Evidence for glycoprotein transport into complex plastids

CLEM, a universal tool for analyzing structural organization in thylakoid membranes

Chlorophyll (Chl) plays a crucial role in photosynthesis, functioning as a photosensitizer. As an integral component of this process, energy absorbed by this pigment is partly emitted as red fluorescence. This signal can be readily imaged by fluorescence microscopy and provides a visualization of photosynthetic activity. However, due to limited resolution, signals cannot be assigned to specific subcellular/organellar membrane structures. By correlating fluorescence micrographs with transmission electron microscopy, researchers can identify sub-cellular compartments and membranes, enabling the monitoring of Chl distribution within thylakoid membrane substructures in cyanobacteria, algae, and higher plant single cells. Here, we describe a simple and effective protocol for correlative light-electron microscopy (CLEM) based on the autofluorescence of Chl and demonstrate its application to selected photosynthetic model organisms. Our findings illustrate the potential of this technique to identify areas of high Chl concentration and photochemical activity, such as grana regions in vascular plants, by mapping stacked thylakoids.

Thylakoid attachment to the plasma membrane in Synechocystis sp. PCC 6803 requires the AncM protein

Thylakoids are the highly specialized internal membrane systems that harbor the photosynthetic electron transport machinery in cyanobacteria and in chloroplasts. In Synechocystis sp. PCC 6803, thylakoid membranes (TMs) are arranged in peripheral sheets that occasionally converge on the plasma membrane (PM) to form thylakoid convergence membranes (TCMs). TCMs connect several thylakoid sheets and form local contact sites called thylapses between the two membrane systems, at which the early steps of photosystem II (PSII) assembly occur. The protein CurT is one of the main drivers of TCM formation known so far. Here, we identify, by whole-genome sequencing of a curT- suppressor strain, the protein anchor of convergence membranes (AncM) as a factor required for the attachment of thylakoids to the PM at thylapses. An ancM- mutant is shown to have a photosynthetic phenotype characterized by reductions in oxygen-evolution rate, PSII accumulation, and PS assembly. Moreover, the ancM- strain exhibits an altered thylakoid ultrastructure with additional sheets and TCMs detached from the PM. By combining biochemical studies with fluorescence and correlative light-electron microscopy-based approaches, we show that AncM is an integral membrane protein located in biogenic TCMs that form thylapses. These data suggest an antagonistic function of AncM and CurT in shaping TM ultrastructure.

Global Profiling of N-Glycoproteins and N-Glycans in the Diatom Phaeodactylum tricornutum

N-glycosylation is an important posttranslational modification in all eukaryotes, but little is known about the N-glycoproteins and N-glycans in microalgae. Here, N-glycoproteomic and N-glycomic approaches were used to unveil the N-glycoproteins and N-glycans in the model diatom Phaeodactylum tricornutum. In total, 863 different N-glycopeptides corresponding to 639 N-glycoproteins were identified from P. tricornutum. These N-glycoproteins participated in a variety of important metabolic pathways in P. tricornutum. Twelve proteins participating in the N-glycosylation pathway were identified as N-glycoproteins, indicating that the N-glycosylation of these proteins might be important for the protein N-glycosylation pathway. Subsequently, 69 N-glycans corresponding to 59 N-glycoproteins were identified and classified into high mannose and hybrid type N-glycans. High mannose type N-glycans contained four different classes, such as Man-5, Man-7, Man-9, and Man-10 with a terminal glucose residue. Hybrid type N-glycan harbored Man-4 with a terminal GlcNAc residue. The identification of N-glycosylation on nascent proteins expanded our understanding of this modification at a N-glycoproteomic scale, the analysis of N-glycan structures updated the N-glycan database in microalgae. The results obtained from this study facilitate the elucidation of the precise function of these N-glycoproteins and are beneficial for future designing the microalga to produce the functional humanized biopharmaceutical N-glycoproteins for the clinical therapeutics.

