A unique caleosin serving as the major integral protein in oil bodies isolated from Chlorella sp. cells cultured with limited nitrogen

Lipid Droplets in Endosymbiotic Symbiodiniaceae spp. Associated with Corals

Symbiodiniaceae species is a dinoflagellate that plays a crucial role in maintaining the symbiotic mutualism of reef-building corals in the ocean. Reef-building corals, as hosts, provide the nutrition and habitat to endosymbiotic Symbiodiniaceae species and Symbiodiniaceae species transfer the fixed carbon to the corals for growth. Environmental stress is one of the factors impacting the physiology and metabolism of the corals-dinoflagellate association. The environmental stress triggers the metabolic changes in Symbiodiniaceae species resulting in an increase in the production of survival organelles related to storage components such as lipid droplets (LD). LDs are found as unique organelles, mainly composed of triacylglycerols surrounded by phospholipids embedded with some proteins. To date, it has been reported that investigation of lipid droplets significantly present in animals and plants led to the understanding that lipid droplets play a key role in lipid storage and transport. The major challenge of investigating endosymbiotic Symbiodiniaceae species lies in overcoming the strategies in isolating lesser lipid droplets present in its intercellular cells. Here, we review the most recent highlights of LD research in endosymbiotic Symbiodiniaceae species particularly focusing on LD biogenesis, mechanism, and major lipid droplet proteins. Moreover, to comprehend potential novel ways of energy storage in the symbiotic interaction between endosymbiotic Symbiodiniaceae species and its host, we also emphasize recent emerging environmental factors such as temperature, ocean acidification, and nutrient impacting the accumulation of lipid droplets in endosymbiotic Symbiodiniaceae species.

Lipid Droplets from Plants and Microalgae: Characteristics, Extractions, and Applications

Plant and algal LDs are gaining popularity as a promising non-chemical technology for the production of lipids and oils. In general, these organelles are composed of a neutral lipid core surrounded by a phospholipid monolayer and various surface-associated proteins. Many studies have shown that LDs are involved in numerous biological processes such as lipid trafficking and signaling, membrane remodeling, and intercellular organelle communications. To fully exploit the potential of LDs for scientific research and commercial applications, it is important to develop suitable extraction processes that preserve their properties and functions. However, research on LD extraction strategies is limited. This review first describes recent progress in understanding the characteristics of LDs, and then systematically introduces LD extraction strategies. Finally, the potential functions and applications of LDs in various fields are discussed. Overall, this review provides valuable insights into the properties and functions of LDs, as well as potential approaches for their extraction and utilization. It is hoped that these findings will inspire further research and innovation in the field of LD-based technology.

Multiomics and optobiotechnological approaches for the development of microalgal strain for production of aviation biofuel and biorefinery

Demand and consumption of fossil fuels is increasing daily, and oil reserves are depleting. Technological developments are required towards developing sustainable renewable energy sources and microalgae are emerging as a potential candidate for various application-driven research. Molecular understanding attained through omics and system biology approach empowering researchers to modify various metabolic pathways of microalgal system for efficient extraction of biofuel and important biomolecules. This review furnish insight into different "advanced approaches" like optogenetics, systems biology and multi-omics for enhanced production of FAS (Fatty Acid Synthesis) and lipids in microalgae and their associated challenges. These new approaches would be helpful in the path of developing microalgae inspired technological platforms for optobiorefinery, which could be explored as source material to produce biofuels and other valuable bio-compounds on a large scale.

Multiple caleosins have overlapping functions in oil accumulation and embryo development

Caleosins are lipid droplet- and endoplasmic reticulum-associated proteins. To investigate their functions in oil accumulation, expression levels of caleosins in developing seeds of Arabidopsis thaliana were examined and four seed-expressed caleosins (CLO1, CLO2, CLO4, and CLO6) were identified. The four single mutants showed similar minor changes of fatty acid composition in seeds. Two double mutants (clo1 clo2 and clo1×clo2) demonstrated distinct changes of fatty acid composition, a 16-23% decrease of oil content, and a 10-13% decrease of seed weight. Moreover, a 40% decrease of oil content, further fatty acid changes, and misshapen membranes of smaller lipid droplets were found in seeds of quadruple CLO RNAi lines. Notably, ~40% of quadruple CLO RNAi T1 seeds failed to germinate, and deformed embryos and seedlings were also observed. Complementation experiments showed that CLO1 rescued the phenotype of clo1 clo2. Overexpression of CLO1 in seedlings and BY2 cells increased triacylglycerol content up to 73.6%. Transcriptome analysis of clo1 clo2 developing seeds showed that expression levels of some genes related to lipid, embryo development, calcium signaling, and stress responses were affected. Together, these results suggest that the major seed-expressed caleosins have overlapping functions in oil accumulation and show pleiotropic effects on embryo development.

Isolation of Lipid Droplets for Protein and Lipid Analysis

Cytosolic lipid droplets (LDs) are organelles which emulsify a variety of hydrophobic molecules in the aqueous cytoplasm of essentially all plant cells. Most familiar are the LDs from oilseeds or oleaginous fruits that primarily store triacylglycerols and serve a storage function. However, similar hydrophobic particles are found in cells of plant tissues that package terpenoids, sterol esters, wax esters, or other types of nonpolar lipids. The various hydrophobic lipids inside LDs are coated with a phospholipid monolayer, mostly derived from membrane phospholipids during their ontogeny. Various proteins have been identified to be associated with LDs, and these may be cell-type, tissue-type, or even species specific. While major LD proteins like oleosins have been known for decades, more recently a growing list of LD proteins has been identified, primarily by proteomics analyses of isolated LDs and confirmation of their localization by confocal microscopy. LDs, unlike other organelles, have a density less than that of water, and consequently can be isolated and enriched in cellular fractions by flotation centrifugation for composition studies. However, due to its deep coverage, modern proteomics approaches are also prone to identify contaminants, making control experiments necessary. Here, procedures for the isolation of LDs, and analysis of LD components are provided as well as methods to validate the LD localization of proteins.

Biochemical and Molecular Aspects of Phosphorus Limitation in Diatoms and Their Relationship with Biomolecule Accumulation

Simple Summary

Phosphorus (P) is a key nutrient involved in the transfer of energy and the synthesis of several cellular components. It has been reported that P limitation in diatoms induces the synthesis of biomolecules and the accumulation of storage compounds, such as pigments, carbohydrates and lipids, with diverse biological activities, which can be used in diverse biotechnological applications. However, the molecular and biochemical mechanisms related to how diatoms cope with P deficiency are not clear, and research into this has been limited to a few species. The integration of results obtained from omics sciences could provide a broad understanding of the response of diatoms to P limitation, and the information obtained could help to solve challenges such as biomass production, by-products yield and genetic improvement of strains.

