists have also turned their attention to examining the value of specific dietary nutrients, in particular polyunsaturated fatty acids (PUFA), among which the omega-3 PUFA, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) play a central role as essential components of neuronal cell membranes in many organisms. This review focuses on a new neuro-ecological approach stemming from the biochemical (mechanistic) and physiological (functional) role of DHA in neuronal cell membranes, in particular in conjunction with G-protein coupled receptors (GPCRs). We link the co-evolution of these neurological functions to metabolic dependency on dietary omega-3 PUFA. We outline ways in which deficiencies in dietary DHA supply may affect, cognition, vision, and behaviour, and ultimately, the biological fitness of consumers. We then review emerging evidence that changes in access to dietary omega-3 PUFA may ultimately have profound impacts on trophic interactions leading to potential changes in community structure and ecosystem functioning that, in turn, may affect the supply of DHA within and across ecosystems, including the supply for human consumption.

Dynamic Changes in Protein-Membrane Association for Regulating Photosynthetic Electron Transport

Photosynthesis has to work efficiently in contrasting environments such as in shade and full sun. Rapid changes in light intensity and over-reduction of the photosynthetic electron transport chain cause production of reactive oxygen species, which can potentially damage the photosynthetic apparatus. Thus, to avoid such damage, photosynthetic electron transport is regulated on many levels, including light absorption in antenna, electron transfer reactions in the reaction centers, and consumption of ATP and NADPH in different metabolic pathways. Many regulatory mechanisms involve the movement of protein-pigment complexes within the thylakoid membrane. Furthermore, a certain number of chloroplast proteins exist in different oligomerization states, which temporally associate to the thylakoid membrane and modulate their activity. This review starts by giving a short overview of the lipid composition of the chloroplast membranes, followed by describing supercomplex formation in cyclic electron flow. Protein movements involved in the various mechanisms of non-photochemical quenching, including thermal dissipation, state transitions and the photosystem II damage–repair cycle are detailed. We highlight the importance of changes in the oligomerization state of VIPP and of the plastid terminal oxidase PTOX and discuss the factors that may be responsible for these changes. Photosynthesis-related protein movements and organization states of certain proteins all play a role in acclimation of the photosynthetic organism to the environment.

Increased ratio of galactolipid MGDG : DGDG induces jasmonic acid overproduction and changes chloroplast shape

Galactolipids monogalactosyl diacylglycerol (MGDG) and digalactosyl diacylglycerol (DGDG) constitute c. 50% and c. 30% of chloroplast membrane lipids, respectively. They are important for photosynthesis and stress tolerance. Mutations in DGD1, the major DGDG-synthesizing enzyme, severely reduce DGDG content and induce jasmonic acid (JA) overproduction, resulting in stunted growth. However, how DGDG reduction leads to JA overproduction is unknown. We introduced an inducible microRNA (ami-MGD1) into an Arabidopsis dgd1 mutant to reduce MGDG synthesis, thereby further diminishing galactolipid content, but partially restoring the MGDG : DGDG ratio. Galactolipid and Chl contents, expression of JA-biosynthesis and JA-responsive genes, photosystem II (PSII) maximum quantum efficiency, and chloroplast shape were investigated. Expression of JA-biosynthesis and JA-responsive genes were reduced in amiR-MGD1-transformed dgd1 plants. Stunted growth caused by JA overproduction was also partially rescued, but Chl reduction and PSII impairment remained similar to the original dgd1 mutant. Altered chloroplast shape, which is another defect observed in dgd1 but is not caused by JA overproduction, was also partially rescued. Our results reveal that an increased MGDG : DGDG ratio is the primary cause of JA overproduction. The ratio is also important for determining chloroplast shapes, whereas reduced Chl and photosynthesis are most likely a direct consequence of insufficient DGDG.

