COMPARTMENTATION OF PROTEINS IN THE ENDOMEMBRANE SYSTEM OF PLANT CELLS

Involvement of a rice mutation in storage protein biogenesis in endosperm and its genomic location

MAIN CONCLUSION

A mutation was first found to cause the great generation of glutelin precursors (proglutelins) in rice (Oryza sativa L.) endosperm, and thus referred to as GPGG1. The GPGG1 was involved in synthesis and compartmentation of storage proteins. The PPR-like gene in GPGG1-mapped region was determined as its candidate gene. In the wild type rice, glutelins and prolamins are synthesized on respective subdomains of rough endoplasmic reticulum (ER) and intracellularly compartmentalized into different storage protein bodies. In this study, a storage protein mutant was obtained and characterized by the great generation of proglutelins combining with the lacking of 13 kD prolamins. A dominant genic-mutation, referred to as GPGG1, was clarified to result in the proteinous alteration. Novel saccular composite-ER was shown to act in the synthesis of proglutelins and 14 kD prolamins in the mutant. Additionally, a series of organelles including newly occurring several compartments were shown to function in the transfer, trans-plasmalemmal transport, delivery, deposition and degradation of storage proteins in the mutant. The GPGG1 gene was mapped to a 67.256 kb region of chromosome 12, the pentatricopeptide repeat (PPR)-like gene in this region was detected to contain mutational sites.

A century journey of organelles research in the plant endomembrane system

We are entering an exciting century in the study of the plant organelles in the endomembrane system. Over the past century, especially within the past 50 years, tremendous advancements have been made in the complex plant cell to generate a much clearer and informative picture of plant organelles, including the molecular/morphological features, dynamic/spatial behavior, and physiological functions. Importantly, all these discoveries and achievements in the identification and characterization of organelles in the endomembrane system would not have been possible without: (1) the innovations and timely applications of various state-of-art cell biology tools and technologies for organelle biology research; (2) the continuous efforts in developing and characterizing new organelle markers by the plant biology community; and (3) the landmark studies on the identification and characterization of the elusive organelles. While molecular aspects and results for individual organelles have been extensively reviewed, the development of the techniques for organelle research in plant cell biology is less appreciated. As one of the ASPB Centennial Reviews on "organelle biology," here we aim to take a journey across a century of organelle biology research in plants by highlighting the important tools (or landmark technologies) and key scientists that contributed to visualize organelles. We then highlight the landmark studies leading to the identification and characterization of individual organelles in the plant endomembrane systems.

The seven rice vacuolar sorting receptors localize to prevacuolar compartments

Vacuolar sorting is critically important in plants as it regulates the mobilization of proteins and plays a major role in important agricultural traits like yield and seed protein content. Vacuolar sorting receptors (VSRs) are integral membrane proteins that mediate protein trafficking from the Golgi apparatus to the vacuole via the intermediate membrane-bound prevacuolar compartment (PVC)/multivesicular body (MVB). VSR proteins, such as an 80 kD (BP-80) from pea, also serve as markers for PVC/MVB. Dissecting VSR-mediated protein trafficking pathways may provide ways to enhance agronomic traits and crop yield. Green fluorescence protein (GFP) fusions with the seven Arabidopsis (Arabidopsis thaliana) VSRs were previously shown to localize to PVCs in transgenic tobacco BY-2 cells. The Rice (Oryza sativa) genome contains seven VSRs (OsVSR1-7), but little is known about their subcellular localizations. Here we studied the subcellular localization of OsVSR1-7 b y using a reporter approach, in which GFP-OsVSR1-7 fusions containing the transmembrane domain (TMD) and cytoplasmic tail (CT) of individual OsVSR were expressed in the protoplasts of rice, transgenic tobacco BY-2 cells and transgenic rice plants. Immunofluorescent labelling studies and confocal laser scanning microscope observation demonstrated that the seven OsVSRs are localized to PVCs and form ring-like structures upon wortmannin treatment. Therefore, we have verified the subcellular localization of OsVSR1-7 in this study. The OsVSRs tagged with GFP can serve as PVCs/MVBs markers in rice for the future studies.