Abstract

Diatoms are the most abundant group of phytoplankton, and their success lies in their significant adaptation ability to stress conditions, such as nutrient limitation. Phosphorus (P) is a key nutrient involved in the transfer of energy and the synthesis of several cellular components. Molecular and biochemical mechanisms related to how diatoms cope with P deficiency are not clear, and research into this has been limited to a few species. Among the molecular responses that have been reported in diatoms cultured under P deficient conditions is the upregulation of genes encoding enzymes related to the transport, assimilation, remobilization and recycling of this nutrient. Regarding biochemical responses, due to the reduction of the requirements for carbon structures for the synthesis of proteins and phospholipids, more CO₂ is fixed than is consumed by the Calvin cycle. To deal with this excess, diatoms redirect the carbon flow toward the synthesis of storage compounds such as triacylglycerides and carbohydrates, which are excreted as extracellular polymeric substances. This review aimed to gather all current knowledge regarding the biochemical and molecular mechanisms of diatoms related to managing P deficiency in order to provide a wider insight into and understanding of their responses, as well as the metabolic pathways affected by the limitation of this nutrient.

Current perspectives on integrated approaches to enhance lipid accumulation in microalgae

In recent years, research initiatives on renewable bioenergy or biofuels have been gaining momentum, not only due to fast depletion of finite reserves of fossil fuels but also because of the associated concerns for the environment and future energy security. In the last few decades, interest is growing concerning microalgae as the third-generation biofuel feedstock. The CO₂ fixation ability and conversion of it into value-added compounds, devoid of challenging food and feed crops, make these photosynthetic microorganisms an optimistic producer of biofuel from an environmental point of view. Microalgal-derived fuels are currently being considered as clean, renewable, and promising sustainable biofuel. Therefore, most research targets to obtain strains with the highest lipid productivity and a high growth rate at the lowest cultivation costs. Different methods and strategies to attain higher biomass and lipid accumulation in microalgae have been extensively reported in the previous research, but there are fewer inclusive reports that summarize the conventional methods with the modern techniques for lipid enhancement and biodiesel production from microalgae. Therefore, the current review focuses on the latest techniques and advances in different cultivation conditions, the effect of different abiotic and heavy metal stress, and the role of nanoparticles (NPs) in the stimulation of lipid accumulation in microalgae. Techniques such as genetic engineering, where particular genes associated with lipid metabolism, are modified to boost lipid synthesis within the microalgae, the contribution of "Omics" in metabolic pathway studies. Further, the contribution of CRISPR/Cas9 system technique to the production of microalgae biofuel is also briefly described.

OsClo5 functions as a transcriptional co-repressor by interacting with OsDi19-5 to negatively affect salt stress tolerance in rice seedlings

Caleosins constitute a small protein family with one calcium-binding EF-hand motif. They are involved in the regulation of development and response to abiotic stress in plants. Nevertheless, how they impact salt stress tolerance in rice is largely unknown. Thereby, biochemical and molecular genetic experiments were carried out, and the results revealed that OsClo5 was able to bind calcium and phospholipids in vitro and localized in the nucleus and endoplasmic reticulum in rice protoplasts. At the germination and early seedlings stages, overexpression transgenic lines and T-DNA mutant lines exhibited reduced and increased tolerance to salt stress, respectively, compared with the wild-type. Yeast two-hybrid, bimolecular fluorescence complementation and in vitro pull-down assays demonstrated that the EF-hand motif of OsClo5 was essential for the interactions with itself and OsDi19-5. Yeast one-hybrid, electrophoretic migration shift and dual-luciferase reporter assays identified OsDi19-5 as a transcriptional repressor via the TACART cis-element in the promoters of two salt stress-related target genes, OsUSP and OsMST. In addition, OsClo5 enhanced the inhibitory effect of OsDi19-5 in the tobacco transient system, which was confirmed by qRT-PCR analysis in rice seedlings under salt stress. The collective results deepen the understanding of the molecular mechanism underlying the roles of caleosin in the salt stress response. These findings will also inform efforts to improve salt tolerance of rice.

Identification and characterization of caleosin in Cycas revoluta pollen

Oil bodies are essential energy storage organelles that are generally present in the seeds of plants. Caleosin protein has been identified in the seed oil bodies of Cycas revolutaseed. In this study, mature Cycas revoluta pollen grains were collected from cycad elliptical cones. However, the isolation and identification of oil bodies protein from mature Cycas revoluta grains have never been experimentally determined. Ultrastructural studies have shown that the oil bodies were present in pollen Cycas revoluta grains. Lipid analysis showed that oil bodies are predominantly composed of triacylglycerol. Complete cDNA fragments encoding the caleosin were obtained by PCR cloning. Phylogenetic tree analyzes showed that cycad pollen caleosin is closely related to the caleosin of Cycas revoluta seeds. Fresh Cycas revoluta pollen grains were allowed to germinate for 48 h in a germination medium containing 2% sucrose and 0.01% H₃BO_3. Pollen germination were recorded periodically from day one to day 10 and the results showed that the Cycas revoluta pollen elongate the tube and increasing of triacylglycerol(TAG) after 4 days.

Protein Profiles of Lipid Droplets during the Hypersensitive Defense Response of Arabidopsis against Pseudomonas Infection

Lipid droplets (LDs) have classically been viewed as seed storage particles, yet they are now emerging as dynamic organelles associated with developmental and stress responses. Nevertheless, their involvement in plant immunity has still been little studied. Here, we found LD accumulation in Arabidopsis thaliana leaves that induced a hypersensitive response (HR) after Pseudomonas infection. We established a protocol to reproducibly isolate LDs and to analyze their protein content. The expression of GFP fusion proteins in Nicotiana benthamiana and in transgenic Arabidopsis lines validated the LD localization of glycerol-3-phosphate acyltransferase 4 (GPAT4) and 8 (GPAT8), required for cutin biosynthesis. Similarly, we showed LD localization of α-dioxygenase1 (α-DOX1) and caleosin3 (CLO3), involved in the synthesis of fatty acid derivatives, and that of phytoalexin-deficient 3 (PAD3), which is involved in camalexin synthesis. We found evidence suggesting the existence of different populations of LDs, with varying protein contents and distributions. GPAT4 and GPAT8 were associated with LDs inside stomata and surrounding cells of untreated leaves, yet they were mainly confined to LDs in guard cells after bacterial inoculation. By contrast, α-DOX1 and PAD3 were associated with LDs in the epidermal cells of HR-responding leaves, with PAD3 mostly restricted to cells near dead tissue, while CLO3 had a more ubiquitous distribution. As such, the nature of the proteins identified, together with the phenotypic examination of selected mutants, suggests that LDs participate in lipid changes and in the production and transport of defense components affecting the interaction of plants with invading pathogens.

