Protein sorting to the storage vacuoles of plants: a critical appraisal

Storage globulins pass through the Golgi apparatus and multivesicular bodies in the absence of dense vesicle formation during early stages of cotyledon development in mung bean

During seed development and maturation, large amounts of storage proteins are synthesized and deposited in protein storage vacuoles (PSVs). Multiple mechanisms have been proposed to be responsible for transporting storage proteins to PSVs in developing seeds. In this study, a specific antibody was raised against the mung bean (Vigna radiata) seed storage protein 8S globulin and its deposition was followed via immunogold electron microscopy in developing mung bean cotyledons. It is demonstrated that non-aggregated 8S globulins are present in multivesicular bodies (MVBs) in early stages of cotyledon development where neither dense vesicles (DVs) nor a PSV were recognizable. However, at later stages of cotyledon development, condensed globulins were visible in both DVs and distinct MVBs with a novel form of partitioning, with the internal vesicles being pushed to one sector of this organelle. These distinct MVBs were no longer sensitive to wortmannin. This study thus indicates a possible role for MVBs in transporting storage proteins to PSVs during the early stage of seed development prior to the involvement of DVs. In addition, wortmannin treatment is shown to induce DVs to form aggregates and to fuse with the plasma membrane.

Plant and yeast NHX antiporters: roles in membrane trafficking

The plant NHX gene family encodes Na(+)/H(+) antiporters which are crucial for salt tolerance, potassium homeostasis and cellular pH regulation. Understanding the role of NHX antiporters in membrane trafficking is becoming an increasingly interesting subject of study. Membrane trafficking is a central cellular process during which proteins, lipids and polysaccharides are continuously exchanged among membrane compartments. Yeast ScNhx1p, a prevacuole/ vacuolar Na(+)/H(+) antiporter, plays an important role in regulating pH to control trafficking out of the endosome. Evidence begins to accumulate that plant NHX antiporters might function in regulating membrane trafficking in plants.

Subcellular targeting and biosynthesis of cyclotides in plant cells

PREMISE OF THE STUDY

The cyclotide kalata B1 is found in the leaves of Oldenlandia affinis and is a potent insecticidal and nematocidal molecule. This peptide is cleaved from a precursor protein, Oak1, and ligation of the N- and C-termini occurs to form a continuous peptide backbone. The subcellular location of the excision and cyclization reactions is unknown, and there is debate as to which enzyme catalyzes the event. To determine where in the plant cell Oak1 is processed, we prepared constructs encoding GFP (green fluorescent protein) linked to the cyclotide precursor Oak1.

METHODS

The GFP constructs were transiently expressed in the leaves of Nicotiana benthamiana, and GFP fluorescence was observed in living cells using confocal microscopy. A Fei Mao (FM) styryl dye was infiltrated into whole leaves that were still growing and expressing GFP constructs, enabling the plasma membrane and the tonoplast to be highlighted for visualization of the vacuole in living cells.

KEY RESULTS

The full length Oak1 precursor directed GFP to the vacuole, suggesting that excision and cyclization of the cyclotide domain occurs in the vacuole where the cyclotides are then stored. The N-terminal propeptide and N-terminal repeat of Oak1 were both sufficient to target GFP to the vacuole, although the C-terminal propeptide, which is essential for cyclization, was not a targeting signal.

CONCLUSIONS

The vacuolar location of cyclotides supports our hypothesis that the vacuolar processing enzyme, asparaginyl endoproteinase, has a pivotal role in excision and cyclization from cyclotide precursors.

Expression of functional recombinant human growth hormone in transgenic soybean seeds

We produced human growth hormone (hGH), a protein that stimulates growth and cell reproduction, in genetically engineered soybean [Glycine max (L.) Merrill] seeds. Utilising the alpha prime (α') subunit of β-conglycinin tissue-specific promoter from soybean and the α-Coixin signal peptide from Coix lacryma-jobi, we obtained transgenic soybean lines that expressed the mature form of hGH in their seeds. Expression levels of bioactive hGH up to 2.9% of the total soluble seed protein content (corresponding to approximately 9 g kg(-1)) were measured in mature dry soybean seeds. The results of ultrastructural immunocytochemistry assays indicated that the recombinant hGH in seed cotyledonary cells was efficiently directed to protein storage vacuoles. Specific bioassays demonstrated that the hGH expressed in the soybean seeds was fully active. The recombinant hGH protein sequence was confirmed by mass spectrometry characterisation. These results demonstrate that the utilisation of tissue-specific regulatory sequences is an attractive and viable option for achieving high-yield production of recombinant proteins in stable transgenic soybean seeds.

Expression of antibody fragments with a controlled N-glycosylation pattern and induction of endoplasmic reticulum-derived vesicles in seeds of Arabidopsis

Intracellular trafficking and subcellular deposition are critical factors influencing the accumulation and posttranslational modifications of proteins. In seeds, these processes are not yet fully understood. In this study, we set out to investigate the intracellular transport, final destination, N-glycosylation status, and stability of the fusion of recombinant single-chain variable fragments to the crystallizing fragment of an antibody (scFv-Fc) of two antiviral monoclonal antibodies (2G12 and HA78). The scFv-Fcs were expressed in Arabidopsis (Arabidopsis thaliana) seeds and leaves both as secretory molecules and tagged with an endoplasmic reticulum (ER) retention signal. We demonstrate differential proteolytic degradation of scFv-Fcs in leaves versus seeds, with higher degradation in the latter organ. In seeds, we show that secretory versions of HA78 scFv-Fcs are targeted to the extracellular space but are deposited in newly formed ER-derived vesicles upon KDEL tagging. These results are in accordance with the obtained N-glycosylation profiles: complex-type and ER-typical oligomannosidic N-glycans, respectively. HA78 scFv-Fcs, expressed in seeds of an Arabidopsis glycosylation mutant lacking plant-specific N-glycans, exhibit custom-made human-type N-glycosylation. In contrast, 2G12 scFv-Fcs carry exclusively ER-typical oligomannosidic N-glycans and were deposited in newly formed ER-derived vesicles irrespective of the targeting signals. HA78 scFv-Fcs exhibited efficient virus neutralization activity, while 2G12 scFv-Fcs were inactive. We demonstrate the efficient generation of scFv-Fcs with a controlled N-glycosylation pattern. However, our results also reveal aberrant subcellular deposition and, as a consequence, unexpected N-glycosylation profiles. Our attempts to elucidate intracellular protein transport in seeds contributes to a better understanding of this basic cell biological mechanism and is a step toward the versatile use of Arabidopsis seeds as an alternative expression platform for pharmaceutically relevant proteins.