COPII vesicles in plant autophagy and endomembrane trafficking

In eukaryotes, the endomembrane system allows for spatiotemporal compartmentation of complicated cellular processes. The plant endomembrane system consists of the endoplasmic reticulum (ER), the Golgi apparatus (GA), the trans-Golgi network (TGN), the multivesicular body (MVB), and the vacuole. Anterograde traffic from the ER to GA is mediated by coat protein complex II (COPII) vesicles. Autophagy, an evolutionarily conserved catabolic process that turns over cellular materials upon nutrient deprivation or in adverse environments, exploits double-membrane autophagosomes to recycle unwanted constituents in the lysosome/vacuole. Accumulating evidence reveals novel functions of plant COPII vesicles in autophagy and their regulation by abiotic stresses. Here, we summarize current knowledge about plant COPII vesicles in the endomembrane trafficking and then highlight recent findings showing their distinct roles in modulating the autophagic flux and stress responses.

Protein bodies of the endoplasmic reticulum in Arabidopsis thaliana (Brassicaceae): origin, structural and biochemical features, functional significance

History of the discovery, formation, structural and biochemical traits of the protein bodies, derivatives of the granular endoplasmic reticulum (GER) that are known as ER-bodies, are reviewed. The functions of ER-bodies in cell vital activity mainly in Arabidopsis thaliana are reported. The highly specific component of ER-bodies, β-glucosidase enzyme, is described and its protecting role for plants under effect of abiotic and biotic factors is characterized. Based on the analytical review of the literature, it is shown that ER-bodies and the transcription factor NAI2 are unique to species of the family Brassicaceae. The specificity of the system GER – ER-bodies for Brassicaceae and thus the fundamental and applied importance of future research of mechanisms of its functioning in A. thaliana and other Brassicaceae species are emphasized.

Are unusual ultrastructural features occurring in the pollen endomembrane system of Cyperaceae and other angiosperms?

Cyperaceae representatives present peculiar microsporogenesis and microgametogenesis, which raises the question of how regular the sedge pollen is. In order to answer this question, preanthesis pollen grains of Cyperaceae individuals were analyzed under different tools such as light and transmission electron microscopy, which included cytochemistry and immunogold procedures. The results showed that maturing pollen in Cyperaceae presents some unusual endomembrane behaviors. Endoplasmic reticulum and dictyosomes are concerned in classic secretion pathways in vegetative cells, and possibly the late breakdown of degenerative microspores. However, cortical and concentric endoplasmic reticulum are also present and are possibly related to other functions aside the biosynthetic pathway. Unconventional secretion of large membrane-bound bodies containing cell wall precursors was also observed and confirmed by immunogold. However, since these bodies most likely receive material from dictyosomes, as observed in silver nitrate reaction, the "unconventional" status of this secretion is debatable. Reports of the literature show that these unusual endomembrane formations are not exclusive of the sedge pollen, but little attention have been given to them so far. This could represent an opportunity to re-examine our understanding on the endomembrane system in pollen cells in general.

High resolution structural and functional analysis of a hemopexin motif protein from Dolichos

It is increasingly evident that seed proteins exhibit specific functions in plant physiology. However, many proteins remain yet to be functionally characterized. We have screened the seed proteome of Dolichos which lead to identification and purification of a protein, DC25. The protein was monomeric and highly thermostable in extreme conditions of pH and salt. It was crystallized and structure determined at 1.28 Å resolution using x-ray crystallography. The high-resolution structure of the protein revealed a four-bladed β-propeller hemopexin-type fold containing pseudo four-fold molecular symmetry at the central channel. While the structure exhibited homology with 2S albumins, variations in the loops connecting the outermost strands and the differences in surface-charge distribution may be relevant for distinct functions. Comparative study of the protein with other seed hemopexins revealed the presence of four conserved water molecules in between the blades which cross-link them and maintain the tertiary structure. The protein exhibited intrinsic peroxidase activity, which could be inhibited by binding of a heme analog. The identification of redox-sensitive cysteine and inhibition of peroxidase activity by iodoacetamide facilitated characterization of the possible active site. The determined peroxidase activity of DC25 may be responsible for rescuing germinating seeds from oxidative stress.