The Puzzling Conservation and Diversification of Lipid Droplets from Bacteria to Eukaryotes

Membrane compartments are amongst the most fascinating markers of cell evolution from prokaryotes to eukaryotes, some being conserved and the others having emerged via a series of primary and secondary endosymbiosis events. Membrane compartments comprise the system limiting cells (one or two membranes in bacteria, a unique plasma membrane in eukaryotes) and a variety of internal vesicular, subspherical, tubular, or reticulated organelles. In eukaryotes, the internal membranes comprise on the one hand the general endomembrane system, a dynamic network including organelles like the endoplasmic reticulum, the Golgi apparatus, the nuclear envelope, etc. and also the plasma membrane, which are linked via direct lateral connectivity (e.g. between the endoplasmic reticulum and the nuclear outer envelope membrane) or indirectly via vesicular trafficking. On the other hand, semi-autonomous organelles, i.e. mitochondria and chloroplasts, are disconnected from the endomembrane system and request vertical transmission following cell division. Membranes are organized as lipid bilayers in which proteins are embedded. The budding of some of these membranes, leading to the formation of the so-called lipid droplets (LDs) loaded with hydrophobic molecules, most notably triacylglycerol, is conserved in all clades. The evolution of eukaryotes is marked by the acquisition of mitochondria and simple plastids from Gram-positive bacteria by primary endosymbiosis events and the emergence of extremely complex plastids, collectively called secondary plastids, bounded by three to four membranes, following multiple and independent secondary endosymbiosis events. There is currently no consensus view of the evolution of LDs in the Tree of Life. Some features are conserved; others show a striking level of diversification. Here, we summarize the current knowledge on the architecture, dynamics, and multitude of functions of the lipid droplets in prokaryotes and in eukaryotes deriving from primary and secondary endosymbiosis events.

Chlorella vulgaris genome assembly and annotation reveals the molecular basis for metabolic acclimation to high light conditions

Chlorella vulgaris is a fast-growing fresh-water microalga cultivated on the industrial scale for applications ranging from food to biofuel production. To advance our understanding of its biology and to establish genetics tools for biotechnological manipulation, we sequenced the nuclear and organelle genomes of Chlorella vulgaris 211/11P by combining next generation sequencing and optical mapping of isolated DNA molecules. This hybrid approach allowed us to assemble the nuclear genome in 14 pseudo-molecules with an N50 of 2.8 Mb and 98.9% of scaffolded genome. The integration of RNA-seq data obtained at two different irradiances of growth (high light, HL versus low light, LL) enabled us to identify 10 724 nuclear genes, coding for 11 082 transcripts. Moreover, 121 and 48 genes, respectively, were found in the chloroplast and mitochondrial genome. Functional annotation and expression analysis of nuclear, chloroplast and mitochondrial genome sequences revealed particular features of Chlorella vulgaris. Evidence of horizontal gene transfers from chloroplast to mitochondrial genome was observed. Furthermore, comparative transcriptomic analyses of LL versus HL provided insights into the molecular basis for metabolic rearrangement under HL versus LL conditions leading to enhanced de novo fatty acid biosynthesis and triacylglycerol accumulation. The occurrence of a cytosolic fatty acid biosynthetic pathway could be predicted and its upregulation upon HL exposure was observed, consistent with the increased lipid amount under HL conditions. These data provide a rich genetic resource for future genome editing studies, and potential targets for biotechnological manipulation of Chlorella vulgaris or other microalgae species to improve biomass and lipid productivity.

Proteomics Analysis of Lipid Droplets from the Oleaginous Alga Chromochloris zofingiensis Reveals Novel Proteins for Lipid Metabolism

Chromochloris zofingiensis represents an industrially relevant and unique green alga, given its capability of synthesizing triacylglycerol (TAG) and astaxanthin simultaneously for storage in lipid droplets (LDs). To further decipher lipid metabolism, the nitrogen deprivation (ND)-induced LDs from C. zofingiensis were isolated, purified, and subjected to proteomic analysis. Intriguingly, many C. zofingiensis LD proteins had no orthologs present in LD proteome of the model alga Chlamydomonas reinhardtii. Seven novel LD proteins (i.e., two functionally unknown proteins, two caleosins, two lipases, and one l-gulonolactone oxidase) and the major LD protein (MLDP), which were all transcriptionally up-regulated by ND, were selected for further investigation. Heterologous expression in yeast demonstrated that all tested LD proteins were localized to LDs and all except the two functionally unknown proteins enabled yeast to produce more TAG. MLDP could restore the phenotype of mldp mutant strain and enhance TAG synthesis in wild-type strain of C. reinhardtii. Although MLDP and caleosins had a comparable abundance in LDs, they responded distinctly to ND at the transcriptional level. The two lipases, instead of functioning as TAG lipases, likely recycled polar lipids to support TAG synthesis. For the first time, we reported that l-gulonolactone oxidase was abundant in LDs and facilitated TAG accumulation. Moreover, we also proposed a novel working model for C. zofingiensis LDs. Taken together, our work unravels the unique characteristics of C. zofingiensis LDs and provides insights into algal LD biogenesis and TAG synthesis, which would facilitate genetic engineering of this alga for TAG improvement.

Marine Natural Products from Microalgae: An -Omics Overview

Over the last decade, genome sequences and other -omics datasets have been produced for a wide range of microalgae, and several others are on the way. Marine microalgae possess distinct and unique metabolic pathways, and can potentially produce specific secondary metabolites with biological activity (e.g., antipredator, allelopathic, antiproliferative, cytotoxic, anticancer, photoprotective, as well as anti-infective and antifouling activities). Because microalgae are very diverse, and adapted to a broad variety of environmental conditions, the chances to find novel and unexplored bioactive metabolites with properties of interest for biotechnological and biomedical applications are high. This review presents a comprehensive overview of the current efforts and of the available solutions to produce, explore and exploit -omics datasets, with the aim of identifying species and strains with the highest potential for the identification of novel marine natural products. In addition, funding efforts for the implementation of marine microalgal -omics resources and future perspectives are presented as well.

Expression profiling of genes coding for abundant proteins in the alkenone body of marine haptophyte alga Tisochrysis lutea

Background

Several abundant proteins have been identified in lipid body of an alkenone-producing marine haptophyte alga Tisochrysis lutea. The gene expression patterns of these proteins were investigated to better understand their roles in alkenone biosynthesis. For this purpose, T. lutea was first cultured in nitrogen-sufficient medium for biomass production and then shifted to nitrogen-deprived medium to induce lipid body formation.