Sub-compartmental organization of Golgi-resident N-glycan processing enzymes in plants

In all eukaryotes, the Golgi apparatus is the main site of protein glycosylation. It is widely accepted that the glycosidases and glycosyltransferases involved in N-glycan processing are found concentrated within the Golgi stack where they provide their function. This means that enzymes catalyzing early steps in the processing pathway are located mainly at the cis-side, whereas late-acting enzymes mostly locate to the trans-side of the stacks, creating a non-uniform distribution along the cis-trans axis of the Golgi. There is compelling evidence that the information for their sorting to specific Golgi cisternae depends on signals encoded in the proteins themselves as well as on the trafficking machinery that recognizes these signals and it is believed that cisternal sub-compartmentalization is achieved and maintained by a combination of retention and retrieval mechanisms. Yet, the signals, mechanism(s), and molecular factors involved are still unknown. Here, we address recent findings and summarize the current understanding of this fundamental process in plant cell biology.

Production of monoclonal antibodies with a controlled N-glycosylation pattern in seeds of Arabidopsis thaliana

Seed-specific expression is an appealing alternative technology for the production of recombinant proteins in transgenic plants. Whereas attractive yields of recombinant proteins have been achieved by this method, little attention has been paid to the intracellular deposition and the quality of such products. Here, we demonstrate a comparative study of two antiviral monoclonal antibodies (mAbs) (HA78 against Hepatitis A virus; 2G12 against HIV) expressed in seeds of Arabidopsis wild-type (wt) plants and glycosylation mutants lacking plant specific N-glycan residues. We demonstrate that 2G12 is produced with complex N-glycans at great uniformity in the wt as well as in the glycosylation mutant, carrying a single dominant glycosylation species, GnGnXF and GnGn, respectively. HA78 in contrast, contains additionally to complex N-glycans significant amounts of oligo-mannosidic structures, which are typical for endoplasmic reticulum (ER)-retained proteins. A detailed subcellular localization study demonstrated the deposition of both antibodies virtually exclusively in the extracellular space, illustrating their efficient secretion. In addition, although a KDEL-tagged version of 2G12 exhibited an ER-typical N-glycosylation pattern, it was surprisingly detected in protein storage vacuoles. The different antibody variants showed different levels of degradation with hardly any degradation products detectable for HA78 carrying GnGnXF glycans. Finally, we demonstrate functional integrity of the HA78 and 2G12 glycoforms using viral inhibition assays. Our data therefore demonstrate the usability of transgenic seeds for the generation of mAbs with a controlled N-glycosylation pattern, thus expanding the possibilities for the production of optimally glycosylated proteins with enhanced biological activities for the use as human therapeutics.

Membrane traffic and fusion at post-Golgi compartments

Complete sequencing of the Arabidopsis genome a decade ago has facilitated the functional analysis of various biological processes including membrane traffic by which many proteins are delivered to their sites of action and turnover. In particular, membrane traffic between post-Golgi compartments plays an important role in cell signaling, taking care of receptor-ligand interaction and inactivation, which requires secretion, endocytosis, and recycling or targeting to the vacuole for degradation. Here, we discuss recent studies that address the identity of post-Golgi compartments, the machinery involved in traffic and fusion or functionally characterized cargo proteins that are delivered to or pass through post-Golgi compartments. We also provide an outlook on future challenges in this area of research.

MAG4/Atp115 is a golgi-localized tethering factor that mediates efficient anterograde transport in Arabidopsis

Seed storage proteins are synthesized on rough endoplasmic reticulum (ER) in a precursor form and then are transported to protein storage vacuoles (PSVs) where they are converted to their mature form. To understand the mechanisms by which storage proteins are transported, we screened Arabidopsis maigo mutants to identify those that abnormally accumulate storage protein precursors. Here we describe a new maigo mutant, maigo 4 (mag4), that abnormally accumulates the precursors of two major storage proteins, 12S globulin and 2S albumin, in dry seeds. Electron microscopy revealed that mag4 seed cells abnormally develop a large number of novel structures that exhibit a highly electron-dense core. Some of these structures were surrounded by ribosomes. Immunogold analysis suggests that the electron-dense core is an aggregate of 2S albumin precursors and that 12S globulins are localized around the core. The MAG4 gene was identified as At3g27530, and the MAG4 protein has domains homologous to those found in bovine vesicular transport factor p115. MAG4 molecules were concentrated at cis-Golgi stacks. Our findings suggest that MAG4 functions in the transport of storage protein precursors from the ER to the Golgi complex in plants. In addition, the mag4 mutant exhibits a dwarf phenotype, suggesting that MAG4 is involved in both the transport of storage proteins and in plant growth and development.

Tonoplast intrinsic proteins and vacuolar identity

TIPs (tonoplast intrinsic proteins) have been traditionally used as markers for vacuolar identity in a variety of plant species and tissues. In the present article, we review recent attempts to compile a detailed map of TIP expression in Arabidopsis, in order to understand vacuolar identity and distribution in this model species. We discuss the general applicability of these findings. We also review the issue of the intracellular targeting of TIPs and propose key emerging questions relative to the cell biology of this protein family.