Proximity proteomics in a marine diatom reveals a putative cell surface-to-chloroplast iron trafficking pathway

Iron is a biochemically critical metal cofactor in enzymes involved in photosynthesis, cellular respiration, nitrate assimilation, nitrogen fixation, and reactive oxygen species defense. Marine microeukaryotes have evolved a phytotransferrin-based iron uptake system to cope with iron scarcity, a major factor limiting primary productivity in the global ocean. Diatom phytotransferrin is endocytosed; however, proteins downstream of this environmentally ubiquitous iron receptor are unknown. We applied engineered ascorbate peroxidase APEX2-based subcellular proteomics to catalog proximal proteins of phytotransferrin in the model marine diatom Phaeodactylum tricornutum. Proteins encoded by poorly characterized iron-sensitive genes were identified including three that are expressed from a chromosomal gene cluster. Two of them showed unambiguous colocalization with phytotransferrin adjacent to the chloroplast. Further phylogenetic, domain, and biochemical analyses suggest their involvement in intracellular iron processing. Proximity proteomics holds enormous potential to glean new insights into iron acquisition pathways and beyond in these evolutionarily, ecologically, and biotechnologically important microalgae.

N-linked glycosylation enzymes in the diatom Thalassiosira oceanica exhibit a diel cycle in transcript abundance and favor for NXT-type sites

N-linked glycosylation is a posttranslational modification affecting protein folding and function. The N-linked glycosylation pathway in algae is poorly characterized, and further knowledge is needed to understand the cell biology of algae and the evolution of N-linked glycosylation. This study investigated the N-linked glycosylation pathway in Thalassiosira oceanica, an open ocean diatom adapted to survive at growth-limiting iron concentrations. Here we identified and annotated the genes coding for the essential enzymes involved in the N-linked glycosylation pathway of T. oceanica. Transcript levels for genes coding for calreticulin, oligosaccharyltransferase (OST), N-acetylglucosaminyltransferase (GnT1), and UDP-glucose glucosyltransferase (UGGT) under high- and low-iron growth conditions revealed diel transcription patterns with a significant decrease of calreticulin and OST transcripts under iron-limitation. Solid-phase extraction of N-linked glycosylated peptides (SPEG) revealed 118 N-linked glycosylated peptides from cells grown in high- and low-iron growth conditions. The identified peptides had 81% NXT-type motifs, with X being any amino acids except proline. The presence of N-linked glycosylation sites in the iron starvation-induced protein 1a (ISIP1a) confirmed its predicted topology, contributing to the biochemical characterization of ISIP1 proteins. Analysis of extensive oceanic gene databases showed a global distribution of calreticulin, OST, and UGGT, reinforcing the importance of glycosylation in microalgae.

2D and 3D immunogold localization on (epoxy) ultrathin sections with and without osmium tetroxide

For nearly 50 years immunogold labeling on ultrathin sections has been successfully used for protein localization in laboratories worldwide. In theory and in practice, this method has undergone continual improvement over time. In this study, we carefully analyzed circulating protocols for postembedding labeling to find out if they are still valid under modern laboratory conditions, and in addition, we tested unconventional protocols. For this, we investigated immunolabeling of Epon-embedded cells, immunolabeling of cells treated with osmium, and the binding behavior of differently sized gold particles. Here we show that (in contrast to widespread belief) immunolabeling of Epon-embedded cells and of cells treated with osmium tetroxide is actually working. Furthermore, we established a "speed protocol" for immunolabeling by reducing antibody incubation times. Finally, we present our results on three-dimensional immunogold labeling.