Results

There were remarkable increases in the volume of alkenone body (AB) and alkenone content in the alga after they were exposed to nitrogen depletion medium. Relative mRNA levels of the genes coding for the identified proteins V-ATPase subunit V_A, V-ATPase subunit V_d, hypothetical protein EMIHUDRAFT_465,517, coccolith scale associated protein-1, cycloartenol-c-24-methyltransferase 1-like and SPFH domain-containing protein were investigated over the culture period. RT-PCR data showed that the expression of all these genes except the gene coding for SPFH domain-containing protein was up-regulated during the transition period from nitrogen-sufficient to nitrogen-deficient medium. Among them, the expression of the coccolith scale associated protein-1 gene was up-regulated 50–650 folds. These up-regulations were consistent with the increased alkenone production in nitrogen-deprived medium, suggesting that these proteins are involved in alkenone biosynthesis in T. lutea.

Conclusions

Expression analysis of the lipoprotein genes suggests that five out of the six genes are up-regulated and are therefore likely to code for the identified lipoproteins associated with alkenone biosynthesis in T. lutea. These data would help better understand alkenone metabolism and engineer for improved biofuel production in T. lutea.

Evolutionary, structural and functional analysis of the caleosin/peroxygenase gene family in the Fungi

BACKGROUND

Caleosin/peroxygenases, CLO/PXG, (designated PF05042 in Pfam) are a group of genes/proteins with anomalous distributions in eukaryotic taxa. We have previously characterised CLO/PXGs in the Viridiplantae. The aim of this study was to investigate the evolution and functions of the CLO/PXGs in the Fungi and other non-plant clades and to elucidate the overall origin of this gene family.

RESULTS

CLO/PXG-like genes are distributed across the full range of fungal groups from the basal clades, Cryptomycota and Microsporidia, to the largest and most complex Dikarya species. However, the genes were only present in 243 out of 844 analysed fungal genomes. CLO/PXG-like genes have been retained in many pathogenic or parasitic fungi that have undergone considerable genomic and structural simplification, indicating that they have important functions in these species. Structural and functional analyses demonstrate that CLO/PXGs are multifunctional proteins closely related to similar proteins found in all major taxa of the Chlorophyte Division of the Viridiplantae. Transcriptome and physiological data show that fungal CLO/PXG-like genes have complex patterns of developmental and tissue-specific expression and are upregulated in response to a range of biotic and abiotic stresses as well as participating in key metabolic and developmental processes such as lipid metabolism, signalling, reproduction and pathogenesis. Biochemical data also reveal that the Aspergillus flavus CLO/PXG has specific functions in sporulation and aflatoxin production as well as playing roles in lipid droplet function.

CONCLUSIONS

In contrast to plants, CLO/PXGs only occur in about 30% of sequenced fungal genomes but are present in all major taxa. Fungal CLO/PXGs have similar but not identical roles to those in plants, including stress-related oxylipin signalling, lipid metabolism, reproduction and pathogenesis. While the presence of CLO/PXG orthologs in all plant genomes sequenced to date would suggest that they have core housekeeping functions in plants, the selective loss of CLO/PXGs in many fungal genomes suggests more restricted functions in fungi as accessory genes useful in particular environments or niches. We suggest an ancient origin of CLO/PXG-like genes in the 'last eukaryotic common ancestor' (LECA) and their subsequent loss in ancestors of the Metazoa, after the latter had diverged from the ancestral fungal lineage.

Evolutionary and genomic analysis of the caleosin/peroxygenase (CLO/PXG) gene/protein families in the Viridiplantae

Bioinformatics analyses of caleosin/peroxygenases (CLO/PXG) demonstrated that these genes are present in the vast majority of Viridiplantae taxa for which sequence data are available. Functionally active CLO/PXG proteins with roles in abiotic stress tolerance and lipid droplet storage are present in some Trebouxiophycean and Chlorophycean green algae but are absent from the small number of sequenced Prasinophyceaen genomes. CLO/PXG-like genes are expressed during dehydration stress in Charophyte algae, a sister clade of the land plants (Embryophyta). CLO/PXG-like sequences are also present in all of the >300 sequenced Embryophyte genomes, where some species contain as many as 10–12 genes that have arisen via selective gene duplication. Angiosperm genomes harbour at least one copy each of two distinct CLO/PX isoforms, termed H (high) and L (low), where H-forms contain an additional C-terminal motif of about 30–50 residues that is absent from L-forms. In contrast, species in other Viridiplantae taxa, including green algae, non-vascular plants, ferns and gymnosperms, contain only one (or occasionally both) of these isoforms per genome. Transcriptome and biochemical data show that CLO/PXG-like genes have complex patterns of developmental and tissue-specific expression. CLO/PXG proteins can associate with cytosolic lipid droplets and/or bilayer membranes. Many of the analysed isoforms also have peroxygenase activity and are involved in oxylipin metabolism. The distribution of CLO/PXG-like genes is consistent with an origin >1 billion years ago in at least two of the earliest diverging groups of the Viridiplantae, namely the Chlorophyta and the Streptophyta, after the Viridiplantae had already diverged from other Archaeplastidal groups such as the Rhodophyta and Glaucophyta. While algal CLO/PXGs have roles in lipid packaging and stress responses, the Embryophyte proteins have a much wider spectrum of roles and may have been instrumental in the colonisation of terrestrial habitats and the subsequent diversification as the major land flora.

Leveraging algal omics to reveal potential targets for augmenting TAG accumulation

Ongoing global efforts to commercialize microalgal biofuels have expedited the use of multi-omics techniques to gain insights into lipid biosynthetic pathways. Functional genomics analyses have recently been employed to complement existing sequence-level omics studies, shedding light on the dynamics of lipid synthesis and its interplay with other cellular metabolic pathways, thus revealing possible targets for metabolic engineering. Here, we review the current status of algal omics studies to reveal potential targets to augment TAG accumulation in various microalgae. This review specifically aims to examine and catalog systems level data related to stress-induced TAG accumulation in oleaginous microalgae and inform future metabolic engineering strategies to develop strains with enhanced bioproductivity, which could pave a path for sustainable green energy.