Seed-based expression systems for plant molecular farming

The evolution of the seed system provides enormous adaptability to the gymnosperms and angiosperms, because of the properties of dormancy, nutrient storage and seedling vigour. Many of the unique properties of seeds can be exploited in molecular farming applications, particularly where it is desirable to produce large quantities of a recombinant protein. Seeds of transgenic plants have been widely used to generate a raw material for the extraction and isolation of proteins and polypeptides, which can be processed into valuable biopharmaceuticals. The factors that control high-level accumulation of recombinant proteins in seed are reviewed in the following paragraphs. These include promoters and enhancers, which regulate transcript abundance. However, it is shown that subcellular trafficking and targeting of the desired polypeptides or proteins play a crucial role in their accumulation at economically useful levels. Seeds have proven to be versatile hosts for recombinant proteins of all types, including peptides or short and long polypeptides as well as complex, noncontiguous proteins like antibodies and other immunoglobulins. The extraction and recovery of recombinant proteins from seeds is greatly assisted by their dormancy properties, because this allows for long-term stability of stored products including recombinant proteins and a decoupling of processing from the growth and harvest cycles. Furthermore, the low water content and relatively low bioload of seeds help greatly in designing cost-effective manufacturing processes for the desired active pharmaceutical ingredient. The development of cGMP processes based on seed-derived materials has only been attempted by a few groups to date, but we provide a review of the key issues and criteria based on interactions with Food and Drug Administration and European Medicines Agency. This article uses 'case studies' to highlight the utility of seeds as vehicles for pharmaceutical production including: insulin, human growth hormone, lysozyme and lactoferrin. These examples serve to illustrate the preclinical and, in one case, clinical information required to move these plant-derived molecules through the research phase and into the regulatory pathway en route to eventual approval.

Sorting of plant vacuolar proteins is initiated in the ER

Transport of soluble cargo molecules to the lytic vacuole of plants requires vacuolar sorting receptors (VSRs) to divert transport of vacuolar cargo from the default secretory route to the cell surface. Just as important is the trafficking of the VSRs themselves, a process that encompasses anterograde transport of receptor-ligand complexes from a donor compartment, dissociation of these complexes upon arrival at the target compartment, and recycling of the receptor back to the donor compartment for a further round of ligand transport. We have previously shown that retromer-mediated recycling of the plant VSR BP80 starts at the trans-Golgi network (TGN). Here we demonstrate that inhibition of retromer function by either RNAi knockdown of sorting nexins (SNXs) or co-expression of mutants of SNX1/2a specifically inhibits the ER export of VSRs as well as soluble vacuolar cargo molecules, but does not influence cargo molecules destined for the COPII-mediated transport route. Retention of soluble cargo despite ongoing COPII-mediated bulk flow can only be explained by an interaction with membrane-bound proteins. Therefore, we examined whether VSRs are capable of binding their ligands in the lumen of the ER by expressing ER-anchored VSR derivatives. These experiments resulted in drastic accumulation of soluble vacuolar cargo molecules in the ER. This demonstrates that the ER, rather than the TGN, is the location of the initial VSR-ligand interaction. It also implies that the retromer-mediated recycling route for the VSRs leads from the TGN back to the ER.

Auxin-mediated ribosomal biogenesis regulates vacuolar trafficking in Arabidopsis

In plants, the mechanisms that regulate the transit of vacuolar soluble proteins containing C-terminal and N-terminal vacuolar sorting determinants (VSDs) to the vacuole are largely unknown. In a screen for Arabidopsis thaliana mutants affected in the trafficking of C-terminal VSD containing proteins, we isolated the ribosomal biogenesis mutant rpl4a characterized by its partial secretion of vacuolar targeted proteins and a plethora of developmental phenotypes derived from its aberrant auxin responses. In this study, we show that ribosomal biogenesis can be directly regulated by auxins and that the exogenous application of auxins to wild-type plants results in vacuolar trafficking defects similar to those observed in rpl4a mutants. We propose that the influence of auxin on ribosomal biogenesis acts as a regulatory mechanism for auxin-mediated developmental processes, and we demonstrate the involvement of this regulatory mechanism in the sorting of vacuolar targeted proteins in Arabidopsis.

Expression and subcellular targeting of human insulin-like growth factor binding protein-3 in transgenic tobacco plants

Human insulin-like growth factor binding protein-3 (hIGFBP-3) is a multifunctional protein which has high affinity for insulin-like growth factor-I (IGF-I). It combines with IGF-I to form a tertiary complex in circulation, thus regulating the activity of IGF-I. Furthermore, recombinant hIGFBP-3 (rhIGFBP-3) has been found to negatively regulate cell proliferation and induce apoptosis. In this study, we have established an efficient plant bioreactor platform for mass production of rhIGFBP-3. Different expression constructs, driven by the seed-specific phaseolin promoter, were designed and transformed into tobacco plant via Agrobacterium. To enhance protein expression level, the signal peptide (SP) and the C-terminal tetrapeptide AFVY of phaseolin were used to direct rhIGFBP-3 to protein storage vacuole (PSV) in tobacco seed for stable accumulation. Western blot analysis showed that rhIGFBP-3 was successfully synthesized in transgenic tobacco seeds, with the highest protein expression of 800 mug/g dry weight. The localization of rhIGFBP-3 in PSV was also evident by confocal immunofluorescence microscopy. Our results indicated that protein sorting sequences could benefit the expression level of rhIGFBP-3 and it is feasible to use plant as "bio-factory" to produce therapeutic recombinant proteins in large quantity.