Galactolipid deficiency disturbs spatial arrangement of the thylakoid network in Arabidopsis thaliana plants

The chloroplast thylakoid network is a dynamic structure which, through possible rearrangements, plays a crucial role in regulation of photosynthesis. Although the importance of the main components of the thylakoid membrane matrix, galactolipids, in the formation of the network of internal plastid membrane was found before, the structural role of monogalactosyldiacylglycerol (MGDG) and digalactosylidacylglycerol (DGDG) is still largely unknown. We elucidated detailed structural modifications of the thylakoid membrane system in Arabidopsis thaliana MGDG- and DGDG-deficient mutants. An altered MGDG/DGDG ratio was structurally reflected by formation of smaller grana, local changes in grana stacking repeat distance, and significant changes in the spatial organization of the thylakoid network compared with wild-type plants. The decrease of the MGDG level impaired the formation of the typical helical grana structure and resulted in a 'helical-dichotomic' arrangement. DGDG deficiency did not affect spatial grana organization but changed the shape of the thylakoid membrane network in situ from lens like into a flattened shape. Such structural disturbances were accompanied by altered composition of carotenoid and chlorophyll-protein complexes, which eventually led to the decreased photosynthetic efficiency of MGDG- and DGDG-deficient plants.

Role of Galactolipids in Plastid Differentiation Before and After Light Exposure

Galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the predominant lipid classes in the thylakoid membrane of chloroplasts. These lipids are also major constituents of internal membrane structures called prolamellar bodies (PLBs) and prothylakoids (PTs) in etioplasts, which develop in the cotyledon cells of dark-grown angiosperms. Analysis of Arabidopsis mutants defective in the major galactolipid biosynthesis pathway revealed that MGDG and DGDG are similarly and, in part, differently required for membrane-associated processes such as the organization of PLBs and PTs and the formation of pigment-protein complexes in etioplasts. After light exposure, PLBs and PTs in etioplasts are transformed into the thylakoid membrane, resulting in chloroplast biogenesis. During the etioplast-to-chloroplast differentiation, galactolipids facilitate thylakoid membrane biogenesis from PLBs and PTs and play crucial roles in chlorophyll biosynthesis and accumulation of light-harvesting proteins. These recent findings shed light on the roles of galactolipids as key facilitators of several membrane-associated processes during the development of the internal membrane systems in plant plastids.

Triacylglycerol Production in the Snow Algae Chlamydomonas nivalis under Different Nutrient Conditions

Eukaryotic microalgae have been known for high competency in the accumulation of triacylglycerol (TAG), a representative class of storage lipid. The snow algal species, Chlamydomonas nivalis, is a unique green eukaryotic microalga that can grow and survive in a wide range of temperatures. Although a few metabolomic studies of C. nivalis were conducted, no study has reported on TAG accumulation in C. nivalis. Herein, the present work aimed to investigate TAG production in C. nivalis under nutrient-starved conditions at 22 °C. Compared to phosphorus starvation, C. nivalis under nitrogen starvation showed a less severe growth defect, greater capacity for TAG production, and simple acyl composition in TAG enriched with 18:1. These features suggest that C. nivalis may be a significant model species to investigate glycerolipid metabolism for basic and applied research.

Chloroplast Galactolipids: The Link Between Photosynthesis, Chloroplast Shape, Jasmonates, Phosphate Starvation and Freezing Tolerance

Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) together constitute approximately 80% of chloroplast lipids. Apart from facilitating the photosynthesis light reaction in the thylakoid membrane, these two lipids are important for maintaining chloroplast morphology and for plant survival under abiotic stresses such as phosphate starvation and freezing. Recently it was shown that severe growth retardation phenotypes of the DGDG-deficient mutant dgd1 were due to jasmonate overproduction, linking MGDG and DGDG homeostasis with phytohormone production and suggesting MGDG as a major substrate for jasmonate biosynthesis. Induction of jasmonate synthesis and jasmonic acid (JA) signaling was also observed under conditions of phosphate starvation. We hypothesize that when DGDG is recruited to substitute for phospholipids in extraplastidic membranes during phosphate deficiency, the altered MGDG to DGDG ratio in the chloroplast envelope triggers the conversion of galactolipids into jasmonates. The conversion may contribute to rebalancing the MGDG to DGDG ratio rapidly to maintain chloroplast shape, and jasmonate production can reduce the growth rate and enhance predator deterrence. We also hypothesize that other conditions, such as suppression of dgd1 phenotypes by trigalactosyldiacylglycerol (tgd) mutations, may all be linked to altered jasmonate production, indicating that caution should be exercised when interpreting phenotypes caused by conditions that may alter the MGDG to DGDG ratio at the chloroplast envelope.