Metabolic Innovations Underpinning the Origin and Diversification of the Diatom Chloroplast

Of all the eukaryotic algal groups, diatoms make the most substantial contributions to photosynthesis in the contemporary ocean. Understanding the biological innovations that have occurred in the diatom chloroplast may provide us with explanations to the ecological success of this lineage and clues as to how best to exploit the biology of these organisms for biotechnology. In this paper, we use multi-species transcriptome datasets to compare chloroplast metabolism pathways in diatoms to other algal lineages. We identify possible diatom-specific innovations in chloroplast metabolism, including the completion of tocopherol synthesis via a chloroplast-targeted tocopherol cyclase, a complete chloroplast ornithine cycle, and chloroplast-targeted proteins involved in iron acquisition and CO₂ concentration not shared between diatoms and their closest relatives in the stramenopiles. We additionally present a detailed investigation of the chloroplast metabolism of the oil-producing diatom Fistulifera solaris, which is of industrial interest for biofuel production. These include modified amino acid and pyruvate hub metabolism that might enhance acetyl-coA production for chloroplast lipid biosynthesis and the presence of a chloroplast-localised squalene synthesis pathway unknown in other diatoms. Our data provides valuable insights into the biological adaptations underpinning an ecologically critical lineage, and how chloroplast metabolism can change even at a species level in extant algae.

Dynamic N-glycoproteome analysis of maize seedling leaves during de-etiolation using Concanavalin A lectin affinity chromatography and a nano-LC-MS/MS-based iTRAQ approach

The identification of N -glycosylated proteins with information about changes in the level of N -glycosylation during de-etiolation provides a database that will aid further research on plant N -glycosylation and de-etiolation. N-glycosylation is one of the most prominent and abundant protein post-translational modifications in all eukaryotes and in plants it plays important roles in development, stress tolerance and immune responses. Because light-induced de-etiolation is one of the most dramatic developmental processes known in plants, seedlings undergoing de-etiolation are an excellent model for investigating dynamic proteomic profiles. Here, we present a comprehensive, quantitative N-glycoproteomic profile of maize seedlings undergoing 12 h of de-etiolation obtained using Concanavalin A (Con A) lectin affinity chromatography enrichment coupled with a nano-LC-MS/MS-based iTRAQ approach. In total, 1084 unique N-glycopeptides carrying 909 N-glycosylation sites and corresponding to 609 proteins were identified and quantified, including 186 N-glycosylation sites from 162 proteins that were significantly regulated over the course of the 12 h de-etiolation period. Based on hierarchical clustering analysis, the significantly regulated N-glycopeptides were divided into seven clusters that showed different N-glycosylation patterns during de-etiolation. We found no obvious difference in the enriched MapMan bincode categories for each cluster, and these clustered significantly regulated N-glycoproteins (SRNPs) are enriched in miscellaneous, protein, cell wall and signaling, indicating that although the N-glycosylation regulation patterns of these SRNPs might differ, they are involved in similar biological processes. Overall, this study represents the first large-scale quantitative N-glycoproteome of the model C4 plant, maize, which is one of the most important cereal and biofuel crops. Our results greatly expand the maize N-glycoproteomic database and also shed light on the potential roles of N-glycosylation modification during the greening of maize leaves.

Review: origin of complex algae by secondary endosymbiosis: a journey through time

Secondary endosymbiosis-the merging of two eukaryotic cells into one photosynthetic cellular unit-led to the evolution of ecologically and medically very important organisms. We review the biology of these organisms, starting from the first proposal of secondary endosymbiosis up to recent phylogenetic models on the origin of secondarily evolved protists. In addition, we discuss the organelle character of the symbionts based on morphological features, gene transfers from the symbiont into the host and re-import of nucleus-encoded plastid proteins. Finally, we hypothesize that secondary endosymbiosis is more than enslaving a eukaryotic, phototrophic cell, but reflects a complex interplay between host and symbiont, leading to the inseparability of the two symbiotic partners generating a cellular entity.

