Native uridine 5'-diphosphate-sulfoquinovose synthase, SQD1, from spinach purifies as a 250-kDa complex

The Role of Chloroplast Membrane Lipid Metabolism in Plant Environmental Responses

Plants are nonmotile life forms that are constantly exposed to changing environmental conditions during the course of their life cycle. Fluctuations in environmental conditions can be drastic during both day–night and seasonal cycles, as well as in the long term as the climate changes. Plants are naturally adapted to face these environmental challenges, and it has become increasingly apparent that membranes and their lipid composition are an important component of this adaptive response. Plants can remodel their membranes to change the abundance of different lipid classes, and they can release fatty acids that give rise to signaling compounds in response to environmental cues. Chloroplasts harbor the photosynthetic apparatus of plants embedded into one of the most extensive membrane systems found in nature. In part one of this review, we focus on changes in chloroplast membrane lipid class composition in response to environmental changes, and in part two, we will detail chloroplast lipid-derived signals.

Sulfoquinovose in the biosphere: occurrence, metabolism and functions

The sulfonated carbohydrate sulfoquinovose (SQ) is produced in quantities estimated at some 10 billion tonnes annually and is thus a major participant in the global sulfur biocycle. SQ is produced by most photosynthetic organisms and incorporated into the sulfolipid sulfoquinovosyl diacylglycerol (SQDG), as well as within some archaea for incorporation into glycoprotein N-glycans. SQDG is found mainly within the thylakoid membranes of the chloroplast, where it appears to be important for membrane structure and function and for optimal activity of photosynthetic protein complexes. SQDG metabolism within the sulfur cycle involves complex biosynthetic and catabolic processes. SQDG biosynthesis is largely conserved within plants, algae and bacteria. On the other hand, two major sulfoglycolytic pathways have been discovered for SQDG degradation, the sulfo-Embden–Meyerhof–Parnas (sulfo-EMP) and sulfo-Entner–Doudoroff (sulfo-ED) pathways, which mirror the major steps in the glycolytic EMP and ED pathways. Sulfoglycolysis produces C3-sulfonates, which undergo biomineralization to inorganic sulfur species, completing the sulfur cycle. This review discusses the discovery and structural elucidation of SQDG and archaeal N-glycans, the occurrence, distribution, and speciation of SQDG, and metabolic pathways leading to the biosynthesis of SQDG and its catabolism through sulfoglycolytic and biomineralization pathways to inorganic sulfur.

Glycerolipids in photosynthesis: composition, synthesis and trafficking

Glycerolipids constituting the matrix of photosynthetic membranes, from cyanobacteria to chloroplasts of eukaryotic cells, comprise monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol. This review covers our current knowledge on the structural and functional features of these lipids in various cellular models, from prokaryotes to eukaryotes. Their relative proportions in thylakoid membranes result from highly regulated and compartmentalized metabolic pathways, with a cooperation, in the case of eukaryotes, of non-plastidic compartments. This review also focuses on the role of each of these thylakoid glycerolipids in stabilizing protein complexes of the photosynthetic machinery, which might be one of the reasons for their fascinating conservation in the course of evolution. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

The biosynthesis of nitrogen-, sulfur-, and high-carbon chain-containing sugars

Carbohydrates serve many structural and functional roles in biology. While the majority of monosaccharides are characterized by the chemical composition (CH2O)n, modifications including deoxygenation, C-alkylation, amination, O- and N-methylation, which are characteristic of many sugar appendages of secondary metabolites, are not uncommon. Interestingly, some sugar molecules are formed via modifications including amine oxidation, sulfur incorporation, and "high-carbon" chain attachment. Most of these unusual sugars have been identified over the past several decades as components of microbially produced natural products, although a few high-carbon sugars are also found in the lipooligosaccharides of the outer cell walls of Gram-negative bacteria. Despite their broad distribution in nature, these sugars are considered "rare" due to their relative scarcity. The biosynthetic steps that underlie their formation continue to perplex researchers to this day and many questions regarding key transformations remain unanswered. This review will focus on our current understanding of the biosynthesis of unusual sugars bearing oxidized amine substituents, thio-functional groups, and high-carbon chains.

An essential role for tomato sulfite oxidase and enzymes of the sulfite network in maintaining leaf sulfite homeostasis