The Nonbilayer Lipid MGDG and the Major Light-Harvesting Complex (LHCII) Promote Membrane Stacking in Supported Lipid Bilayers

The thylakoid membrane of algae and land plants is characterized by its intricate architecture, comprising tightly appressed membrane stacks termed grana. The contributions of individual components to grana stack formation are not yet fully elucidated. As an in vitro model, we use supported lipid bilayers made of thylakoid lipid mixtures to study the effect of major light-harvesting complex (LHCII), different lipids, and ions on membrane stacking, seen as elevated structures forming on top of the planar membrane surface in the presence of LHCII protein. These structures were examined by confocal laser scanning microscopy, atomic force microscopy, and fluorescence recovery after photobleaching, revealing multilamellar LHCII-membrane stacks composed of connected lipid bilayers. Both native-like and non-native interactions between the LHCII complexes may contribute to membrane appression in the supported bilayers. However, applying in vivo-like salt conditions to uncharged glycolipid membranes drastically increased the level of stack formation due to enforced LHCII-LHCII interactions, which is in line with recent crystallographic and cryo-electron microscopic data [Wan, T., et al. (2014) Mol. Plant 7, 916-919; Albanese, P., et al. (2017) Sci. Rep. 7, 10067-10083]. Furthermore, we observed the nonbilayer lipid MGDG to strongly promote membrane stacking, pointing to the long-term proposed function of MGDG in stabilizing the inner membrane leaflet of highly curved margins in the periphery of each grana disc because of its negative intrinsic curvature [Murphy, D. J. (1982) FEBS Lett. 150, 19-26].

Specific Distribution of Phosphatidylglycerol to Photosystem Complexes in the Thylakoid Membrane

The thylakoid membrane is the site of photochemical and electron transport reactions of oxygenic photosynthesis. The lipid composition of the thylakoid membrane, with two galactolipids, one sulfolipid, and one phospholipid, is highly conserved among oxygenic photosynthetic organisms. Besides providing a lipid bilayer matrix, thylakoid lipids are integrated in photosynthetic complexes particularly in photosystems I and II and play important roles in electron transport processes. Thylakoid lipids are differentially allocated to photosynthetic complexes and the lipid bilayer fraction, but distribution of each lipid in the thylakoid membrane is unclear. In this study, based on published crystallographic and biochemical data, we estimated the proportions of photosynthetic complex-associated and bilayer-associated lipids in thylakoid membranes of cyanobacteria and plants. The data suggest that ∼30 mol% of phosphatidylglycerol (PG), the only major phospholipid in thylakoid membranes, is allocated to photosystem complexes, whereas glycolipids are mostly distributed to the lipid bilayer fraction and constitute the membrane lipid matrix. Because PG is essential for the structure and function of both photosystems, PG buried in these complexes might have been selectively conserved among oxygenic phototrophs. The specific and substantial allocation of PG to the deep sites of photosystems may need a unique mechanism to incorporate PG into the complexes possibly in coordination with the synthesis of photosynthetic proteins and pigments.

Roles of Lipids in Photosynthesis

Thylakoid membranes in cyanobacterial cells and chloroplasts of algae and higher plants are the sites of oxygenic photosynthesis. The lipid composition of the thylakoid membrane is unique and highly conserved among oxygenic photosynthetic organisms. Major lipids in thylakoid membranes are glycolipids, monogalactosyldiacylglycerol, digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol, and the phospholipid, phosphatidylglycerol. The identification of almost all genes involved in the biosynthesis of each lipid class over the past decade has allowed the generation and isolation of mutants of various photosynthetic organisms incapable of synthesizing specific lipids. Numerous studies using such mutants have revealed that these lipids play important roles not only in the formation of the lipid bilayers of thylakoid membranes but also in the folding and assembly of the protein subunits in photosynthetic complexes. In addition to the studies with the mutants, recent X-ray crystallography studies of photosynthetic complexes in thylakoid membranes have also provided critical information on the association of lipids with photosynthetic complexes and their activities. In this chapter, we summarize our current understanding about the structural and functional involvement of thylakoid lipids in oxygenic photosynthesis.