The Central Vacuole of the Diatom Phaeodactylum tricornutum: Identification of New Vacuolar Membrane Proteins and of a Functional Di-leucine-based Targeting Motif

Diatoms are unicellular organisms evolved by secondary endosymbiosis. Although studied in many aspects, the functions of vacuolar-like structures of these organisms are rarely investigated. One of these structures is a dominant central vacuole-like compartment with a marbled phenotype, which is supposed to represent a chrysolaminarin-storing and carbohydrate mobilization compartment. However, other functions as well as targeting of proteins to this compartment are not shown experimentally. In order to study trafficking of membrane proteins to the vacuolar membrane, we scanned the genome for intrinsic vacuolar membrane proteins and used one representative for targeting studies. Our work led to the identification of several proteins located in the vacuolar membrane as well as the sub-compartmentalized localization of one protein. In addition, we show that a di-leucine-based motif is an important signal for correct targeting to the central vacuole of diatoms, like it is in plants.

Protein Targeting to the Plastid of Euglena

The lateral transfer of photosynthesis between kingdoms through endosymbiosis is among the most spectacular examples of evolutionary innovation. Euglena, which acquired a chloroplast indirectly through an endosymbiosis with a green alga, represents such an example. As with other endosymbiont-derived plastids from eukaryotes, there are additional membranes that surround the organelle, of which Euglena has three. Thus, photosynthetic genes that were transferred from the endosymbiont to the host nucleus and whose proteins are required in the new plastid, are now faced with targeting and plastid import challenges. Early immunoelectron microscopy data suggested that the light-harvesting complexes, photosynthetic proteins in the thylakoid membrane, are post-translationally targeted to the plastid via the Golgi apparatus, an unexpected discovery at the time. Proteins targeted to the Euglena plastid have complex, bipartite presequences that direct them into the endomembrane system, through the Golgi apparatus and ultimately on to the plastid, presumably via transport vesicles. From transcriptome sequencing, dozens of plastid-targeted proteins were identified, leading to the identification of two different presequence structures. Both have an amino terminal signal peptide followed by a transit peptide for plastid import, but only one of the two classes of presequences has a third domain-the stop transfer sequence. This discovery implied two different transport mechanisms; one where the protein was fully inserted into the lumen of the ER and another where the protein remains attached to, but effectively outside, the endomembrane system. In this review, we will discuss the biochemical and bioinformatic evidence for plastid targeting, discuss the evolution of the targeting system, and ultimately provide a working model for the targeting and import of proteins into the plastid of Euglena.

Fabrication of Surface Protein-Imprinted Biofuel Cell for Sensitive Self-Powered Glycoprotein Detection

Glycoproteins are important biomarkers and therapeutic targets in clinical diagnostics. The conventional analytical methods for glycoprotein are usually faced with some challenges, such as the complex pretreatment of samples, poor availability, and limited stability of antibody, making them not suitable for point-of-care and on-site application. Herein, we demonstrate a novel miniaturized biofuel cells (BFCs)-based self-powered nanosensor for the specific and sensitive determination of glycoproteins in complex samples through the combination of boronate-affinity molecularly imprinted polymer (MIP) and the boronate affinity functionalized biliroxidase-carbon nanotube nanocomposites. The above MIP and the nanocomposites act as both signal probe and biocatalyst at the cathode. The as-obtained self-powered MIP-BFC-based biosensor can detect horseradish peroxidase (a type of glycoprotein) with a wide linear range of 1 ng/mL to 10 μg/mL and a very low detection limit of 1 ng/mL. Especially, it shows high tolerance for different interferences (e.g., sugars and other glycoproteins) and can even measure the α-fetoprotein level in serum samples. Moreover, it exhibits significant advantages over the conventional assays in terms of cost efficiency, stability, and speed, especially inexpensive instrument needed. Our novel approach for construction of the sensor paves a simple and economical way to fabricate portable devices for point-of-care and on-site application.