The rice storage protein mRNAs as a model system for RNA localization in higher plants

The transport and targeting of mRNAs to specific intracellular locations is a ubiquitous process in prokaryotic and eukaryotic organisms. Despite the prevalent nature of RNA localization in guiding development, differentiation, cellular movement and intracellular organization of biochemical activities, only a few examples exist in higher plants. Here, we summarize past studies on mRNA-based protein targeting to specific subdomains of the cortical endoplasmic reticulum (ER) using the rice storage protein mRNAs as a model. Such studies have demonstrated that there are multiple pathways of RNA localization to the cortical ER that are controlled by cis-determinants (zipcodes) on the mRNA. These zipcode sequences are recognized by specific RNA binding proteins organized into multi-protein complexes. The available evidence suggests mRNAs are transported to their destination sites by co-opting membrane trafficking factors. Lastly, we discuss the major gaps in our knowledge on RNA localization and how information on the targeting of storage protein mRNAs can be used to further our understanding on how plant mRNAs are organized into regulons to facilitate protein localization and formation of multi-protein complexes.

The relationship between vacuolation and initiation of PCD in rice (Oryza sativa) aleurone cells

Vacuole fusion is a necessary process for the establishment of a large central vacuole, which is the central location of various hydrolytic enzymes and other factors involved in death at the beginning of plant programmed cell death (PCD). In our report, the fusion of vacuoles has been presented in two ways: i) small vacuoles coalesce to form larger vacuoles through membrane fusion, and ii) larger vacuoles combine with small vacuoles when small vacuoles embed into larger vacuoles. Regardless of how fusion occurs, a large central vacuole is formed in rice (Oryza sativa) aleurone cells. Along with the development of vacuolation, the rupture of the large central vacuole leads to the loss of the intact plasma membrane and the degradation of the nucleus, resulting in cell death. Stabilizing or disrupting the structure of actin filaments (AFs) inhibits or promotes the fusion of vacuoles, which delays or induces PCD. In addition, the inhibitors of the vacuolar processing enzyme (VPE) and cathepsin B (CathB) block the occurrence of the large central vacuole and delay the progression of PCD in rice aleurone layers. Overall, our findings provide further evidence for the rupture of the large central vacuole triggering the PCD in aleruone layers.

Overexpression of the potassium channel TPKb in small vacuoles confers osmotic and drought tolerance to rice

Potassium (K(+) ) is the most important cationic nutrient for all living organisms. Vacuolar two-pore K(+) (TPK) channels are important players in the regulation of cellular levels of K(+) but have not been characterised in rice. In order to assess the role of OsTPKb, a K(+) selective ion channel predominantly expressed in the tonoplast of small vacuoles, we generated overexpressing (OX) lines using a constitutive promoter and compared their phenotypes with control plants. Relative to control plants, OX lines showed better growth when exposed to low-K(+) or water stress conditions. K(+) uptake was greater in OX lines which may be driven by increased AKT1 and HAK1 activity. The enhanced K(+) uptake led to tissue K(+) levels that were raised in roots and shoots. Furthermore, energy dispersive X-ray (EDX) analyses showed a higher cytoplasm: vacuole K(+) ratio which is likely to contribute to the increased stress tolerance. In all, the data suggest that TPKb can alter the K(+) status of small vacuoles, which is important for general cellular K(+) homeostasis which, in turn, affects stress tolerance.

BEACH-domain proteins act together in a cascade to mediate vacuolar protein trafficking and disease resistance in Arabidopsis

Membrane trafficking to the protein storage vacuole (PSV) is a specialized process in seed plants. However, this trafficking mechanism to PSV is poorly understood. Here, we show that three types of Beige and Chediak-Higashi (BEACH)-domain proteins contribute to both vacuolar protein transport and effector-triggered immunity (ETI). We screened a green fluorescent seed (GFS) library of Arabidopsis mutants with defects in vesicle trafficking and isolated two allelic mutants gfs3 and gfs12 with a defect in seed protein transport to PSV. The gene responsible for the mutant phenotype was found to encode a putative protein belonging to group D of BEACH-domain proteins, which possess kinase domains. Disruption of other BEACH-encoding loci in the gfs12 mutant showed that BEACH homologs acted in a cascading manner for PSV trafficking. The epistatic genetic interactions observed among BEACH homologs were also found in the ETI responses of the gfs12 and gfs12 bchb-1 mutants, which showed elevated avirulent bacterial growth. The GFS12 kinase domain interacted specifically with the pleckstrin homology domain of BchC1. These results suggest that a cascade of multiple BEACH-domain proteins contributes to vacuolar protein transport and plant defense.