Floral glycerolipid profiles in homeotic mutants of Arabidopsis thaliana

Flowers have distinct glycerolipid composition, yet its floral organ-specific profile remains elusive in Arabidopsis whose flowers are too tiny to dissect different floral organs. Here, we employed known floral homeotic mutants agamous-1 (ag-1) and apetala3-3 (ap3-3) to facilitate sample preparation enriched in different floral organs. The result of analysis on different polar glycerolipid classes and their fatty acid composition demonstrated that flowers of ap3-3 and ag-1 have distinct glycerolipid composition from that of wild type. Moreover, distinct set of glycerolipid biosynthetic genes is expressed in these mutants by qRT-PCR gene expression analysis. These data suggest that glycerolipid profile is distinct among different floral organs of Arabidopsis thaliana.

Contribution of galactoglycerolipids to the 3-dimensional architecture of thylakoids

Thylakoid membranes, the universal structure where photosynthesis takes place in all oxygenic photosynthetic organisms from cyanobacteria to higher plants, have a unique lipid composition. They contain a high fraction of 2 uncharged glycolipids, the galactoglycerolipids mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), and an anionic sulfolipid, sulfoquinovosediacylglycerol (SQDG). A remarkable feature of the evolution from cyanobacteria to higher plants is the conservation of MGDG, DGDG, SQDG, and phosphatidylglycerol (PG), the major phospholipid of thylakoids. Using neutron diffraction on reconstituted thylakoid lipid extracts, we observed that the thylakoid lipid mixture self-organizes as a regular stack of bilayers. This natural lipid mixture was shown to switch from hexagonal II toward lamellar phase on hydration. This transition and the observed phase coexistence are modulated by the fine-tuning of the lipid profile, in particular the MGDG/DGDG ratio, and by the hydration. Our analysis highlights the critical role of DGDG as a contributing component to the membrane stacking via hydrogen bonds between polar heads of adjacent bilayers. DGDG interactions balance the repulsive electrostatic contribution of the charged lipids PG and SQDG and allow the persistence of regularly stacked membranes at high hydration. In developmental contexts or in response to environmental variations, these properties can contribute to the highly dynamic flexibility of plastid structure.

Importance of the hexagonal lipid phase in biological membrane organization

Domains are present in every natural membrane. They are characterized by a distinctive protein and/or lipid composition. Their size is highly variable from the nano- to the micrometer scale. The domains confer specific properties to the membrane leading to original structure and function. The determinants leading to domain organization are therefore important but remain obscure. This review presents how the ability of lipids to organize into hexagonal II or lamellar phases can promote particular local structures within membranes. Since biological membranes are composed of a mixture of lipids, each with distinctive biophysical properties, lateral and transversal sorting of lipids can promote creation of domains inside the membrane through local modulation of the lipid phase. Lipid biophysical properties have been characterized for long based on in vitro analyses using non-natural lipid molecules; their re-examinations using natural lipids might open interesting perspectives on membrane architecture occurring in vivo in various cellular and physiological contexts.

The dynamic roles of intracellular lipid droplets: from archaea to mammals

During the past decade, there has been a paradigm shift in our understanding of the roles of intracellular lipid droplets (LDs). New genetic, biochemical and imaging technologies have underpinned these advances, which are revealing much new information about these dynamic organelles. This review takes a comparative approach by examining recent work on LDs across the whole range of biological organisms from archaea and bacteria, through yeast and Drosophila to mammals, including humans. LDs probably evolved originally in microorganisms as temporary stores of excess dietary lipid that was surplus to the immediate requirements of membrane formation/turnover. LDs then acquired roles as long-term carbon stores that enabled organisms to survive episodic lack of nutrients. In multicellular organisms, LDs went on to acquire numerous additional roles including cell- and organism-level lipid homeostasis, protein sequestration, membrane trafficking and signalling. Many pathogens of plants and animals subvert their host LD metabolism as part of their infection process. Finally, malfunctions in LDs and associated proteins are implicated in several degenerative diseases of modern humans, among the most serious of which is the increasingly prevalent constellation of pathologies, such as obesity and insulin resistance, which is associated with metabolic syndrome.