The plant endoplasmic reticulum: a cell-wide web

The ER (endoplasmic reticulum) in higher plants forms a pleomorphic web of membrane tubules and small cisternae that pervade the cytoplasm, but in particular form a polygonal network at the cortex of the cell which may be anchored to the plasma membrane. The network is associated with the actin cytoskeleton and demonstrates extensive mobility, which is most likely to be dependent on myosin motors. The ER is characterized by a number of domains which may be associated with specific functions such as protein storage, or with direct interaction with other organelles such as the Golgi apparatus, peroxisomes and plastids. In the present review we discuss the nature of the network, the role of shape-forming molecules such as the recently described reticulon family of proteins and the function of some of the major domains within the ER network.

Plant seeds as bioreactors for recombinant protein production

Plants are attractive expression systems for the economic production of recombinant proteins. Among the different plant-based systems, plant seed is the leading platform and holds several advantages such as high protein yields and stable storage of target proteins. Significant advances in using seeds as bioreactors have occurred in the past decade, which include the first commercialized plant-derived recombinant protein. Here we review the current progress on seeds as bioreactors, with focus on the different food crops as production platforms and comprehensive strategies in optimizing recombinant protein production in seeds.

Post-Golgi traffic in plants

Secretory and endocytic traffic through the post-Golgi endomembrane system regulates the abundance of plasma-membrane proteins such as receptors, transporters and ion channels, modulating the ability of a cell to communicate with its neighbours and to adapt to a changing environment. The major post-Golgi compartments are numerous and appear to be similar to their counterparts in animals. However, endosomes are rather ill defined morphologically but seem to be involved in specific trafficking pathways. Many plasma-membrane proteins cycle constitutively via endosomal compartments. The trans-Golgi network (TGN) appears to be an early endosome where secretory and endocytic traffic meet. Endocytosed proteins that are to be degraded are targeted to the vacuole via the multivesiculate prevacuolar compartment (PVC) whereas cycling proteins pass through recycling endosomes. The trafficking machinery involves the same classes of proteins as in other eukaryotes. However, there are modifications that match the specifics of post-Golgi traffic in plants. Although plants lack epithelia, some plasma-membrane proteins are located on specific faces of the cell which reflects polarized traffic and influences the physiological performance of the tissue. Plants also differentiate highly polarized tip-growing cells in which post-Golgi traffic is adapted to very high rates of targeted exocytosis, endocytosis and recycling.

Production and localization of recombinant pharmaceuticals in transgenic seeds

Among the many plant-based production systems that have been developed for pharmaceutical proteins, seeds have the useful advantage of accumulating proteins in a relatively small volume, and recombinant proteins are very stable in dry seeds allowing long-term storage and facilitating distribution before processing.To take full advantage of the natural ability of endosperm cells to store large amounts of protein in a protected subcellular environment, it is useful to target recombinant proteins to appropriate storage organelles. In this chapter, we describe the distinct types of protein storage organelles in the cereal endosperm and a protocol for the detection of recombinant proteins in these organelles by immunofluorescence and immunogold labelling.The use of food and feed crops for the production of pharmaceutical proteins such as edible vaccines implies the need for strict separation of the transgenic seeds from the food and feed chain. For improved traceability visual markers may be co-expressed with the gene of interest in engineered seeds. DsRed is one example for a fluorescent protein that can be detected with high sensitivity using low tech equipment. We therefore describe the generation of transgenic maize plants expressing DsRed in a constitutive manner, and we point out the advantages of using this marker during the process of transformation and selection of plant tissue and later during breeding of transgenic lines into elite germplasm.

Preventing unintended proteolysis in plant protein biofactories

Numerous reports have been published over the last decade assessing the potential of plants as useful hosts for the heterologous expression of clinically useful proteins. Significant progress has been made, in particular, in optimizing transgene transcription and translation in plants, and in elucidating the complex post-translational modifications of proteins typical of the plant cell machinery. In this article, we address the important issue of recombinant protein degradation in plant expression platforms, which directly impacts on the final yield, homogeneity and overall quality of the resulting protein product. Unlike several more stable and structurally less complex pharmaceuticals, recombinant proteins present a natural tendency to structural heterogeneity, resulting in part from the inherent instability of polypeptide chains expressed in heterologous environments. Proteolytic processing, notably, may dramatically alter the structural integrity and overall accumulation of recombinant proteins in plant expression systems, both in planta during expression and ex planta after extraction. In this article, we describe the current strategies proposed to minimize protein hydrolysis in plant protein factories, including organ-specific transgene expression, organelle-specific protein targeting, the grafting of stabilizing protein domains to labile proteins, protein secretion in natural fluids and the co-expression of companion protease inhibitors.

Intermediate organelles of the plant secretory pathway: identity and function

The secretory pathway of eukaryotic cells comprises a network of organelles that connects three large membranes, the plasma membrane, the vacuole and the endoplasmic reticulum. The Golgi apparatus and the various post-Golgi organelles that control vacuolar sorting, secretion and endocytosis can be regarded as intermediate organelles of the endocytic and biosynthetic routes. Many processes in the secretory pathway have evolved differently in plants and cannot be studied using yeast or mammalian cells as models. The best characterized organelles are the Golgi apparatus and the prevacuolar compartment, but recent work has shed light on the role of the trans Golgi network, which has to be regarded as a separate organelle in plants. In this study, we wish to highlight recent findings regarding the late secretory pathway and its crosstalk with the early secretory pathway as well as the endocytic route in plants. Recently published findings and suggested models are discussed within the context of known features of the equivalent pathway in other eukaryotes.