How vacuolar sorting receptor proteins interact with their cargo proteins: crystal structures of apo and cargo-bound forms of the protease-associated domain from an Arabidopsis vacuolar sorting receptor

In plant cells, soluble proteins are directed to vacuoles because they contain vacuolar sorting determinants (VSDs) that are recognized by vacuolar sorting receptors (VSR). To understand how a VSR recognizes its cargo, we present the crystal structures of the protease-associated domain of VSR isoform 1 from Arabidopsis thaliana (VSR1PA) alone and complexed with a cognate peptide containing the barley (Hordeum vulgare) aleurain VSD sequence of 1ADSNPIRPVT10. The crystal structures show that VSR1PA binds the sequence, Ala-Asp-Ser, preceding the NPIR motif. A conserved cargo binding loop, with a consensus sequence of 95RGxCxF100, forms a cradle that accommodates the cargo-peptide. In particular, Arg-95 forms a hydrogen bond to the Ser-3 position of the VSD, and the essential role of Arg-95 and Ser-3 in receptor-cargo interaction was supported by a mutagenesis study. Cargo binding induces conformational changes that are propagated from the cargo binding loop to the C terminus via conserved residues in switch I-IV regions. The resulting 180° swivel motion of the C-terminal tail is stabilized by a hydrogen bond between Glu-24 and His-181. A mutagenesis study showed that these two residues are essential for cargo interaction and trafficking. Based on our structural and functional studies, we present a model of how VSRs recognize their cargos.

OsVPS9A functions cooperatively with OsRAB5A to regulate post-Golgi dense vesicle-mediated storage protein trafficking to the protein storage vacuole in rice endosperm cells

In the rice endosperm cells, glutelins are synthesized on rough endoplasmic reticulum as proglutelins and are sorted to the protein storage vacuoles (PSVs) called protein body IIs (PBIIs), where they are converted to the mature forms. Dense vesicle (DV)-mediated trafficking of proglutelins in rice seeds has been proposed, but the post-Golgi control of this process is largely unknown. Whether DV can fuse directly with PSV is another matter of debate. In this study, we propose a regulatory mechanism underlying DV-mediated, post-Golgi proglutelin trafficking to PBII (PSV). gpa2, a loss-of-function mutant of OsVPS9A, which encodes a GEF of OsRAB5A, accumulated uncleaved proglutelins. Proglutelins were mis-targeted to the paramural bodies and to the apoplast along the cell wall in the form of DVs, which led to a concomitant reduction in PBII size. Previously reported gpa1, mutated in OsRab5a, has a similar phenotype, while gpa1gpa2 double mutant exacerbated the conditions. In addition, OsVPS9A interacted with OsRAB5A in vitro and in vivo. We concluded that OsVPS9A and OsRAB5A may work together and play a regulatory role in DV-mediated post-Golgi proglutelin trafficking to PBII (PSV). The evidence that DVs might fuse directly to PBII (PSV) to deliver cargos is also presented.

The oxidative protein folding machinery in plant cells

Formation of intra-molecular disulfides and concomitant oxidative protein folding is essential for stability and catalytic function of many soluble and membrane-bound proteins in the endomembrane system, the mitochondrial inter-membrane space and the thylakoid lumen. Disulfide generation from free cysteines in nascent polypeptide chains is generally a catalysed process for which distinct pathways exist in all compartments. A high degree of similarities between highly diverse eukaryotic and bacterial systems for generation of protein disulfides indicates functional conservation of key processes throughout evolution. However, while many aspects about molecular function of enzymatic systems promoting disulfide formation have been demonstrated for bacterial and non-plant eukaryotic organisms, it is now clear that the plant machinery for oxidative protein folding displays distinct details, suggesting that the different pathways have been adapted to plant-specific requirements in terms of compartmentation, molecular function and regulation. Here, we aim to evaluate biological diversity by comparing the plant systems for oxidative protein folding to the respective systems from non-plant eukaryotes.