The relationship between different spectral forms of the protochlorophyllide oxidoreductase complex and the structural organisation of prolamellar bodies isolated from Zea mays

Incubation of prolamellar bodies (PLB) in high-salt media leads to changes in PLB structure and properties of their protochlorophyllide oxidoreductase-protochlorophyllide (POR-PChlide) complex. The paracrystalline organisation typical of PLB is disrupted and NADPH dissociates from photoconvertible POR-PChlide, with absorption maxima at 640 and 650 nm (POR-PChlide (640/650)), and a non-photoconvertible form, with absorption maxima at 635 nm (POR-PChlide (635)), is formed. These effects are strongly dependent on the valence of the cation of the perturbing salt, indicating that they involve surface double layers effects. They are also influenced by the nature of the anion and by high concentrations of non-electrolytes, suggesting the involvement of surface hydration effects. The structural changes are largely, if not entirely, independent of the presence of excess NADPH. Changes to the POR-PChlide complex, however, are strongly inhibited by excess NADPH suggesting that the two sets of changes may not be causally linked. As long as the disruption is not too great, the structural changes seen on incubation of PLB in high salt media lacking excess NADPH are reversed on removal of the high salt perturbation. This reversal is independent of the presence or absence of added NADPH. Reformation of photoconvertible POR-PChlide, however, requires the presence of NADPH. The reformation of paracrystalline PLB in the absence of NADPH strongly indicates that preservation of PLB structure, in isolated PLB preparations at least, is independent of the presence or absence of POR-PChlide (650).

Critical regulation of galactolipid synthesis controls membrane differentiation and remodeling in distinct plant organs and following environmental changes

The plant galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the most abundant lipids in chloroplast membranes, and they constitute the majority of total membrane lipids in plants. MGDG is synthesized by two types of MGDG synthase, type-A (MGD1) and type-B (MGD2, MGD3). These MGDG synthases have distinct roles in Arabidopsis. In photosynthetic organs, Type A MGD is responsible for the bulk of MGDG synthesis, whereas Type B MGD is expressed in non-photosynthetic organs such as roots and flowers and mainly contributes to DGDG accumulation under phosphate deficiency. Similar to MGDG synthesis, DGDG is synthesized by two synthases, DGD1 and DGD2; DGD1 is responsible for the majority of DGDG synthesis, whereas DGD2 makes its main contribution under phosphate deficiency. These galactolipid synthases are regulated by light, plant hormones, redox state, phosphatidic acid levels, and various stress conditions such as drought and nutrient limitation. Maintaining the appropriate ratio of these two galactolipids in chloroplasts is important for stabilizing thylakoid membranes and maximizing the efficiency of photosynthesis. Here we review progress made in the last decade towards a better understanding of the pathways regulating plant galactolipid biosynthesis.

Type A and type B monogalactosyldiacylglycerol synthases are spatially and functionally separated in the plastids of higher plants

Mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively) constitute the bulk of membrane lipids in plant chloroplasts. The final step in MGDG biosynthesis occurs in the plastid envelope and is catalyzed by MGDG synthase. In Arabidopsis, the three MGDG synthases are classified into type A (atMGD1) and type B MGD isoforms (atMGD2 and atMGD3). atMGD1 is an inner envelope membrane-associated protein of chloroplasts and is responsible for the bulk of galactolipid biosynthesis in green tissues. MGD1 function is indispensable for thylakoid membrane biogenesis and embryogenesis. By contrast, type B atMGD2 and atMGD3 are localized in the outer envelopes and have no important role in chloroplast biogenesis or plant development under nutrient-sufficient conditions. These type B MGD genes are, however, strongly induced by phosphate (Pi) starvation and are essential for alternative galactolipid biosynthesis during Pi starvation. MGD1 gene expression is up-regulated by light and cytokinins. By contrast, Pi starvation-dependent expression of atMGD2/3 is suppressed by cytokinins but induced through auxin signaling pathways. These growth factors may control the functional sharing of the inner envelope pathway by atMGD1 and the outer envelope pathway by atMGD2/3 according to the growth environment.