Little is known about the homeostasis of sulfite levels, a cytotoxic by-product of plant sulfur turnover. By employing extended dark to induce catabolic pathways, we followed key elements of the sulfite network enzymes that include adenosine-5'-phosphosulfate reductase and the sulfite scavengers sulfite oxidase (SO), sulfite reductase, UDP-sulfoquinovose synthase, and β-mercaptopyruvate sulfurtransferases. During extended dark, SO was enhanced in tomato (Solanum lycopersicum) wild-type leaves, while the other sulfite network components were down-regulated. SO RNA interference plants lacking SO activity accumulated sulfite, resulting in leaf damage and mortality. Exogenous sulfite application induced up-regulation of the sulfite scavenger activities in dark-stressed or unstressed wild-type plants, while expression of the sulfite producer, adenosine-5'-phosphosulfate reductase, was down-regulated. Unstressed or dark-stressed wild-type plants were resistant to sulfite applications, but SO RNA interference plants showed sensitivity and overaccumulation of sulfite. Hence, under extended dark stress, SO activity is necessary to cope with rising endogenous sulfite levels. However, under nonstressed conditions, the sulfite network can control sulfite levels in the absence of SO activity. The novel evidence provided by the synchronous dark-induced turnover of sulfur-containing compounds, augmented by exogenous sulfite applications, underlines the role of SO and other sulfite network components in maintaining sulfite homeostasis, where sulfite appears to act as an orchestrating signal molecule.

Sulfoquinovose synthase - an important enzyme in the N-glycosylation pathway of Sulfolobus acidocaldarius

Recently, the Surface (S)-layer glycoprotein of the thermoacidophilic crenarchaeote Sulfolobus acidocaldarius was found to be N-glycosylated with a heterogeneous family of glycans, with the largest having a composition Glc(1)Man(2)GlcNAc(2) plus 6-sulfoquinovose. However, genetic analyses of genes involved in the N-glycosylation process in Crenarchaeota were missing so far. In this study we identify a gene cluster involved in the biosynthesis of sulfoquinovose and important for the assembly of the S-layer N-glycans. A successful markerless in-frame deletion of agl3 resulted in a decreased molecular mass of the S-layer glycoprotein SlaA and the flagellin FlaB, indicating a change in the N-glycan composition. Analyses with nanoLC ES-MS/MS confirmed the presence of only a reduced trisaccharide structure composed of Man(1) GlcNAc(2) , missing the sulfoquinovose, a mannose and glucose. Biochemical studies of the recombinant Agl3 confirmed the proposed function as a UDP-sulfoquinovose synthase. Furthermore, S. acidocaldarius cells lacking agl3 had a significantly lower growth rate at elevated salt concentrations compared with the background strain, underlining the importance of the N-glycosylation to maintain an intact and stable cell envelope, to enable the survival of S. acidocaldarius in its extreme environment.

Biosynthesis and functions of the plant sulfolipid

Higher-plant chloroplast membranes are composed primarily of four characteristic lipids, namely monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol. Among them, SQDG is the only sulfur-containing anionic glycerolipid and is the least prevalent component of photosynthetic membrane lipids. SQDG biosynthesis is mostly mediated by UDP-sulfoquinovose synthase (SQD1) and SQDG synthase (SQD2). Recently, another essential gene for SQDG synthesis, UGP3, was identified using transcriptome coexpression analysis and reverse genetics. UGP3 is a novel plastid UDP-glucose pyrophosphorylase that supplies UDP-glucose to SQD1 in plastids. In Arabidopsis, SQDG is dispensable under normal growth conditions but important in certain environments, particularly phosphate-depleted conditions. The function of SQDG under phosphate-limited growth conditions is highly correlated with the regulation of other plant glycerolipid biosyntheses. This review summarizes recent research defining the mechanism for SQDG biosynthesis and its biological function in higher plants, particularly under phosphate-starved conditions.

A chloroplastic UDP-glucose pyrophosphorylase from Arabidopsis is the committed enzyme for the first step of sulfolipid biosynthesis

Plants synthesize a sulfur-containing lipid, sulfoquinovosyldiacylglycerol, which is one of three nonphosphorus glycerolipids that provide the bulk of the structural lipids in photosynthetic membranes. Here, the identification of a novel gene, UDP-glucose pyrophosphorylase3 (UGP3), required for sulfolipid biosynthesis is described. Transcriptome coexpression analysis demonstrated highly correlated expression of UGP3 with known genes for sulfolipid biosynthesis in Arabidopsis thaliana. Liquid chromatography-mass spectrometry analysis of leaf lipids in two Arabidopsis ugp3 mutants revealed that no sulfolipid was accumulated in these mutants, indicating the participation of UGP3 in sulfolipid biosynthesis. From the deduced amino acid sequence, UGP3 was presumed to be a UDP-glucose pyrophosphorylase (UGPase) involved in the generation of UDP-glucose, serving as the precursor of the polar head of sulfolipid. Recombinant UGP3 was able to catalyze the formation of UDP-glucose from glucose-1-phosphate and UTP. A transient assay using fluorescence fusion proteins and UGPase activity in isolated chloroplasts indicated chloroplastic localization of UGP3. The transcription level of UGP3 was increased by phosphate starvation. A comparative genomics study on UGP3 homologs across different plant species suggested the structural and functional conservation of the proteins and, thus, a committing role for UGP3 in sulfolipid synthesis.