Microglomerular Synaptic Complexes in the Sky-Compass Network of the Honeybee Connect Parallel Pathways from the Anterior Optic Tubercle to the Central Complex

While the ability of honeybees to navigate relying on sky-compass information has been investigated in a large number of behavioral studies, the underlying neuronal system has so far received less attention. The sky-compass pathway has recently been described from its input region, the dorsal rim area (DRA) of the compound eye, to the anterior optic tubercle (AOTU). The aim of this study is to reveal the connection from the AOTU to the central complex (CX). For this purpose, we investigated the anatomy of large microglomerular synaptic complexes in the medial and lateral bulbs (MBUs/LBUs) of the lateral complex (LX). The synaptic complexes are formed by tubercle-lateral accessory lobe neuron 1 (TuLAL1) neurons of the AOTU and GABAergic tangential neurons of the central body’s (CB) lower division (TL neurons). Both TuLAL1 and TL neurons strongly resemble neurons forming these complexes in other insect species. We further investigated the ultrastructure of these synaptic complexes using transmission electron microscopy. We found that single large presynaptic terminals of TuLAL1 neurons enclose many small profiles (SPs) of TL neurons. The synaptic connections between these neurons are established by two types of synapses: divergent dyads and divergent tetrads. Our data support the assumption that these complexes are a highly conserved feature in the insect brain and play an important role in reliable signal transmission within the sky-compass pathway.

Phylogenomic analysis supports the ancestral presence of LPS-outer membranes in the Firmicutes

One of the major unanswered questions in evolutionary biology is when and how the transition between diderm (two membranes) and monoderm (one membrane) cell envelopes occurred in Bacteria. The Negativicutes and the Halanaerobiales belong to the classically monoderm Firmicutes, but possess outer membranes with lipopolysaccharide (LPS-OM). Here, we show that they form two phylogenetically distinct lineages, each close to different monoderm relatives. In contrast, their core LPS biosynthesis enzymes were inherited vertically, as in the majority of bacterial phyla. Finally, annotation of key OM systems in the Halanaerobiales and the Negativicutes shows a puzzling combination of monoderm and diderm features. Together, these results support the hypothesis that the LPS-OMs of Negativicutes and Halanaerobiales are remnants of an ancient diderm cell envelope that was present in the ancestor of the Firmicutes, and that the monoderm phenotype in this phylum is a derived character that arose multiple times independently through OM loss.

DOI:http://dx.doi.org/10.7554/eLife.14589.001

The cell envelope is one of the evolutionarily oldest parts of a bacterium. This structure – made up of a cell wall and either one or two cell membranes – surrounds the bacterial cell, maintaining the cell’s structure and providing an interface through which bacteria can sense their environment and communicate.

Bacteria can be broadly classed based on the number of cell membranes that their envelope consists of. Bacteria that have a single cell membrane are known as “monoderm”, whereas those with two membranes are termed “diderm”. The number of membranes that bacteria have can affect how well they resist antibacterial compounds. When, how and why bacteria switched between monoderm and diderm cell envelopes are some of the major unanswered questions in evolutionary biology.

The textbook example of a monoderm cell envelope can be found in bacteria called Firmicutes. This group includes some notoriously harmful bacteria such as Staphylococcus, which can cause conditions ranging from abscesses to pneumonia. However, some Firmicutes possess two cell membranes. It was unclear how these unusual diderm Firmicutes developed a second membrane, and how they are related to their monoderm relatives.

Antunes, Poppleton et al. set out to answer these questions by analyzing the information contained in the thousands of bacterial genomes that have already been described. The results indicate that Firmicutes originally had diderm envelopes, and that species with monoderm envelopes arose independently several times through the loss of their outermost membrane.

Future work is needed to investigate the driving forces and the precise mechanism that led most Firmicutes to lose their outer membrane. Also, further characterization of diderm Firmicutes will provide key information about the biology of these poorly understood bacteria.