A guanine nucleotide exchange factor for Rab5 proteins is essential for intracellular transport of the proglutelin from the Golgi apparatus to the protein storage vacuole in rice endosperm

Rice (Oryza sativa) glutelins are synthesized on the endoplasmic reticulum as a precursor, which are then transported via the Golgi to protein storage vacuoles (PSVs), where they are proteolytically processed into acidic and basic subunits. The glutelin precursor mutant6 (glup6) accumulates abnormally large amounts of proglutelin. Map-base cloning studies showed that glup6 was a loss-of-function mutant of guanine nucleotide exchange factor (GEF), which activates Rab GTPase, a key regulator of membrane trafficking. Immunofluorescence studies showed that the transport of proglutelins and α-globulins to PSV was disrupted in glup6 endosperm. Secreted granules of glutelin and α-globulin were readily observed in young glup6 endosperm, followed by the formation of large dilated paramural bodies (PMBs) containing both proteins as the endosperm matures. The PMBs also contained membrane biomarkers for the Golgi and prevacuolar compartment as well as the cell wall component, β-glucan. Direct evidence was gathered showing that GLUP6/GEF activated in vitro GLUP4/Rab5 as well as several Arabidopsis (Arabidopsis thaliana) Rab5 isoforms to the GTP-bound form. Therefore, loss-of-function mutations in GEF or Rab5 disrupt the normal transport of proglutelin from the Golgi to PSVs, resulting in the initial extracellular secretion of these proteins followed, in turn, by the formation of PMBs. Overall, our results indicate that GLUP6/GEF is the activator of Rab5 GTPase and that the cycling of GTP- and GDP-bound forms of this regulatory protein is essential for the intracellular transport of proglutelin and α-globulin from the Golgi to PSVs and in the maintenance of the general structural organization of the endomembrane system in rice seeds.

Fluorescence imaging-based forward genetic screens to identify trafficking regulators in plants

Coordinated, subcellular trafficking of proteins is one of the fundamental properties of the multicellular eukaryotic organisms. Trafficking involves a large diversity of compartments, pathways, cargo molecules, and vesicle-sorting events. It is also crucial in regulating the localization and, thus, the activity of various proteins, but the process is still poorly genetically defined in plants. In the past, forward genetics screens had been used to determine the function of genes by searching for a specific morphological phenotype in the organism population in which mutations had been induced chemically or by irradiation. Unfortunately, these straightforward genetic screens turned out to be limited in identifying new regulators of intracellular protein transport, because mutations affecting essential trafficking pathways often lead to lethality. In addition, the use of these approaches has been restricted by functional redundancy among trafficking regulators. Screens for mutants that rely on the observation of changes in the cellular localization or dynamics of fluorescent subcellular markers enable, at least partially, to circumvent these issues. Hence, such image-based screens provide the possibility to identify either alleles with weak effects or components of the subcellular trafficking machinery that have no strong impact on the plant growth.

Proteomic characterisation of endoplasmic reticulum-derived protein bodies in tobacco leaves

BACKGROUND

The N-terminal proline-rich domain (Zera) of the maize storage protein γ-zein, is able to induce the formation of endoplasmic reticulum (ER)-derived protein bodies (PBs) when fused to proteins of interest. This encapsulation enables a recombinant fused protein to escape from degradation and facilitates its recovery from plant biomass by gradient purification. The aim of the present work was to evaluate if induced PBs encapsulate additional proteins jointly with the recombinant protein. The exhaustive analysis of protein composition of PBs is expected to facilitate a better understanding of PB formation and the optimization of recombinant protein purification approaches from these organelles.

RESULTS

We analysed the proteome of PBs induced in Nicotiana benthamiana leaves by transient transformation with Zera fused to a fluorescent marker protein (DsRed). Intact PBs with their surrounding ER-membrane were isolated on iodixanol based density gradients and their integrity verified by confocal and electron microscopy. SDS-PAGE analysis of isolated PBs showed that Zera-DsRed accounted for around 85% of PB proteins in term of abundance. Differential extraction of PBs was performed for in-depth analysis of their proteome and structure. Besides Zera-DsRed, 195 additional proteins were identified including a broad range of proteins resident or trafficking through the ER and recruited within the Zera-DsRed polymer.

CONCLUSIONS

This study indicates that Zera-protein fusion is still the major protein component of the new formed organelle in tobacco leaves. The analysis also reveals the presence of an unexpected diversity of proteins in PBs derived from both the insoluble Zera-DsRed polymer formation, including ER-resident and secretory proteins, and a secretory stress response induced most likely by the recombinant protein overloading. Knowledge of PBs protein composition is likely to be useful to optimize downstream purification of recombinant proteins in molecular farming applications.

