Patatin-like lipolytic acyl hydrolases and galactolipid metabolism in marine diatoms of the genus Pseudo-nitzschia.

Diatoms are eukaryotic microalgae that play a pivotal role in biological and geochemical marine cycles. These microorganisms are at the basis of the trophic chain and their lipids are essential components (e.g. eicosapentaenoic acid, EPA) of aquatic food webs. Galactolipids are the primary lipid components of plastid membranes and form the largest lipid family of diatoms. As source of polyunsaturated fatty acids (PUFAs), these compounds are also involved in the synthesis of lipoxygenase (LOX) products such as non-volatile oxylipins and polyunsaturated aldehydes. Here, we report the first identification of two genes, namely PmLAH1 and PaLAH1, coding for lipolytic enzymes in two diatoms of the genus Pseudo-nitzschia. Functional and modeling studies evidence a patatin-like domain endowed with galactolipase and phospholipase activity at the C-terminus of both proteins. Homologues of Pseudo-nitzschia LAH1 genes were retrieved in other diatom species so far sequenced in agreement with conservation of the functional role of these proteins within the lineage.

Role of MGDG and Non-bilayer Lipid Phases in the Structure and Dynamics of Chloroplast Thylakoid Membranes.

In this chapter we focus our attention on the enigmatic structural and functional roles of the major, non-bilayer lipid monogalactosyl-diacylglycerol (MGDG) in the thylakoid membrane. We give an overview on the state of the art on the role of MGDG and non-bilayer lipid phases in the xanthophyll cycles in different organisms. We also discuss data on the roles of MGDG and other lipid molecules found in crystal structures of different photosynthetic protein complexes and in lipid-protein assemblies, as well as in the self-assembly of the multilamellar membrane system. Comparison and critical evaluation of different membrane models--that take into account and capitalize on the special properties of non-bilayer lipids and/or non-bilayer lipid phases, and thus to smaller or larger extents deviate from the 'standard' Singer-Nicolson model--will conclude this review. With this chapter the authors hope to further stimulate the discussion about, what we think, is perhaps the most exciting question of membrane biophysics: the why and wherefore of non-bilayer lipids and lipid phases in, or in association with, bilayer biological membranes.

[Sulfogalactosylglycerolipid in spermatogenesis and fertilization].

Sulfogalactosylglycerolipid (SGG) is the main glycolipid in male mammalian germ cells, which is selectively and highly expressed in mammalian testes and helps form the lipid bilayer of cell membrane. In the process of spermatogenesis, SGG is involved in the meiosis of spermiocytes. Either deficiency or accumulation of SGG will lead to male infertility. SGG homeostasis in the testis is the premise of normal spermatogenesis. In the process of sperm-zona binding, SGG becomes a component of lipid raft and provides a platform for signal transduction. The SGG binding protein plays a role in sperm-egg recognition and membrane fusion. SGG has a great research value and application prospect in male reproduction.

Mono- and digalactosyldiacylglycerol lipids function nonredundantly to regulate systemic acquired resistance in plants.

The plant galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) have been linked to the anti-inflammatory and cancer benefits of a green leafy vegetable diet in humans due to their ability to regulate the levels of free radicals like nitric oxide (NO). Here, we show that DGDG contributes to plant NO as well as salicylic acid biosynthesis and is required for the induction of systemic acquired resistance (SAR). In contrast, MGDG regulates the biosynthesis of the SAR signals azelaic acid (AzA) and glycerol-3-phosphate (G3P) that function downstream of NO. Interestingly, DGDG is also required for AzA-induced SAR, but MGDG is not. Notably, transgenic expression of a bacterial glucosyltransferase is unable to restore SAR in dgd1 plants even though it does rescue their morphological and fatty acid phenotypes. These results suggest that MGDG and DGDG are required at distinct steps and function exclusively in their individual roles during the induction of SAR.

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.

Effect of moderate and high light on photosystem II function in Arabidopsis thaliana depleted in digalactosyl-diacylglycerol.

The response of the heat-sensitive dgd1-2 and dgd1-3 Arabidopsis mutants depleted in the galactolipid DGDG to photoinhibition of chloroplasts photosystem II was studied to verify if there is a relationship between heat stress vulnerability due to depletion in DGDG and the susceptibility to photoinhibitory damage. Non-photochemical quenching (NPQ) is known to dissipate excessive absorbed light energy as heat to protect plants against photodamage. The main component of NPQ is dependent of the transthylakoid pH gradient and is modulated by zeaxanthin (Zx) synthesis. These processes together with chlorophyll fluorescence induction were used to characterize the response of the genotypes. The mutants were more sensitive to photoinhibition to a small extent but this was more severe for dgd1-3 especially at high light intensity. It was deduced that DGDG was not a main factor to influence photoinhibition but other lipid components could affect PSII sensitivity towards photoinhibition in relation to the physical properties of the thylakoid membrane. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Unprecedented pathogen-inducible complex oxylipins from flax--linolipins A and B.