DOI:http://dx.doi.org/10.7554/eLife.14589.002

Protein-protein interactions indicate composition of a 480 kDa SELMA complex in the second outermost membrane of diatom complex plastids

Most secondary plastids of red algal origin are surrounded by four membranes and nucleus-encoded plastid proteins have to traverse these barriers. Translocation across the second outermost plastid membrane, the periplastidal membrane (PPM), is facilitated by a ERAD-(ER-associated degradation) derived machinery termed SELMA (symbiont-specific ERAD-like machinery). In the last years, important subunits of this translocator have been identified, which clearly imply compositional similarities between SELMA and ERAD. Here we investigated, via protein-protein interaction studies, if the composition of SELMA is comparable to the known ERAD complex. As a result, our data suggest that the membrane proteins of SELMA, the derlin proteins, are linked to the soluble sCdc48 complex via the UBX protein sUBX. This is similar to the ERAD machinery whereas the additional SELMA components, sPUB und a second Cdc48 copy might indicate the influence of functional constraints in developing a translocation machinery from ERAD-related factors. In addition, we show for the first time that a rhomboid protease is a central interaction partner of the membrane proteins of the SELMA system in complex plastids.

Review/N-glycans: The making of a varied toolbox

Asparagine (N)-linked protein glycosylation is one of the most crucial, prevalent, and complex co- and post-translational protein modifications. It plays a pivotal role in protein folding, quality control, and endoplasmic reticulum (ER)-associated degradation (ERAD) as well as in protein sorting, protein function, and in signal transduction. Furthermore, glycosylation modulates many important biological processes including growth, development, morphogenesis, and stress signaling processes. As a consequence, aberrant or altered N-glycosylation is often associated with reduced fitness, diseases, and disorders. The initial steps of N-glycan synthesis at the cytosolic side of the ER membrane and in the lumen of the ER are highly conserved. In contrast, the final N-glycan processing in the Golgi apparatus is organism-specific giving rise to a wide variety of carbohydrate structures. Despite our vast knowledge on N-glycans in yeast and mammals, the modus operandi of N-glycan signaling in plants is still largely unknown. This review will elaborate on the N-glycosylation biosynthesis pathway in plants but will also critically assess how N-glycans are involved in different signaling cascades, either active during normal development or upon abiotic and biotic stresses.

Protein import into complex plastids: Cellular organization of higher complexity

Many protists with high ecological and medical relevance harbor plastids surrounded by four membranes. Thus, nucleus-encoded proteins of these complex plastids have to traverse these barriers. Here we report on the identification of the protein translocators located in two of the plastid surrounding membranes and present recent findings on the mechanisms of protein import into the plastids of diatoms.

N-terminal lysines are essential for protein translocation via a modified ERAD system in complex plastids

Nuclear-encoded pre-proteins being imported into complex plastids of red algal origin have to cross up to five membranes. Thereby, transport across the second outermost or periplastidal membrane (PPM) is facilitated by SELMA (symbiont-specific ERAD-like machinery), an endoplasmic reticulum-associated degradation (ERAD)-derived machinery. Core components of SELMA are enzymes involved in ubiquitination (E1-E3), a Cdc48 ATPase complex and Derlin proteins. These components are present in all investigated organisms with four membrane-bound complex plastids of red algal origin, suggesting a ubiquitin-dependent translocation process of substrates mechanistically similar to the process of retro-translocation in ERAD. Even if, according to the current model, translocation via SELMA does not end up in the classical poly-ubiquitination, transient mono-/oligo-ubiquitination of pre-proteins might be required for the mechanism of translocation. We investigated the import mechanism of SELMA and were able to show that protein transport across the PPM depends on lysines in the N-terminal but not in the C-terminal part of pre-proteins. These lysines are predicted to be targets of ubiquitination during the translocation process. As proteins lacking the N-terminal lysines get stuck in the PPM, a 'frozen intermediate' of the translocation process could be envisioned and initially characterized.