Glycolipid Membranes through Atomistic Simulations: Effect of Glucose and Galactose Head Groups on Lipid Bilayer Properties

Though glycolipids are involved in a multitude of cellular functions, the understanding of their atom-scale properties in lipid membranes has remained very limited due to the lack of atomistic simulations. In this work, we employ extensive simulations to characterize one-component membranes comprised of glycoglycerolipids, focusing on two common glyco head groups, namely glucose and galactose. The properties of these two glycoglycerolipid bilayers are compared in a systematic manner with membranes consisting of phosphatidylcholine (PC) or phosphatidylethanolamine (PE) lipids, whose structures aside from the head group are identical with those of the two glycolipids. We find that the glycolipid systems are characterized by a substantial number of hydrogen bonds in the head group region, leading to membrane packing that is stronger than in a PC but less significant than that in a PE bilayer. The role played by the glyco head group is especially evident in the electrostatic membrane potential, which is particularly large in the glycolipid membranes. For the same reason, the interfacial forces near glycolipid bilayers are significantly different from those found in PC and PE bilayers, affecting, e.g., the ordering of water close to the membrane. These effects are particularly important for the case of galactose, an important component in thylacoids.

Digalactosyl-diacylglycerol deficiency impairs the capacity for photosynthetic intersystem electron transport and state transitions in Arabidopsis thaliana due to photosystem I acceptor-side limitations

Compared with wild type, the dgd1 mutant of Arabidopsis thaliana exhibited a lower amount of PSI-related Chl-protein complexes and lower abundance of the PSI-associated polypeptides, PsaA, PsaB, PsaC, PsaL and PsaH, with no changes in the levels of Lhca1-4. Functionally, the dgd1 mutant exhibited a significantly lower light-dependent, steady-state oxidation level of P700 (P700(+)) in vivo, a higher intersystem electron pool size, restricted linear electron transport and a higher rate of reduction of P700(+) in the dark, indicating an increased capacity for PSI cyclic electron transfer compared with the wild type. Concomitantly, the dgd1 mutant exhibited a higher sensitivity to and incomplete recovery of photoinhibition of PSI. Furthermore, dgd1 exhibited a lower capacity to undergo state transitions compared with the wild type, which was associated with a higher reduction state of the plastoquinone (PQ) pool. We conclude that digalactosyl-diacylglycerol (DGDG) deficiency results in PSI acceptor-side limitations that alter the flux of electrons through the photosynthetic electron chain and impair the regulation of distribution of excitation energy between the photosystems. These results are discussed in terms of thylakoid membrane domain reorganization in response to DGDG deficiency in A. thaliana.

Digalactosyldiacylglycerol is a major glycolipid in floral organs of Petunia hybrida

In higher plants, glycolipids such as monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are major components of chloroplast membranes in leaves. A recent study identified an isoform of MGDG synthase that is expressed specifically in floral organs, suggesting a novel function for glycolipids in flowers. To elucidate the localization and developmental changes of glycolipids and their biosynthetic activities in flowers, we carried out a series of analytical studies with Petunia hybrida. The results showed that the biosynthetic activities of galactolipid synthesis, particularly for DGDG, increased during flower development. Among the floral organs, the pistil had the highest galactolipid synthetic activity. Its specific activity for incorporation of UDP-galactose to yield galactolipids was estimated to be more than twice that of leaves, which are the major site of galactolipid synthesis in plant tissues. Analysis of lipid contents of pistils revealed that they contained higher amounts of galactolipids than other floral organs. Moreover, DGDG was more abundant than MGDG in both pistils and petals. These results show that DGDG is a major glycolipid in floral organs and that DGDG biosynthetic activity is highly upregulated in the pistils and petals of Petunia flowers.

Light and cytokinin play a co-operative role in MGDG synthesis in greening cucumber cotyledons

The current research investigated the regulation of monogalactosyldiacylglycerol (MGDG) biosynthesis, catalyzed by MGDG synthase (MGD) (UDP-galactose:1,2-diacylglycerol 3-beta-D-galactosyltransferase; EC 2.4.1.46), during chloroplast development in cucumbers (Cucumis sativus L. cv. Aonagajibai). In etiolated seedlings, white light induced a transient increase in MGD mRNA, followed by a subsequent increase in enzyme activity. MGDG, digalactosyldiacylglycerol (DGDG), and linolenic acid (18 : 3) of both MGDG and DGDG accumulated in a light-dependent manner. Early light-dependent induction of MGD protein was also identified in isolated chloroplasts. When cotyledons were detached from seedlings, these light-induced changes diminished. However, when a synthetic cytokinin, benzyladenine, was added to the detached cotyledons, a transient increase in MGD mRNA and a linear increase in the enzyme activity were induced even in the dark. Galactolipids subsequently accumulated to some extent and 18 : 3 content also increased. MGDG fully accumulated in detached cotyledons with co-treatment of light and a cytokinin. Red light (>600 nm) and far-red light (>700 nm) both induced an increase in MGD mRNA and enzyme activity but far-red light did not induce an accumulation of MGDG. These results suggest that (1). galactolipid biosynthesis is regulated by the cooperation of light and a cytokinin; (2). the accumulation of MGDG requires cytokinin in addition to light; (3). a red light (600-700 nm) dependent factor is necessary for the maximal galactolipid accumulation in addition to increase in MGD transcript and activity.