Questions remaining in sulfolipid biosynthesis: a historical perspective

The plant sulfolipid sulfoquinovosyldiacylglycerol was discovered by A.A. Benson in the late 1950s. The increasing availability of radioisotope-containing biological substrates such as (35)S-sulfate provided the means to discover novel biological compounds and to sketch out their biosynthetic pathways. During this time the structure of sulfolipid with its 6-deoxy-6-sulfo-alpha-D: -glucose (sulfoquinovose) headgroup was determined. Immediately, the origin of this unusual biological sulfonic acid mystified the scientific community and several proposals for its biosynthesis were developed and tested. Strong supportive evidence for the nucleotide pathway of sulfolipid biosynthesis became available with the discovery of the bacterial and plant genes encoding the enzymes of sulfolipid biosynthesis during the 1990s. This latter work was based on the foundations laid by A.A. Benson and confirmed one initial hypothesis on sulfolipid biosynthesis. An abbreviated summary of the turning points in defining the mechanism for sulfolipid biosynthesis and remaining issues in sulfolipid biochemistry are provided.

Ferredoxin-dependent glutamate synthase moonlights in plant sulfolipid biosynthesis by forming a complex with SQD1

UDP-sulfoquinovose synthase, SQD1, catalyzes the transfer of sulfite to UDP-glucose giving rise to UDP-sulfoquinovose, which is the head group donor for the biosynthesis of the plant sulfolipid sulfoquinovosyldiacylglyerol. The native SQD1 enzyme of spinach exists as a 250 kDa heteroprotein complex with much higher affinity for the substrate sulfite than the recombinant SQD1 protein itself. The SQD1 protein co-purified with nine proteins. Likely binding partners included rubisco activase, HSP70, and ferredoxin-dependent glutamate synthase (FdGOGAT). While the first two proteins are known to interact with many other proteins, the identification of FdGOGAT was most intriguing because this 160kDa protein contains an FMN cofactor known to bind sulfite in vitro. Using different constructs expressing recombinant forms of the multidomain protein FdGOGAT, it was demonstrated that the FMN-binding domain of FdGOGAT is essential for specific binding of the protein to SQD1. A model suggests that FdGOGAT could channel sulfite to SQD1.

Phosphatidylglycerol and sulfoquinovosyldiacylglycerol: anionic membrane lipids and phosphate regulation

Photosynthetic membranes of organisms from cyanobacteria to seed plants are characterized by the neutral galactolipids and the anionic glycerolipids sulfoquinovosyldiacylglycerol and phosphatidylglycerol. Recent findings have brought new insights into the biosynthesis of the anionic membrane lipids, the evolutionary origin of the enzymes involved in this process, and the importance of phosphatidylglycerol and sulfoquinovosyldiacylgycerol in photosynthesis. Photosynthetic membranes require a defined level of anionic membrane lipids for proper function, and phosphatidylglycerol and sulfoquinovosyldiacylglycerol can substitute for each other to a certain extent. A defined level of phosphatidylglycerol is, however, indispensable for photoautotrophic growth. On the other hand, sulfoquinovosyldiacylglycerol plays a conditionally important role in enabling photosynthetic organisms to survive the phosphate-limiting conditions frequently encountered in natural habitats.

The sulfolipids 2'-O-acyl-sulfoquinovosyldiacylglycerol and sulfoquinovosyldiacylglycerol are absent from a Chlamydomonas reinhardtii mutant deleted in SQD1

The biosynthesis of thylakoid lipids in eukaryotic photosynthetic organisms often involves enzymes in the endoplasmic reticulum (ER) and the chloroplast envelopes. Two pathways of thylakoid lipid biosynthesis, the ER and the plastid pathways, are present in parallel in many species, including Arabidopsis, but in other plants, e.g. grasses, only the ER pathway is active. The unicellular alga Chlamydomonas reinhardtii diverges from plants like Arabidopsis in a different way because its membranes do not contain phosphatidylcholine, and most thylakoid lipids are derived from the plastid pathway. Here, we describe an acylated derivative of sulfolipid, 2'-O-acyl-sulfoquinovosyldiacylglycerol (ASQD), which is present in C. reinhardtii. Although the fatty acids of sulfoquinovosyldiacylglycerol (SQDG) were mostly saturated, ASQD molecular species carried predominantly unsaturated fatty acids. Moreover, directly attached to the head group of ASQD was preferentially an 18-carbon fatty acid with four double bonds. High-throughput robotic screening led to the isolation of a plasmid disruption mutant of C. reinhardtii, designated Deltasqd1, which lacks ASQD as well as SQDG. In this mutant, the SQD1 ortholog was completely deleted and replaced by plasmid sequences. It is proposed that ASQD arises from the sugar nucleotide pathway of sulfolipid biosynthesis by acylation of the 2'-hydroxyl of the sulfoquinovosyl head group. At the physiological level, the mutant showed increased sensitivity to a diuron herbicide and reduced growth under phosphate limitation, suggesting a role for SQDG and/or ASQD in photosynthesis as conducted by C. reinhardtii, particularly under phosphate-limited conditions.