Functional specialization of Medicago truncatula leaves and seeds does not affect the subcellular localization of a recombinant protein

A number of recent reports suggest that the functional specialization of plant cells in storage organs can influence subcellular protein sorting, so that the fate of a recombinant protein tends to differ between seeds and leaves. In order to test the general applicability of this hypothesis, we investigated the fate of a model recombinant glycoprotein in the leaves and seeds of a leguminous plant, Medicago truncatula. Detailed analysis of immature seeds by immunofluorescence and electron microscopy showed that recombinant phytase carrying a signal peptide for entry into the endoplasmic reticulum was efficiently secreted from storage cotyledon cells. A second version of the protein carrying a C-terminal KDEL tag for retention in the endoplasmic reticulum was predominantly retained in the ER of seed cotyledon cells, but some of the protein was secreted to the apoplast and some was deposited in storage vacuoles. Importantly, the fate of the recombinant protein in the leaves was nearly identical to that in the seeds from the same plant. This shows that in M. truncatula, the unanticipated partial vacuolar delivery and secretion is not a special feature of seed cotyledon tissue, but are conserved in different specialized tissues. Further investigation revealed that the unexpected fate of the tagged variant of phytase likely resulted from partial loss of the KDEL tag in both leaves and seeds. Our results indicate that the previously observed aberrant deposition of recombinant proteins into storage organelles of seed tissue is not a general reflection of functional specialization, but also depends on the species of plant under investigation. This discovery will have an impact on the production of recombinant pharmaceutical proteins in plants.

Identification of eukaryotic secreted and cell surface proteins using the yeast secretion trap screen

Secreted and cell surface proteins play essential roles in numerous essential biological processes in eukaryotic organisms, but are often more difficult to isolate and identify than proteins that are localized in intracellular compartments. However, several high-throughput 'gene-trap' techniques have been developed to characterize these 'secretomes', including the yeast secretion trap (YST) screen. This method involves fusing cDNA libraries from the tissue or cell type of interest to a yeast (Saccharomyces cerevisiae) invertase reporter gene, transforming the resulting fusion library into an invertase-deficient yeast strain and plating the transformants on a medium containing sucrose as the sole carbon source. A yeast cell with a transgene encoding a secreted or cell surface protein can synthesize a secreted invertase fusion protein that can rescue the mutant, and the plasmid DNA can then be sequenced to identify the gene that encodes it. We describe a recently improved version of this screen, which allows the identification of genes encoding secreted proteins in 1-2 months.

Ectopic expression of phosphoenolpyruvate carboxylase in Vicia narbonensis seeds: effects of improved nutrient status on seed maturation and transcriptional regulatory networks

Seed maturation responds to endogenous and exogenous signals like nutrient status, energy and hormones. We recently showed that phosphoenolpyruvate carboxylase (PEPC) overexpression in Vicia narbonensis seeds alters seed metabolism and channels carbon into organic acids, resulting in greater seed storage capacity and increased protein content. Thus, these lines represent models with altered sink strength and improved nutrient status. Here we analyse seed developmental and metabolic parameters, and C/N partitioning in these seeds. Transgenic embryos take up more carbon and nitrogen. Changes in dry to FW ratio, seed fill duration and major seed components indicate altered seed development. Array-based gene expression analysis of embryos reveals upregulation of seed metabolism, especially during the transition phase and at late maturation, in terms of protein storage and processing, amino acid metabolism, primary metabolism and transport, energy and mitochondrial activity, transcriptional and translational activity, stress tolerance, photosynthesis, cell proliferation and elongation, signalling and hormone action and regulated protein degradation. Stimulated cell elongation is in accordance with upregulated signalling pathways related to gibberellic acid/brassinosteroids. We discuss that activated organic and amino acid production leads to a wide-range activation of nitrogen metabolism, including the machinery of storage protein synthesis, amino acid synthesis, protein processing and deposition, translational activity and the methylation cycle. We suggest that alpha-ketoglutarate (alpha-KG) and/or oxalacetate provide signals for coordinate upregulation of amino acid biosynthesis. Activation of stress tolerance genes indicates partial overlap between nutrient, stress and abscisic acid (ABA) signals, indicating a common interacting or regulatory mechanism between nutrients, stress and ABA. In conclusion, analysis of PEPC overexpressing seeds identified pathways responsive to metabolic and nutrient control on the transcriptional level and its underlying signalling mechanisms.

Localization of vacuolar transport receptors and cargo proteins in the Golgi apparatus of developing Arabidopsis embryos

Using immunogold electron microscopy, we have investigated the relative distribution of two types of vacuolar sorting receptors (VSR) and two different types of lumenal cargo proteins, which are potential ligands for these receptors in the secretory pathway of developing Arabidopsis embryos. Interestingly, both cargo proteins are deposited in the protein storage vacuole, which is the only vacuole present during the bent-cotyledon stage of embryo development. Cruciferin and aleurain do not share the same pattern of distribution in the Golgi apparatus. Cruciferin is mainly detected in the cis and medial cisternae, especially at the rims where storage proteins aggregate into dense vesicles (DVs). Aleurain is found throughout the Golgi stack, particularly in the trans cisternae and trans Golgi network where clathrin-coated vesicles (CCVs) are formed. Nevertheless, aleurain was detected in both DV and CCV. VSR-At1, a VSR that recognizes N-terminal vacuolar sorting determinants (VSDs) of the NPIR type, localizes mainly to the trans Golgi and is hardly detectable in DV. Receptor homology-transmembrane-RING H2 domain (RMR), a VSR that recognizes C-terminal VSDs, has a distribution that is very similar to that of cruciferin and is found in DV. Our results do not support a role for VSR-At1 in storage protein sorting, instead RMR proteins because of their distribution similar to that of cruciferin in the Golgi apparatus and their presence in DV are more likely candidates. Aleurain, which has an NPIR motif and seems to be primarily sorted via VSR-At1 into CCV, also possesses putative hydrophobic sorting determinants at its C-terminus that could allow the additional incorporation of this protein into DV.