Accumulation of plant galactolipid affects cell morphology of Escherichia coli

Monogalactosyldiacylglycerol (MGDG) is a major constituent of thylakoid membrane in chloroplasts. Therefore, it is considered to have an important role in the maintenance of the complicated structure of the thylakoid membrane. We have succeeded in cloning the enzyme for MGDG synthesis and overexpressed it in Escherichia coli. In this study we analyzed the morphology of the E. coli harboring the gene. The fatty acid composition of its membrane lipids did not differ between the wild type and transformant, except for the appearance of MGDG. However, transformant cells appeared to be elongated. DAPI staining revealed the entire intracellular region of filamentous cells to be stained; therefore, the elongation of the cells is probably due to a defect in cell division. Atomic force microscopy revealed that the transformant had a smooth but scratched surface. It was concluded that the excessive accumulation of a non-bilayer lipid, MGDG, interfered with the translocation of proteins across the plasma membrane, including those for cell division.

Role of thylakoid lipids in the structural flexibility of lamellar aggregates of the isolated light-harvesting chlorophyll a/b complex of photosystem II

We studied the role of added thylakoid lipids in the light-induced reversible structural changes in isolated macroaggregates of the main light-harvesting chlorophyll a/b complex of photosystem II (LHCII). Loosely stacked lamellar macroaggregates were earlier shown to undergo light-induced reversible structural changes and changes in the photophysical pathways, which resembled those in thylakoid membranes exposed to excess light [Barzda, V., et al. (1996) Biochemistry 35, 8981-8985]. This structural flexibility of LHCII depends critically on the lipid content of the preparations [Simidjiev, I., et al. (1997) Anal. Biochem. 250, 169-175]. It is now reported that lamellar aggregates of LHCII are capable of incorporating substantial amounts of different thylakoid lipids. The long-range order of the chromophores is retained, while the ultrastructure of the lipid-protein macroaggregates can be modified significantly. Addition of thylakoid lipids to the preparations significantly enhances the ability of the LHCII macroaggregates to undergo light-induced structural changes. The lipid environment of the LHCII complexes therefore plays a significant role in determining the structural flexibility of the macroaggregates. As concerns the mechanism of these changes, it is proposed that the absorption of light and the dissipation of its energy in the macrodomains induces thermal fluctuations which bring about changes in the shape or in the stacking interactions of the membranes, this in turn affecting the long-range order of the embedded chromophores. In thylakoids, a similar mechanism is likely to explain the light-induced structural changes which are largely independent of the photochemical activity of the membranes.

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.

Protein conjugates with water-soluble poly(alkylene oxide)s entrapped in hydrated reversed micelles

Conjugates of alpha-chymotrypsin (ChT) with poly(ethylene glycol) (PEG) and block copolymers of ethylene and propylene oxides (proxanols) have been synthesized. The molecular mass of the polymers used was 2 kDa. The conjugates contained five to seven polymer chains per enzyme molecule. Hydrolysis of N-trans-cinnamoylimidazole catalyzed by the conjugates of ChT with poly(alkylene oxide)s was studied in 0.05 M Tris-HCl buffer at pH 8.0 and in the system of the hydrated reversed micelles of aerosol OT (AOT) in octane at 25 degrees C. The deacylation rate constant k3 for the conjugates in buffer solution was 1.5-1.8-fold higher than that for native ChT. The value of the [H2O]:[AOT] ratio corresponding to the maximum on k3 versus [H2O]:[AOT] curves for the conjugates (ca. 16) allows the dimensions of their molecules to be evaluated. The radius of the conjugate molecules was found to be about 2.8 nm. The value of k3 for the conjugate of ChT with PEG, as in the case of native ChT, remains constant when the concentration of AOT is varied. However, the deacylation rate constant for the conjugates of ChT with proxanols decreases with the increase in AOT concentration, which indicates that these conjugates are able to interact with the micellar matrix and therefore may be considered membranotropic compounds.