Structure and properties of oil bodies in diatoms

Diatoms accumulate triacylglycerols in spherical organelles called oil bodies when exposed to nutrient deprivation conditions. Oil body biology in diatoms has attracted significant attention due to the complexity of the intracellular organelles and the unique combination of genes generated by the evolutionary history of secondary endosymbiosis. The demand for biofuel production has further increased the interest in and importance of a better understanding of oil body biology in diatoms, because it could provide targets for genetic engineering to further enhance their promising lipid accumulation. This review describes recent progress in studies of the structure and properties of diatom oil bodies. Firstly, the general features of diatom oil bodies are described, in particular, their number, size and morphology, as well as the quantity and quality of lipids they contain. Subsequently, the diatom oil body-associated proteins, which were recently discovered through oil body proteomics, are introduced. Then, the metabolic pathways responsible for the biogenesis and degradation of diatom oil bodies are summarized. During biogenesis and degradation, oil bodies interact with other organelles, including chloroplasts, the endoplasmic reticulum and mitochondria, suggesting their dynamic nature in response to environmental changes. Finally, the functions of oil bodies in diatoms are discussed.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Analysis of the lipid body proteome of the oleaginous alga Lobosphaera incisa

BACKGROUND

Lobosphaera incisa (L. incisa) is an oleaginous microalga that stores triacylglycerol (TAG) rich in arachidonic acid in lipid bodies (LBs). This organelle is gaining attention in algal research, since evidence is accumulating that proteins attached to its surface fulfill important functions in TAG storage and metabolism.

RESULTS

Here, the composition of the LB proteome in L incisa was investigated by comparing different cell fractions in a semiquantitative proteomics approach. After applying stringent filters to the proteomics data in order to remove contaminating proteins from the list of possible LB proteins (LBPs), heterologous expression of candidate proteins in tobacco pollen tubes, allowed us to confirm 3 true LBPs: A member of the algal Major Lipid Droplet Protein family, a small protein of unknown function and a putative lipase. In addition, a TAG lipase that belongs to the SUGAR DEPENDENT 1 family of TAG lipases known from oilseed plants was identified. Its activity was verified by functional complementation of an Arabidopsis thaliana mutant lacking the major seed TAG lipases.

CONCLUSIONS

Here we describe 3 LBPs as well as a TAG lipase from the oleaginous microalga L. incisa and discuss their possible involvement in LB metabolism. This study highlights the importance of filtering LB proteome datasets and verifying the subcellular localization one by one, so that contaminating proteins can be recognized as such. Our dataset can serve as a valuable resource in the identification of additional LBPs, shedding more light on the intriguing roles of LBs in microalgae.

Identification of caleosin and oleosin in oil bodies of pine pollen

Unique proteins including steroleosin, caleosin, oleosin-L, and oleosin-G have been identified in seed oil bodies of pine (Pinus massoniana). In this study, mature pollen grains with wing-like bladders were collected from pine (Pinus elliottii). Ultrastructural studies showed that oil bodies were present in pollen grains, but not the attached bladders, and the presence of oil bodies was further confirmed by fluorescent staining with BODIPY 493/503. Stable oil bodies were successfully purified from pine pollen grains, and analyzed to be mainly composed of triacylglycerols. Putative oleosin and caleosin in pine pollen oil bodies were detected by immunoassaying with antibodies against sesame seed caleosin and lily pollen oleosin. Complete cDNA fragments encoding these two pollen oil-body proteins were obtained by PCR cloning. Sequence alignment showed that pine pollen caleosin (27 kDa) was highly homologous to pine seed caleosin (28 kDa) except for the lack of an appendix of eight residues at the C-terminus in accord with the 1 kDa difference in their molecular masses. Pine pollen oleosin (15 kDa) was highly homologous to pine seed oleosin-G (14 kDa) except for an insertion of eight residues at the N-terminus in accord with the 1 kDa difference in their molecular masses.

Identification of steroleosin in oil bodies of pine megagametophytes

Three classes of integral proteins termed oleosin, caleosin and steroleosin have been identified in seed oil bodies of diverse angiosperm species. Recently, two oleosin isoforms and one caleosin were identified in megagametophyte oil bodies of pine (Pinus massoniana), a representative gymnosperm species. In this study, a putative steroleosin of approximately 41 kDa was observed in isolated oil bodies of pine megagametophytes, and its corresponding cDNA fragment was obtained by PCR cloning and further confirmed by mass spectrometric analysis. Phylogenetic tree analysis showed that pine steroleosin was evolutionarily more closely-related to steroleosin-B than steroleosin-A found in angiosperm seed oil bodies. As expected, artificial oil bodies constituted with recombinant steroleosin over-expressed in Escherichia coli were less stable and larger than native pine oil bodies. Filipin staining of artificial oil bodies sheltered by recombinant steroleosin with or without its sterol binding domain showed that the sterol binding domain was responsible for the sterol binding capability of steroleosin. Sterol-coupling dehydrogenase activity was demonstrated in artificial oil bodies constituted with recombinant steroleosin as well as in purified pine oil bodies.

Application of high-content image analysis for quantitatively estimating lipid accumulation in oleaginous yeasts with potential for use in biodiesel production

Biodiesel from oleaginous microorganisms is a viable substitute for a fossil fuel. Current methods for microorganism lipid productivity evaluation do not analyze lipid dynamics in single cells. Here, we described a high-content image analysis (HCA) as a promising strategy for screening oleaginous microorganisms for biodiesel production, while generating single-cell lipid dynamics data in large cell density. Rhodotorula slooffiae yeast were grown in standard (CTL) or lipid trigger medium (LTM), and lipid droplet (LD) accumulation was analyzed in deconvolved confocal microscopy images of cells stained with the lipophilic fluorescent Nile red (NR) dye using automated cell and LD segmentation. The 'vesicle segmentation' method yielded valid morphometric results for limited lipid accumulation in smaller LDs (CTL samples) and for high lipid accumulation in larger LDs (LTM samples), and detected LD localization changes. Thus, HCA can be used to analyze the lipid accumulation patterns likely to be encountered in screens for biodiesel production.

Identification of a Major Lipid Droplet Protein in a Marine Diatom Phaeodactylum tricornutum

Various kinds of organisms, including microalgae, accumulate neutral lipids in distinct intracellular compartments called lipid droplets. Generally, lipid droplets are generated from the endoplasmic reticulum, and particular proteins localize on their surface. Some of these proteins function as structural proteins to prevent fusion between the lipid droplets, and the others could have an enzymatic role or might be involved in intracellular membrane trafficking. However, information about lipid droplet proteins in microalgae is scarce as compared with that in animals and land plants. We focused on the oil-producing, marine, pennate diatom Phaeodactylum tricornutum that forms lipid droplets during nitrogen deprivation and we investigated the proteins located on the lipid droplets. After 6 d of cultivation in a nitrate-deficient medium, the mature lipid droplets were isolated by sucrose density gradient centrifugation. Proteomic analyses revealed five proteins, with Stramenopile-type lipid droplet protein (StLDP) being the most abundant protein in the lipid droplet fraction. Although the primary sequence of StLDP did not have homology to any known lipid droplet proteins, StLDP had a central hydrophobic domain. This structural feature is also detected in oleosin of land plants and in lipid droplet surface protein (LDSP) of Nannochloropsis species. As a proline knot motif of oleosin, conservative proline residues existed in the hydrophobic domain. StLDP was up-regulated during nitrate deprivation, and fluctuations of StLDP expression levels corresponded to the size of the lipid droplets.