A bioinformatic approach to the identification of a conserved domain in a sugarcane legumain that directs GFP to the lytic vacuole

Sugarcane is an ideal candidate as a biofactory for the production of alternate higher value products. One way of achieving this is to direct useful proteins into the vacuoles within the sugarcane storage parenchyma tissue. By bioinformatic analysis of gene sequences from putative sugarcane vacuolar proteins a motif has been identified that displays high conservation across plant legumain homologues that are known to function within vacuolar compartments. This five amino acid motif, represented by the sequence IRLPS in sugarcane is shown to direct an otherwise secreted GFP fusion protein into a large acidic and proteolytic vacuole in sugarcane callus cells as well as in diverse plant species. In mature sugarcane transgenic plants, the stability of GFP appeared to be dependent on cell type, suggesting that the vacuolar environment can be hostile to introduced proteins. This targeting motif will be a valuable tool for engineering plants such as sugarcane for production of novel products.

Coated vesicles in plant cells

Coated vesicles represent vital transport intermediates in all eukaryotic cells. While the basic mechanisms of membrane exchange are conserved through the kingdoms, the unique topology of the plant endomembrane system is mirrored by several differences in the genesis, function and regulation of coated vesicles. Efforts to unravel the complex network of proteins underlying the behaviour of these vesicles have recently benefited from the application in planta of several molecular tools used in mammalian systems, as well as from advances in imaging technology and the ongoing analysis of the Arabidopsis genome. In this review, we provide an overview of the roles of coated vesicles in plant cells and highlight salient new developments in the field.

The Arabidopsis AAA ATPase SKD1 is involved in multivesicular endosome function and interacts with its positive regulator LYST-INTERACTING PROTEIN5

In yeast and mammals, the AAA ATPase Vps4p/SKD1 (for Vacuolar protein sorting 4/SUPPRESSOR OF K(+) TRANSPORT GROWTH DEFECT1) is required for the endosomal sorting of secretory and endocytic cargo. We identified a VPS4/SKD1 homolog in Arabidopsis thaliana, which localizes to the cytoplasm and to multivesicular endosomes. In addition, green fluorescent protein-SKD1 colocalizes on multivesicular bodies with fluorescent fusion protein endosomal Rab GTPases, such as ARA6/RabF1, RHA1/RabF2a, and ARA7/RabF2b, and with the endocytic marker FM4-64. The expression of SKD1(E232Q), an ATPase-deficient version of SKD1, induces alterations in the endosomal system of tobacco (Nicotiana tabacum) Bright Yellow 2 cells and ultimately leads to cell death. The inducible expression of SKD1(E232Q) in Arabidopsis resulted in enlarged endosomes with a reduced number of internal vesicles. In a yeast two-hybrid screen using Arabidopsis SKD1 as bait, we isolated a putative homolog of mammalian LYST-INTERACTING PROTEIN5 (LIP5)/SKD1 BINDING PROTEIN1 and yeast Vta1p (for Vps twenty associated 1 protein). Arabidopsis LIP5 acts as a positive regulator of SKD1 by increasing fourfold to fivefold its in vitro ATPase activity. We isolated a knockout homozygous Arabidopsis mutant line with a T-DNA insertion in LIP5. lip5 plants are viable and show no phenotypic alterations under normal growth conditions, suggesting that basal SKD1 ATPase activity is sufficient for plant development and growth.

Function and evolution of the vacuolar compartment in green algae and land plants (Viridiplantae)

Plant vacuoles perform several different functions and are essential for the plant cell. The large central vacuoles of mature plant cells provide structural support, and they serve other functions, such as protein degradation and turnover, waste disposal, storage of metabolites, and cell growth. A unique feature of the plant vacuolar system is the presence of different types of vacuoles within the same cell. The current knowledge about the vacuolar compartments in plants and green algae is summarized and a hypothesis is presented to explain the origin of multiple types of vacuoles in plants.

A patatin-like protein protects Toxoplasma gondii from degradation in activated macrophages

The apicomplexan parasite Toxoplasma gondii is able to suppress nitric oxide production in activated macrophages. A screen of over 6000 T. gondii insertional mutants identified two clones, which were consistently unable to suppress nitric oxide production from activated macrophages. One strain, called 89B7, grew at the same rate as wild-type parasites in naïve macrophages, but unlike wild type, the mutant was degraded in activated macrophages. This degradation was marked by a reduction in the number of parasites within vacuoles over time, the loss of GRA4 and SAG1 protein staining by immunofluorescence assay, and the vesiculation and breakdown of the internal parasite ultrastructure by electron microscopy. The mutagenesis plasmid in the 89B7 clone disrupts the promoter of a 3.4 kb mRNA that encodes a predicted 68 kDa protein with a cleavable signal peptide and a patatin-like phospholipase domain. Genetic complementation with the genomic locus of this patatin-like protein restores the parasites ability to suppress nitric oxide and replicate in activated macrophages. A haemagglutinin-tagged version of this patatin-like protein shows punctate localization into atypical T. gondii structures within the parasite. This is the first study that defines a specific gene product that is needed for parasite survival in activated but not naïve macrophages.

The type IV secretion system of Sinorhizobium meliloti strain 1021 is required for conjugation but not for intracellular symbiosis

The type IV secretion system (T4SS) of the plant intracellular symbiont Sinorhizobium meliloti 1021 is required for conjugal transfer of DNA. However, it is not required for host invasion and persistence, unlike the T4SSs of closely related mammalian intracellular pathogens. A comparison of the requirement for a bacterial T4SS in plant versus animal host invasion suggests an important difference in the intracellular niches occupied by these bacteria.

Traffic between the plant endoplasmic reticulum and Golgi apparatus: to the Golgi and beyond

Significant advances have been made in recent years that have increased our understanding of the trafficking to and from membranes that are functionally linked to the Golgi apparatus in plants. New routes from the Golgi to organelles outside the secretory pathway are now being identified, revealing the importance of the Golgi apparatus as a major sorting station in the plant cell. This review discusses our current perception of Golgi structure and organization as well as the molecular mechanisms that direct traffic in and out of the Golgi.