Cloning of the gene for monogalactosyldiacylglycerol synthase and its evolutionary origin

Monogalactosyldiacylglycerol (MGDG) synthase (UDPgalactose:1,2-diacylglycerol 3-beta-D-galactosyltransferase; EC 2.4.1.46) catalyzes formation of MGDG, a major structural lipid of chloroplast. We cloned a cDNA for the synthase from cucumber cDNA library. The full-length cDNA clone was 2142 bp, and it contains a 1575-bp open reading frame encoding 525 aa. The open reading frame consists of the regions for a mature protein (422 aa; Mr of 46,552) and transit peptide to chloroplast (103 aa). Although the molecular weight of mature protein region matched that purified from cucumber cotyledons, it was quite different from those purified from spinach (approximately 20 kDa) reported by other groups. The mature region of the protein was expressed in Escherichia coli as a fusion protein with glutathione S-transferase. The expression in E. coli showed that the protein catalyzed MGDG synthesis very efficiently. Therefore, we concluded that the cDNA encodes MGDG synthase in cucumber. In addition, the deduced amino acid sequence of the MGDG synthase cDNA showed homology with MurG of Bacillus subtilis and E. coli, which encode a glycosyltransferase catalyzing the last step of peptidoglycan synthesis in bacteria. This sequence homology implies that the machinery of chloroplast membrane biosynthesis is evolutionarily derived from that of cell wall biosynthesis in bacteria. This is consistent with the endosymbiotic hypothesis of chloroplast formation.

The second E.C. Slater lecture. Micellar enzymology: its relation to membranology

Micellar enzymology, a new trend in molecular biology, studies catalysis by enzymes entrapped in hydrated reversed micelles composed of surfactants (phospholipids, detergents) in organic solvents. The key research problems of micellar enzymology and its relation to enzyme membranology are discussed.

Kinetics of soybean lipoxygenase reaction in hydrated reversed micelles

The rate of linoleic acid peroxidation catalysed by soybean lipoxygenase I was studied as a function of the hydration degree of aerosol OT (bis(2-ethylhexyl) sulfosuccinate sodium salt) reversed micelles in octane. Lipoxygenase reaction parameters for the micelle-bound substrate were spectrophotometrically determined. The linoleic acid distribution between the micelles and octane was detected by the sedimentation method, with the concentration of linoleic acid in supernatant after settling of micelles (i.e. the concentration of free linoleic acid) being estimated by the enzymatic method. The apparent constant of linoleic acid distribution (the ratio of the bound and free substrate concentrations) was enhanced with increasing hydration of reversed micelles. The dependence of the enzymatic reaction rate on the bound substrate concentration obeyed the empiric Hill equation. The Hill coefficient remained practically constant (h = 1.34) as the hydration degree changed. Parameters of the lipoxygenase reaction, enzyme reaction limiting rate V and semi-saturation substrate concentration [S]0.5 increased with increasing degree of hydration and reached the optimum at [H2O]/[AOT] approximately 30, where dimensions of the micellar internal cavity coincided with those of the enzyme molecule. Some aspects of kinetic behavior of membrane-bound enzymes participating in chemical transformation of non-polar compounds dispersed in lipid phase are discussed.

Structural and functional consequences of galactolipids on thylakoid membrane organization

Photosynthetic membranes of higher plant chloroplasts are composed primarily of polar, but uncharged, galactolipids unlike most mammalian membranes which contain large amounts of phosphatidylcholine. It is unclear what role(s) the galactolipids play in maintaining the differentiated thylakoid membranes, or in stabilizing the photosynthetically active enzyme complexes. Some of the membrane complexes show no lipid selectivity for maintaining structural or functional integrity. Others are poisoned or dissociated in the presence of high concentrations of a trace lipid class. The efficiency of energy transfer and the reconstitution of protein complexes into liposomes are dependent on the lipid class employed. The lipids are asymmetrically arranged along and across the thylakoid membranes but not as distinctly as the proteins.