Proteomic analysis of lipid body from the alkenone-producing marine haptophyte alga Tisochrysis lutea

Lipid body (LB) is recognized as the cellular carbon and energy storage organelle in many organisms. LBs have been observed in the marine haptophyte alga Tisochrysis lutea that produces special lipids such as long‐chain (C₃₇‐C₄₀) ketones (alkenones) with 2–4 trans‐type double bonds. In this study, we succeeded in developing a modified method to isolate LB from T. lutea. Purity of isolated LBs was confirmed by the absence of chlorophyll auto‐fluorescence and no contamination of the most abundant cellular protein ribulose‐1,5‐bisphosphate carboxylase/oxygenase. As alkenones predominated in the LB by GC‐MS analysis, the LB can be more appropriately named as “alkenone body (AB).” Extracted AB‐containing proteins were analyzed by the combination of 1DE (SDS‐PAGE) and MS/MS for confident protein identification and annotated using BLAST tools at National Center for Biotechnology Information. Totally 514 proteins were identified at the maximum. The homology search identified three major proteins, V‐ATPase, a hypothetical protein EMIHUDRAFT_465517 found in other alkenone‐producing haptophytes, and a lipid raft‐associated SPFH domain‐containing protein. Our data suggest that AB of T. lutera is surrounded by a lipid membrane originating from either the ER or the ER‐derived four layer‐envelopes chloroplast and function as the storage site of alkenones and alkenes.

Leaf oil bodies are subcellular factories producing antifungal oxylipins

Oil bodies act as lipid storage compartments in plant cells. In seeds they supply energy for germination and early seedling growth. Oil bodies are also present in the leaves of many vascular plants, but their function in leaves has been poorly understood. Recent studies with oil bodies from senescent Arabidopsis thaliana leaves identified two enzymes, peroxygenase (CLO3) and α-dioxygenase (α-DOX), which together catalyze a coupling reaction to produce an antifungal compound (2-hydroxy-octadecanoic acid) from α-linolenic acid. Leaf oil bodies also have other enzymes including lipoxygenases, phospholipases, and triacylglycerol lipases. Hence, leaf oil bodies might function as intracellular factories to efficiently produce stable compounds via unstable intermediates by concentrating the enzymes and hydrophobic substrates.

N-terminus of seed caleosins is essential for lipid droplet sorting but not for lipid accumulation

Caleosin, a calcium-binding protein associated with plant lipid droplets, stimulates lipid accumulation when heterologously expressed in Saccharomyces cerevisiae. Accumulated lipids are stored in cytoplasmic lipid droplets that are stabilised by incorporated caleosin. We designed a set of mutants affecting putative crucial sites for caleosin function and association with lipid droplets, i.e. the N-terminus, the EF-hand motif and the proline-knot motif. We investigated the effect of introduced mutations on caleosin capacity to initiate lipid accumulation and on caleosin sorting within cell as well as on its association with lipid droplets. Our results strongly suggest that the N-terminal domain is essential for proper protein sorting and targeting to lipid droplets but not for enhancing lipid accumulation.

Examination of metabolic responses to phosphorus limitation via proteomic analyses in the marine diatom Phaeodactylum tricornutum

Phosphorus (P) is an essential macronutrient for the survival of marine phytoplankton. In the present study, phytoplankton response to phosphorus limitation was studied by proteomic profiling in diatom Phaeodactylum tricornutum in both cellular and molecular levels. A total of 42 non-redundant proteins were identified, among which 8 proteins were found to be upregulated and 34 proteins were downregulated. The results also showed that the proteins associated with inorganic phosphate uptake were downregulated, whereas the proteins involved in organic phosphorus uptake such as alkaline phosphatase were upregulated. The proteins involved in metabolic responses such as protein degradation, lipid accumulation and photorespiration were upregulated whereas energy metabolism, photosynthesis, amino acid and nucleic acid metabolism tend to be downregulated. Overall our results showed the changes in protein levels of P. tricornutum during phosphorus stress. This study preludes for understanding the role of phosphorous in marine biogeochemical cycles and phytoplankton response to phosphorous scarcity in ocean. It also provides insight into the succession of phytoplankton community, providing scientific basis for elucidating the mechanism of algal blooms.

Microalgal lipid droplets: composition, diversity, biogenesis and functions

Lipid droplet is the major site of neutral lipid storage in eukaryotic cells, and increasing evidence show its involvement in numerous cellular processes such as lipid homeostasis, signaling, trafficking and inter-organelle communications. Although the biogenesis, structure, and functions of lipid droplets have been well documented for seeds of vascular plants, mammalian adipose tissues, insects and yeasts, relative little is known about lipid droplets in microalgae. Over the past 5 years, the growing interest of microalgae as a platform for biofuel, green chemicals or value-added polyunsaturated fatty acid production has brought algal lipid droplets into spotlight. Studies conducted on the green microalga Chlamydomonas reinhardtii and other model microalgae such as Haematococcus and Nannochloropsis species have led to the identification of proteins associated with lipid droplets, which include putative structural proteins different from plant oleosins and animal perilipins, as well as candidate proteins for lipid biosynthesis, mobilization, trafficking and homeostasis. Biochemical and microscopy studies have also started to shed light on the role of chloroplasts in the biogenesis of lipid droplets in Chlamydomonas.

Caleosin from Chlorella vulgaris TISTR 8580 is salt-induced and heme-containing protein

Physiological and functional properties of lipid droplet-associated proteins in algae remain scarce. We report here the caleosin gene from Chlorella vulgaris encodes a protein of 279 amino acid residues. Amino acid sequence alignment showed high similarity to the putative caleosins from fungi, but less to plant caleosins. When the C. vulgaris TISTR 8580 cells were treated with salt stress (0.3 M NaCl), the level of triacylglycerol increased significantly. The mRNA contents for caleosin in Chlorella cells significantly increased under salt stress condition. Caleosin gene was expressed in E. coli. Crude extract of E. coli cells exhibited the cumene hydroperoxide-dependent oxidation of aniline. Absorption spectroscopy showed a peak around 415 nm which was decreased upon addition of cumene hydroperoxide. Native polyacrylamide gel electrophoresis suggests caleosin existed as the oligomer. These data indicate that a fresh water C. vulgaris TISTR 8580 contains a salt-induced heme-protein caleosin.