Localization of green fluorescent protein fusions with the seven Arabidopsis vacuolar sorting receptors to prevacuolar compartments in tobacco BY-2 cells

We have previously demonstrated that vacuolar sorting receptor (VSR) proteins are concentrated on prevacuolar compartments (PVCs) in plant cells. PVCs in tobacco (Nicotiana tabacum) BY-2 cells are multivesicular bodies (MVBs) as defined by VSR proteins and the BP-80 reporter, where the transmembrane domain (TMD) and cytoplasmic tail (CT) sequences of BP-80 are sufficient and specific for correct targeting of the reporter to PVCs. The genome of Arabidopsis (Arabidopsis thaliana) contains seven VSR proteins, but little is known about their individual subcellular localization and function. Here, we study the subcellular localization of the seven Arabidopsis VSR proteins (AtVSR1-7) based on the previously proven hypothesis that the TMD and CT sequences correctly target individual VSR to its final destination in transgenic tobacco BY-2 cells. Toward this goal, we have generated seven chimeric constructs containing signal peptide (sp) linked to green fluorescent protein (GFP) and TMD/CT sequences (sp-GFP-TMD/CT) of the seven individual AtVSR. Transgenic tobacco BY-2 cell lines expressing these seven sp-GFP-TMD-CT fusions all exhibited typical punctate signals colocalizing with VSR proteins by confocal immunofluorescence. In addition, wortmannin caused the GFP-marked prevacuolar organelles to form small vacuoles, and VSR antibodies labeled these enlarged MVBs in transgenic BY-2 cells. Wortmannin also caused VSR-marked PVCs to vacuolate in other cell types, including Arabidopsis, rice (Oryza sativa), pea (Pisum sativum), and mung bean (Vigna radiata). Therefore, the seven AtVSRs are localized to MVBs in tobacco BY-2 cells, and wortmannin-induced vacuolation of PVCs is a general response in plants.

The proteolytic processing of seed storage proteins in Arabidopsis embryo cells starts in the multivesicular bodies

We have investigated the transport of storage proteins, their processing proteases, and the Vacuolar Sorting Receptor-1/Epidermal Growth Factor Receptor-Like Protein1 (VSR-1/ATELP1) receptor during the formation of protein storage vacuoles in Arabidopsis thaliana embryos by means of high-pressure freezing/freeze substitution, electron tomography, immunolabeling techniques, and subcellular fractionation. The storage proteins and their processing proteases are segregated from each other within the Golgi cisternae and packaged into separate vesicles. The storage protein-containing vesicles but not the processing enzyme-containing vesicles carry the VSR-1/ATELP1 receptor. Both types of secretory vesicles appear to fuse into a type of prevacuolar multivesicular body (MVB). We have also determined that the proteolytic processing of the 2S albumins starts in the MVBs. We hypothesize that the compartmentalized processing of storage proteins in the MVBs may allow for the sequential activation of processing proteases as the MVB lumen gradually acidifies.

AtVPS29, a putative component of a retromer complex, is required for the efficient sorting of seed storage proteins

Seed storage proteins are synthesized on rough endoplasmic reticulum (ER) as larger precursors and are sorted to protein storage vacuoles, where they are converted into the mature forms. We report here an Arabidopsis mutant, maigo 1 (mag1), which abnormally accumulates the precursors of two major storage proteins, 12S globulin and 2S albumin, in dry seeds. Electron microscopy revealed that mag1 seeds mis-sort storage proteins by secreting them from cells. mag1 seeds have smaller protein storage vacuoles in the seeds than do wild-type seeds. The MAG1 gene encodes a homolog of the yeast (Saccharomyces cerevisiae) protein VPS29. VPS29 is a component of a retromer complex for recycling a vacuolar sorting receptor VPS10 from the pre-vacuolar compartment to the Golgi complex. Our findings suggest that MAG1/AtVPS29 protein is involved in recycling a plant receptor for the efficient sorting of seed storage proteins. The mag1 mutant exhibits a dwarf phenotype. A plant retromer complex plays a significant role in plant growth and development.

A conserved region in the EBL proteins is implicated in microneme targeting of the malaria parasite Plasmodium falciparum

The proliferation of the malaria parasite Plasmodium falciparum within the human host is dependent upon invasion of erythrocytes. This process is accomplished by the merozoite, a highly specialized form of the parasite. Secretory organelles including micronemes and rhoptries play a pivotal role in the invasion process by storing and releasing parasite proteins. The mechanism of protein sorting to these compartments is unclear. Using a transgenic approach we show that trafficking of the most abundant micronemal proteins (members of the EBL-family: EBA-175, EBA-140/BAEBL, and EBA-181/JSEBL) is independent of their cytoplasmic and transmembrane domains, respectively. To identify the minimal sequence requirements for microneme trafficking, we generated parasites expressing EBA-GFP chimeric proteins and analyzed their distribution within the infected erythrocyte. This revealed that: (i) a conserved cysteine-rich region in the ectodomain is necessary for protein trafficking to the micronemes and (ii) correct sorting is dependent on accurate timing of expression.