Oxylipins constitute a large family of bioregulators, biosynthesized via unsaturated fatty acid oxidation. This study reports the detection of an unprecedented family of complex oxylipins from flax leaves. Two major members of this family, compounds 1 and 2, were isolated and purified. Their structures were evaluated using NMR and MS analyses. Both compounds were identified as monogalactosyldiacylglycerol species. Compound 1 contains one alpha-linolenoyl residue and one residue of (9Z,11E,1'Z,3'Z)-12-(1',3'-hexadienyloxy)-9,11-dodecadienoic, (omega5Z)-etherolenic acid at positions sn-1 and sn-2, respectively. Compound 2 possesses (omega5Z)-etherolenic acid residues at both position sn-1 and position sn-2. We suggest the trivial names linolipin A and linolipin B for compounds 1 and 2, respectively, and the collective name linolipins for this new family of complex oxylipins. The linolipin content of flax leaves increased significantly in response to pathogenesis. Thus, accumulation of esterified antimicrobial divinyl ethers may be of relevance to plant defense.

The role of diglycosyl lipids in photosynthesis and membrane lipid homeostasis in Arabidopsis.

The galactolipid digalactosyldiacylglycerol (DGD) is an abundant thylakoid lipid in chloroplasts. The introduction of the bacterial lipid glucosylgalactosyldiacylglycerol (GGD) from Chloroflexus aurantiacus into the DGD-deficient Arabidopsis (Arabidopsis thaliana) dgd1 mutant was previously shown to result in complementation of growth, but photosynthetic efficiency was only partially restored. Here, we demonstrate that GGD accumulation in the double mutant dgd1dgd2, which is totally devoid of DGD, also complements growth at normal and high-light conditions, but photosynthetic efficiency in the GGD-containing dgd1dgd2 line remains decreased. This is attributable to an increased susceptibility of photosystem II to photodamage, resulting in reduced photosystem II accumulation already at normal light intensities. The chloroplasts of dgd1 and dgd1dgd2 show alterations in thylakoid ultrastructure, a phenotype that is restored in the GGD-containing lines. These data suggest that the strong growth retardation of the DGD-deficient lines dgd1 and dgd1dgd2 can be primarily attributed to a decreased capacity for chloroplast membrane assembly and proliferation and, to a smaller extent, to photosynthetic deficiency. During phosphate limitation, GGD increases in plastidial and extraplastidial membranes of the transgenic lines to an extent similar to that of DGD in the wild type, indicating that synthesis and transport of the bacterial lipid (GGD) and of the authentic plant lipid (DGD) are subject to the same mechanisms of regulation.

Lipid-assisted protein-protein interactions that support photosynthetic and other cellular activities.

Glycoglycerolipids are dominant lipids of photosynthetic organisms, i.e. higher plants and cyanobacteria. X-ray crystallographic localization of glycerolipids revealed that they are present at functionally and structurally important sites of both the PS I and PS II reaction centres. Phosphatidylglycerol (PG) is an indispensible member of glycerolipids, including the formation of functionally active oligomers of the reaction centres both PS I and PS II. Lipids assist in the assembly of protein subunits of the photosynthetic machinery by pasting the individual protein components together. PG is needed to glue CP43 to the reaction centre core. PG and digalactosyldiacylglycerol (DGDG) interact in photosynthetic processes: PG alone controls electron transport at the acceptor site of PS II, and together with DGDG is involved in electron transport at the donor site of PS II. PG is crucial for the formation of division rings and is implicated in the fission of cyanobacteria.

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.

Functional roles of the major chloroplast lipids in the violaxanthin cycle.