Proteome analysis of cytoplasmatic and plastidic β-carotene lipid droplets in Dunaliella bardawil

The halotolerant green alga Dunaliella bardawil is unique in that it accumulates under stress two types of lipid droplets: cytoplasmatic lipid droplets (CLD) and β-carotene-rich (βC) plastoglobuli. Recently, we isolated and analyzed the lipid and pigment compositions of these lipid droplets. Here, we describe their proteome analysis. A contamination filter and an enrichment filter were utilized to define core proteins. A proteome database of Dunaliella salina/D. bardawil was constructed to aid the identification of lipid droplet proteins. A total of 124 and 42 core proteins were identified in βC-plastoglobuli and CLD, respectively, with only eight common proteins. Dunaliella spp. CLD resemble cytoplasmic droplets from Chlamydomonas reinhardtii and contain major lipid droplet-associated protein and enzymes involved in lipid and sterol metabolism. The βC-plastoglobuli proteome resembles the C. reinhardtii eyespot and Arabidopsis (Arabidopsis thaliana) plastoglobule proteomes and contains carotene-globule-associated protein, plastid-lipid-associated protein-fibrillins, SOUL heme-binding proteins, phytyl ester synthases, β-carotene biosynthesis enzymes, and proteins involved in membrane remodeling/lipid droplet biogenesis: VESICLE-INDUCING PLASTID PROTEIN1, synaptotagmin, and the eyespot assembly proteins EYE3 and SOUL3. Based on these and previous results, we propose models for the biogenesis of βC-plastoglobuli and the biosynthesis of β-carotene within βC-plastoglobuli and hypothesize that βC-plastoglobuli evolved from eyespot lipid droplets.

SLDP: a novel protein related to caleosin is associated with the endosymbiotic Symbiodinium lipid droplets from Euphyllia glabrescens

Intracellular lipid droplets (LDs) have been proposed to play a key role in the mutualistic endosymbiosis between reef-building corals and the dinoflagellate endosymbiont Symbiodinium spp. This study investigates and identifies LD proteins in Symbiodinium from Euphyllia glabrescens. Discontinuous Percoll gradient centrifugation was used to separate Symbiodinium cells from E. glabrescens tentacles. Furthermore, staining with a fluorescent probe, Nile red, indicated that lipids accumulated in that freshly isolated Symbiodinium cells and lipid analyses further showed polyunsaturated fatty acids (PUFA) was abundant. The stable LDs were purified from endosymbiotic Symbiodinium cells. The structural integrity of the Symbiodinium LDs was maintained via electronegative repulsion and steric hindrance possibly provided by their surface proteins. Protein extracts from the purified LDs revealed a major protein band with a molecular weight of 20 kDa, which was termed Symbiodinium lipid droplet protein (SLDP). Interestingly, immunological cross-recognition analysis revealed that SLDP was detected strongly by the anti-sesame and anti-cycad caleosin antibodies. It was suggested that the stable Symbiodinium LDs were sheltered by this unique structural protein and was suggested that SLDP might be homologous to caleosin to a certain extent.

Nutrient limitation in Auxenochlorella protothecoides induces qualitative changes of fatty acid and expression of caleosin as a membrane protein associated with oil bodies

Oil bodies formed in Auxenochlorella protothecoides induced during limited nutrition had a coating of caleosin. The total lipid content obtained from A. protothecoides in unstressed cultures (first week) was ~210 mg/g compared to the 231 mg/g obtained in the third week (nutrient limited) and 290 mg/g obtained in the fourth week (nutrient limited). The proportion of total saturated fatty acids increased from 28 to 46 %, whereas that of total polyunsaturated fatty acids decreased from 52 to 35 %. The expression levels of the 28 kDa caleosin protein in A. protothecoides rose to a maximum up to 4 weeks; immunolocalization studies showed that caleosin was predominantly associated with the membranes of oil bodies.

Identification of caleosin and two oleosin isoforms in oil bodies of pine megagametophytes

Numerous oil bodies of 0.2-2 μm occupied approximately 80% of intracellular space in mature pine (Pinus massoniana) megagametophytes. They were stably isolated and found to comprise mostly triacylglycerols as examined by thin layer chromatography analysis and confirmed by both Nile red and BODIPY stainings. Fatty acids released from the triacylglycerols of pine oil bodies were mainly unsaturated, including linoleic acid (60%), adrenic acid (12.3%) and vaccenic acid (9.7%). Proteins extracted from pine oil bodies were subjected to immunological cross-recognition, and the results showed that three proteins of 28, 16 and 14 kDa were detected by antibodies against sesame seed caleosin, sesame oleosin-L and lily pollen oleosin-P, respectively. Complete cDNA fragments encoding these three pine oil-body proteins, tentatively named caleosin, oleosin-L and oleosin-G, were obtained by PCR cloning and further confirmed by mass spectrometric analysis. Consistently, phylogenetic tree analyses showed that pine caleosin was closely-related to the caleosin of cycad megagametophyte among known caleosin sequences. While pine oleosin-L was found clustered with seed oleosin isoforms of angiosperm species, oleosin-G was distinctively grouped with the oleosin-P of lily pollen. The oleosin-G identified in pine megagametophytes seems to represent a new class of seed oleosin isoform evolutionarily close to the pollen oleosin-P.

Nitrogen deprivation induces lipid droplet accumulation and alters fatty acid metabolism in symbiotic dinoflagellates isolated from Aiptasia pulchella

The stability of cnidarian-dinoflagellate (genus Symbiodinium spp.) endosymbioses depends on the regulation of nutrient transport between Symbiodinium populations and their hosts. Previously, we successfully induced the production of lipid droplets in the free-living cultured Symbiodinium (clade B) under the nitrogen-deprivation condition for 5 days. Therefore, the present study aimed at understanding the disruption of the endosymbiotic relationship between the cnidarians and dinoflagellates by nitrogen deprivation using Aiptasia pulchella as an example. Transmission electron micrographs revealed the formation of lipid droplets induced by nitrogen deprivation, and the lipid analyses further showed that polyunsaturated fatty acids were drastically enriched in Symbiodinium after 30 days of nitrogen deprivation, although these were unaffected after 5 days of nitrogen starvation. The present study also suggested that the host provided nitrogen to the symbiotic cells during short-term environmental stress. However, the relationship started to deteriorate after 30 days. These findings provide a more detailed understanding of the mechanisms of the symbiotic relationship between the symbiotic dinoflagellates in terms of the nitrogen source, which might provide more information for the explanation of the regulatory mechanism underlying endosymbiotic associations.