Targeting of the plant vacuolar sorting receptor BP80 is dependent on multiple sorting signals in the cytosolic tail

Although signals for vacuolar sorting of soluble proteins are well described, we have yet to learn how the plant vacuolar sorting receptor BP80 reaches its correct destination and recycles. To shed light on receptor targeting, we used an in vivo competition assay in which a truncated receptor (green fluorescent protein-BP80) specifically competes with sorting machinery and causes hypersecretion of BP80-ligands from tobacco (Nicotiana tabacum) leaf protoplasts. We show that both the transmembrane domain and the cytosolic tail of BP80 contain information necessary for efficient progress to the prevacuolar compartment (PVC). Furthermore, the tail must be exposed on the correct membrane surface to compete with sorting machinery. Mutational analysis of conserved residues revealed that multiple sequence motifs are necessary for competition, one of which is a typical Tyr-based motif (YXXPhi). Substitution of Tyr-612 for Ala causes partial retention in the Golgi apparatus, mistargeting to the plasma membrane (PM), and slower progress to the PVC. A role in Golgi-to-PVC transport was confirmed by generating the corresponding mutation on full-length BP80. The mutant receptor was partially mistargeted to the PM and induced the secretion of a coexpressed BP80-ligand. Further mutants indicate that the cytosolic tail is likely to contain other information besides the YXXPhi motif, possibly for endoplasmic reticulum export, endocytosis from the PM, and PVC-to-Golgi recycling.

The intracellular fate of a recombinant protein is tissue dependent

Recombinant proteins directed to the secretory pathway in plants require a signal peptide for entry into the endoplasmic reticulum. In the absence of further targeting information, such proteins are generally secreted via the default pathway to the apoplast. This has been well documented in protoplasts and leaf tissue, but the trafficking of recombinant proteins in seeds and other storage tissues has rarely been investigated. We used Aspergillus niger phytase as a model glycoprotein to compare the intracellular fate of a recombinant protein in the leaves and seeds of rice (Oryza sativa). Using fluorescence and electron microscopy we showed that the recombinant protein was efficiently secreted from leaf cells as expected. In contrast, within endosperm cells it was retained in endoplasmic reticulum-derived prolamin bodies and protein storage vacuoles. Consistent with our immunolocalization data, the phytase produced in endosperm cells possessed oligomannose and vacuolar-type N-glycans [Man(3)(Xyl)(Fuc)GlcNAc(2)], whereas the phytase produced in leaves contained predominantly secretion-type N-glycans [GlcNAc(2)Man(3)(Xyl)(Fuc)GlcNAc(2)]. The latter could not be detected in preparations of the endosperm-derived phytase. Our results show that the intracellular deposition and modification of a recombinant protein is tissue dependent.

Plant retromer, localized to the prevacuolar compartment and microvesicles in Arabidopsis, may interact with vacuolar sorting receptors

Receptors for acid hydrolases destined for the lytic compartment in yeast and mammalian cells are retrieved from intermediate, endosomal organelles with the help of a pentameric protein complex called the retromer. We cloned the Arabidopsis thaliana homologs of the three yeast proteins (Vps35, Vps29, and Vps26) constituting the larger subunit of retromer and prepared antisera against them. With these antibodies, we demonstrated the presence of a retromer-like protein complex in salt extracts prepared from Arabidopsis microsomes. This complex is associated with membranes that coequilibrate with prevacuolar compartment markers and with high-density sedimenting membranes. Immunogold negative staining identified these membranes as 90-nm-diameter coated microvesicles. Confocal laser scanning immunofluorescence studies performed on tobacco (Nicotiana tabacum) BY-2 cells revealed high degrees of colabeling between all three retromer antisera and the prevacuolar compartment (PVC) markers PEP12 and vacuolar sorting receptor VSR(At-1). The presence of plant retromer at the surface of multivesicular bodies was also demonstrated by immunogold labeling of sections obtained from high-pressure frozen/freeze-substituted specimens. Treatment of BY-2 cells with wortmannin led to swelling of the PVC and a separation of the VPS35 and VSR signals. Preliminary data suggesting that retromer interacts with the cytosolic domain of a VSR were obtained by immunoprecipitation experiments performed on detergent-solubilized microsomes with Vps35 antibodies.

The role of mRNA and protein sorting in seed storage protein synthesis, transport, and deposition

Rice synthesizes and accumulates high levels of 2 distinct classes of seed storage proteins and sorts them to separate intracellular compartments, making it an ideal model system for studying the mechanisms of storage protein synthesis, transport, and deposition. In rice, RNA localization dictates the initial site of storage protein synthesis on specific subdomains of the cortical endoplasmic reticulum (ER), and there is a direct relation between the RNA localization site and the final destination of the encoded protein within the endomembrane system. Current data support the existence of 3 parallel RNA localization pathways leading from the nucleus to the actively synthesizing cortical ER. Additional pathways may exist for the synthesis of cytoplasmic and nuclear-encoded proteins targeted to organelles, the latter located in a stratified arrangement in developing endosperm cells. The study of rice mutants, which accumulate unprocessed glutelin precursors, indicates that these multiple pathways prevent nonproductive interactions between different classes of storage proteins that would otherwise disrupt protein sorting. Indeed, it appears that the prevention of disruptive interactions between different classes of storage proteins plays a key role in their biosynthesis in rice. In addition to highlighting the unique features of the plant endomembrane system and describing the relation between RNA and protein localization, this minireview will attempt to address a number of questions raised by recent studies on these processes.

Plant Prevacuolar/Endosomal Compartments

Multivesicular endosomes or prevacuolar compartments (PVCs) are membrane-bound organelles that play an important role in mediating protein traffic in the secretory and endocytic pathways of eukaryotic cells. PVCs function as an intermediate compartment for sorting proteins from the Golgi apparatus to vacuoles, sending missorted proteins back to the Golgi from the PVC, and receiving proteins from plasma membrane in the endocytic pathway. PVCs have been identified as multivesicular bodies in mammalian cells and yeast and more recently in plant cells. Whereas much is known about PVC-mediated protein trafficking and PVC biogenesis in mammalian cells and yeast, relatively little is known about the molecular mechanism of plant PVCs. In this review, we summarize and discuss our understanding of the plant PVC and compare it with its counterparts in yeast and mammalian cells.