New mode of energy metabolism in the seventh order of methanogens as revealed by comparative genome analysis of “Candidatus methanoplasma termitum”

The recently discovered seventh order of methanogens, the Methanomassiliicoccales (previously referred to as “Methanoplasmatales”), so far consists exclusively of obligately hydrogen-dependent methylotrophs. We sequenced the complete genome of “Candidatus Methanoplasma termitum” from a highly enriched culture obtained from the intestinal tract of termites and compared it with the previously published genomes of three other strains from the human gut, including the first isolate of the order. Like all other strains, “Ca. Methanoplasma termitum” lacks the entire pathway for CO2 reduction to methyl coenzyme Mand produces methane by hydrogen-dependent reduction of methanol or methylamines, which is consistent with additional physiological data. However, the shared absence of cytochromes and an energy-converting hydrogenase for the reoxidation of the ferredoxin produced by the soluble heterodisulfide reductase indicates that Methanomassiliicoccales employ a new mode of energy metabolism, which differs from that proposed for the obligately methylotrophic Methanosphaera stadtmanae. Instead, all strains possess a novel complex that is related to the F420:methanophenazine oxidoreductase (Fpo) of Methanosarcinales butlacks an F420-oxidizing module, resembling the apparently ferredoxin-dependent Fpo-like homolog in Methanosaeta thermophila. Since all Methanomassiliicoccales also lack the subunit E of the membrane-bound heterodisulfide reductase (HdrDE), wepropose that the Fpo-like complex interacts directly with subunit D, forming an energy-converting ferredoxin: heterodisulfideoxidoreductase. The dual function of heterodisulfide in Methanomassiliicoccales, which serves both in electron bifurcation and as terminal acceptor in a membrane-associated redox process, may be a unique characteristic of the novel order.

Biogenesis of thylakoid membranes

Thylakoids mediate photosynthetic electron transfer and represent one of the most elaborate energy-transducing membrane systems. Despite our detailed knowledge of its structure and function, much remains to be learned about how the machinery is put together. The concerted synthesis and assembly of lipids, proteins and low-molecular-weight cofactors like pigments and transition metal ions require a high level of spatiotemporal coordination. While increasing numbers of assembly factors are being functionally characterized, the principles that govern how thylakoid membrane maturation is organized in space are just starting to emerge. In both cyanobacteria and chloroplasts, distinct production lines for the fabrication of photosynthetic complexes, in particular photosystem II, have been identified. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Chloroplast-targeting protein expression in the oleaginous diatom Fistulifera solaris JPCC DA0580 toward metabolic engineering

The chloroplast plays critical roles in lipid metabolism of microalgae, thus it is recognized as an attractive target of metabolic engineering to enhance biofuel production. It has been well known that recombinant protein expression in microalgal chloroplasts needs specific signal sequence which governs the transition manner of nuclear-encoded polypeptides within the subcellular compartments. However certain microalgae, including diatoms, have complex membrane systems surrounding the chloroplast, and thus chloroplast-targeting protein expression with the signal sequence has rarely been demonstrated except for a few model non-oleaginous diatoms. In this study, we performed recombinant green fluorescence protein (GFP) expression and transportation into the chloroplast of the oleaginous marine diatom, Fistulifera solaris JPCC DA0580. The signal sequence of ATP synthetase gamma subunit, which was predicted to localize in the chloroplast according to a bioinformatics analysis pipeline, was employed as a key factor of this technique. As a result, specific localization of GFP in the chloroplast was observed. It would be useful to engineer the lipid synthesis pathways existing in the chloroplast. Furthermore, intensive gathering of GFP in the rod-like structure was also detected, which has not been observed in model diatom studies. As comparing with electron microscopic observation, the structure was estimated to be a pyrenoid.

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.

Glycoprotein import: a common feature of complex plastids?

Complex plastids evolved by secondary endosymbiosis and are, in contrast to primary plastids, surrounded by 3 or 4 envelope membranes. Recently, we provided evidence that in diatoms proteins exist that get N-glycosylated during transport across the outermost membrane of the complex plastid. This gives rise to unique questions on the transport mechanisms of these bulky proteins, which get transported across up to 3 further membranes into the plastid stroma. Here we discuss our results in an evolutionary context and speculate about the existence of plastidal glycoproteins in other organisms with complex plastids.