Oleosome-associated protein of the oleaginous diatom Fistulifera solaris contains an endoplasmic reticulum-targeting signal sequence

Microalgae tend to accumulate lipids as an energy storage material in the specific organelle, oleosomes. Current studies have demonstrated that lipids derived from microalgal oleosomes are a promising source of biofuels, while the oleosome formation mechanism has not been fully elucidated. Oleosome-associated proteins have been identified from several microalgae to elucidate the fundamental mechanisms of oleosome formation, although understanding their functions is still in infancy. Recently, we discovered a diatom-oleosome-associated-protein 1 (DOAP1) from the oleaginous diatom, Fistulifera solaris JPCC DA0580. The DOAP1 sequence implied that this protein might be transported into the endoplasmic reticulum (ER) due to the signal sequence. To ensure this, we fused the signal sequence to green fluorescence protein. The fusion protein distributed around the chloroplast as like a meshwork membrane structure, indicating the ER localization. This result suggests that DOAP1 could firstly localize at the ER, then move to the oleosomes. This study also demonstrated that the DOAP1 signal sequence allowed recombinant proteins to be specifically expressed in the ER of the oleaginous diatom. It would be a useful technique for engineering the lipid synthesis pathways existing in the ER, and finally controlling the biofuel quality.

Origin of β-carotene-rich plastoglobuli in Dunaliella bardawil

The halotolerant microalgae Dunaliella bardawil accumulates under nitrogen deprivation two types of lipid droplets: plastoglobuli rich in β-carotene (βC-plastoglobuli) and cytoplasmatic lipid droplets (CLDs). We describe the isolation, composition, and origin of these lipid droplets. Plastoglobuli contain β-carotene, phytoene, and galactolipids missing in CLDs. The two preparations contain different lipid-associated proteins: major lipid droplet protein in CLD and the Prorich carotene globule protein in βC-plastoglobuli. The compositions of triglyceride (TAG) molecular species, total fatty acids, and sn-1+3 and sn-2 positions in the two lipid pools are similar, except for a small increase in palmitic acid in plastoglobuli, suggesting a common origin. The formation of CLD TAG precedes that of βC-plastoglobuli, reaching a maximum after 48 h of nitrogen deprivation and then decreasing. Palmitic acid incorporation kinetics indicated that, at early stages of nitrogen deprivation, CLD TAG is synthesized mostly from newly formed fatty acids, whereas in βC-plastoglobuli, a large part of TAG is produced from fatty acids of preformed membrane lipids. Electron microscopic analyses revealed that CLDs adhere to chloroplast envelope membranes concomitant with appearance of small βC-plastoglobuli within the chloroplast. Based on these results, we propose that CLDs in D. bardawil are produced in the endoplasmatic reticulum, whereas βC-plastoglobuli are made, in part, from hydrolysis of chloroplast membrane lipids and in part, by a continual transfer of TAG or fatty acids derived from CLD.

Nitrogen-deprivation elevates lipid levels in Symbiodinium spp. by lipid droplet accumulation: morphological and compositional analyses

Stable cnidarian-dinoflagellate (genus Symbiodinium) endosymbioses depend on the regulation of nutrient transport between Symbiodinium populations and their hosts. It has been previously shown that the host cytosol is a nitrogen-deficient environment for the intracellular Symbiodinium and may act to limit growth rates of symbionts during the symbiotic association. This study aimed to investigate the cell proliferation, as well as ultrastructural and lipid compositional changes, in free-living Symbiodinium spp. (clade B) upon nitrogen (N)-deprivation. The cell proliferation of the N-deprived cells decreased significantly. Furthermore, staining with a fluorescent probe, boron dipyrromethane 493/503 (BODIPY 493/503), indicated that lipid contents progressively accumulated in the N-deprived cells. Lipid analyses further showed that both triacylglycerol (TAG) and cholesterol ester (CE) were drastically enriched, with polyunsaturated fatty acids (PUFA; i.e., docosahexaenoic acid, heneicosapentaenoic acid, and oleic acid) became more abundant. Ultrastructural examinations showed that the increase in concentration of these lipid species was due to the accumulation of lipid droplets (LDs), a cellular feature that have previously shown to be pivotal in the maintenance of intact endosymbioses. Integrity of these stable LDs was maintained via electronegative repulsion and steric hindrance possibly provided by their surface proteins. Proteomic analyses of these LDs identified proteins putatively involved in lipid metabolism, signaling, stress response and energy metabolism. These results suggest that LDs production may be an adaptive response that enables Symbiodinium to maintain sufficient cellular energy stores for survival under the N-deprived conditions in the host cytoplasm.

Proteomics analysis of oil body-associated proteins in the oleaginous diatom

For biodiesel production from microalgae, it is desirable to understand the entire triacylglycerol (TAG) metabolism. TAG accumulation occurs in oil bodies, and although oil body-associated proteins could play important roles in TAG metabolism, only a few microalgal species have been studied by a comprehensive analysis. Diatoms are microalgae that are promising producers of biodiesel, on which such proteomics analysis has not been conducted to date. Herein, we identified oil body-associated proteins in the oleaginous diatom Fistulifera sp. strain JPCC DA0580. The oil body fraction was separated by cell disruption with beads beating and subsequent ultracentrifugation. Contaminating factors could be removed by comparing proteins from the oil body and the soluble fractions. This novel strategy successfully revealed 15 proteins as oil body-associated protein candidates. Among them, two proteins, which were parts of proteins predicted to have transmembrane domains, were indeed confirmed to specifically localize to the oil bodies in this strain by observation of GFP fusion proteins. One (predicted to be a potassium channel) was also detected from the ER, suggesting that oil bodies might originate from the ER. By utilizing this novel subtraction method, we succeeded in identifying the oil body-associated proteins in the diatom for the first time.

Integral Proteins in Plant Oil Bodies

Hydrophobic storage neutral lipids are stably preserved in specialized organelles termed oil bodies in the aqueous cytosolic compartment of plant cells via encapsulation with surfactant molecules including phospholipids and integral proteins. To date, three classes of integral proteins, termed oleosin, caleosin, and steroleosin, have been identified in oil bodies of angiosperm seeds. Proposed structures, targeting traffic routes, and biological functions of these three integral oil-body proteins were summarized and discussed. In the viewpoint of evolution, isoforms of oleosin and caleosin are found in oil bodies of pollens as well as those of more primitive species; moreover, caleosin- and steroleosin-like proteins are also present in other subcellular locations besides oil bodies. Technically, artificial oil bodies of structural stability similar to native ones were successfully constituted and seemed to serve as a useful tool for both basic research studies and biotechnological applications.