Monogalactosyldiacylglyceride (MGDG) and digalactosyldiacylglyceride (DGDG) are the major membrane lipids of chloroplasts. The question of the specialized functions of these unique lipids has received limited attention. One function is to support violaxanthin de-epoxidase (VDE) activity, an enzyme of the violaxanthin cycle. To understand better the properties of this system, the effects of galactolipids and phosphatidylcholines on VDE activity were examined by two independent methods. The results show that the micelle-forming lipid (MGDG) and bilayer forming lipids (DGDG and phosphatidylcholines) support VDE activity differently. MGDG supported rapid and complete de-epoxidation starting at a threshold lipid concentration (10 microM) coincident with complete solubilization of violaxanthin. In contrast, DGDG supported slow but nevertheless complete to nearly complete de-epoxidation at a lower lipid concentration (6.7 microM) that did not completely solubilize violaxanthin. Phosphotidylcholines showed similar effects as DGDG except that de-epoxidation was incomplete. Since VDE requires solubilized violaxanthin, aggregated violaxanthin in DGDG at low concentration must become solubilized as de-epoxidation proceeds. High lipid concentrations had lower activity possibly due to formation of multilayered structures (liposomes) that restrict accessibility of violaxanthin to VDE. MGDG micelles do not present such restrictions. The results indicate VDE operates throughout the lipid phase of the single bilayer thylakoid membrane and is not limited to putative MGDG micelle domains. Additionally, the results also explain the differential partitioning of violaxanthin between the envelope and thylakoid as due to the relative solubilities of violaxanthin and zeaxanthin in MGDG, DGDG and phospholipids. The violaxanthin cycle is hypothesized to be a linked system of the thylakoid and envelope for signal transduction of light stress.

Sperm capacitation induces an increase in lipid rafts having zona pellucida binding ability and containing sulfogalactosylglycerolipid.

Sperm gain full ability to bind to the zona(e) pellucida(e) (ZP) during capacitation. Since lipid rafts are implicated in cell adhesion, we determined whether capacitated sperm lipid rafts had affinity for the ZP. We demonstrated that lipid rafts, isolated as low-density detergent resistant membranes (DRMs), from capacitated pig sperm had ability to bind to homologous ZP. This binding was dependent on pig ZPB glycoprotein, a major participant in sperm binding. Capacitated sperm DRMs were also enriched in the male germ cell specific sulfogalactosylglycerolipid (SGG), which contributed to DRMs-ZP binding. Furthermore, SGG may participate in the formation of sperm DRMs due to its interaction with cholesterol, an integral component of lipid rafts, as shown by infrared spectroscopic studies. Since sperm capacitation is associated with cholesterol efflux from the sperm membrane, we questioned whether the formation of DRMs was compromised in capacitated sperm. Our studies indeed revealed that capacitation induced increased levels of sperm DRMs, with an enhanced ZP affinity. These results corroborated the implication of lipid rafts and SGG in cell adhesion and strongly suggested that the enhanced ZP binding ability of capacitated sperm may be attributed to increased levels and a greater ZP affinity of lipid rafts in the sperm plasma membrane.

[Relationship between mycoplasma infection and germ cell sulfogalactosylglycerolipid].

It is well known that mycoplasma can cause infection in the male reproductive tract. Some studies indicate that Ureaplasma urealyticum (Uu), a species of mycoplasma, is associated with male infertility. Sulfogalactosylglycerolipid(SGG) is the major mammalian male germ cell glycolipid, synthesized via sulfation of galactosylglycerolipid in early primary spermatocytes. Some experiments have proved that SGG is implicated in sperm-egg binding by linking arylsulfatase A (AS-A), SGG's ligand on the egg. SGG can be desulfated by binding mycoplasmas and transformed galactosylglycerolipid, which doesn't bind AS-A. So the binding and degradation of the sperm SGG by mycoplasmas may play a role in the induction of male infertility. As a kind of mycoplasma, Uu can also bind SGG, which offers another explantion for the association of Uu infection with male infertility caused by Uu infection.

[Role of sulfogalactosylglycerolipid and sulfolipidimmobilizing protein 1 in sperm-egg plasma membrane interaction].

Sperm-egg plasma membrane interaction is one of the important steps of mammalian fertilization. Many sperm and egg surface proteins are reported to be involved in sperm-zona pellucida interaction. Sulfogalactosylglycerolipid(SGG) is the major sulfoglycolipid in the germ cells of mammalian and lower vertebrates, mainly in the sperm head. It is a differentiation marker in spermatogenesis restricted to the zygotene and early pachytene spermatocytes. Sulfolipidimmobilizing protein 1 (SLIP1) is the major sulfoglycolipid of mammalian germ cells and eggs, with the same localization as SGG in the sperm. SLIP1 binds specificity to SGG, both playing a vital role in sperm-egg interaction. This article is aimed at reviewing the localization of SGG and SLIP1 in the germ cell surface and their role and related mechanism in gamete formation.