Plant defense mechanisms are activated during biotrophic and necrotrophic development of Colletotricum graminicola in maize

Hemibiotrophic plant pathogens first establish a biotrophic interaction with the host plant and later switch to a destructive necrotrophic lifestyle. Studies of biotrophic pathogens have shown that they actively suppress plant defenses after an initial microbe-associated molecular pattern-triggered activation. In contrast, studies of the hemibiotrophs suggest that they do not suppress plant defenses during the biotrophic phase, indicating that while there are similarities between the biotrophic phase of hemibiotrophs and biotrophic pathogens, the two lifestyles are not analogous. We performed transcriptomic, histological, and biochemical studies of the early events during the infection of maize (Zea mays) with Colletotrichum graminicola, a model pathosystem for the study of hemibiotrophy. Time-course experiments revealed that mRNAs of several defense-related genes, reactive oxygen species, and antimicrobial compounds all begin to accumulate early in the infection process and continue to accumulate during the biotrophic stage. We also discovered the production of maize-derived vesicular bodies containing hydrogen peroxide targeting the fungal hyphae. We describe the fungal respiratory burst during host infection, paralleled by superoxide ion production in specific fungal cells during the transition from biotrophy to a necrotrophic lifestyle. We also identified several novel putative fungal effectors and studied their expression during anthracnose development in maize. Our results demonstrate a strong induction of defense mechanisms occurring in maize cells during C. graminicola infection, even during the biotrophic development of the pathogen. We hypothesize that the switch to necrotrophic growth enables the fungus to evade the effects of the plant immune system and allows for full fungal pathogenicity.

Protein disulfide isomerase-2 of Arabidopsis mediates protein folding and localizes to both the secretory pathway and nucleus, where it interacts with maternal effect embryo arrest factor

Protein disulfide isomerase (PDI) is a thiodisulfide oxidoreductase that catalyzes the formation, reduction and rearrangement of disulfide bonds in proteins of eukaryotes. The classical PDI has a signal peptide, two CXXC-containing thioredoxin catalytic sites (a,a'), two noncatalytic thioredoxin fold domains (b,b'), an acidic domain (c) and a C-terminal endoplasmic reticulum (ER) retention signal. Although PDI resides in the ER where it mediates the folding of nascent polypeptides of the secretory pathway, we recently showed that PDI5 of Arabidopsis thaliana chaperones and inhibits cysteine proteases during trafficking to vacuoles prior to programmed cell death of the endothelium in developing seeds. Here we describe Arabidopsis PDI2, which shares a primary structure similar to that of classical PDI. Recombinant PDI2 is imported into ER-derived microsomes and complements the E. coli protein-folding mutant, dsbA. PDI2 interacted with proteins in both the ER and nucleus, including ER-resident protein folding chaperone, BiP1, and nuclear embryo transcription factor, MEE8. The PDI2-MEE8 interaction was confirmed to occur in vitro and in vivo. Transient expression of PDI2-GFP fusions in mesophyll protoplasts resulted in labeling of the ER, nucleus and vacuole. PDI2 is expressed in multiple tissues, with relatively high expression in seeds and root tips. Immunoelectron microscopy with GFP- and PDI2-specific antisera on transgenic seeds (PDI2-GFP) and wild type roots demonstrated that PDI2 was found in the secretory pathway (ER, Golgi, vacuole, cell wall) and the nuclei. Our results indicate that PDI2 mediates protein folding in the ER and has new functional roles in the nucleus.

The small GTPase Rab5a is essential for intracellular transport of proglutelin from the Golgi apparatus to the protein storage vacuole and endosomal membrane organization in developing rice endosperm

Rice (Oryza sativa) glutelins are synthesized on the endoplasmic reticulum as larger precursors, which are then transported via the Golgi to the protein storage vacuole (PSV), where they are processed into acidic and basic subunits. Three independent glutelin precursor mutant4 (glup4) rice lines, which accumulated elevated levels of proglutelin over the wild type, were identified as loss-of-function mutants of Rab5a, the small GTPase involved in vesicular membrane transport. In addition to the plasma membrane, Rab5a colocalizes with glutelins on the Golgi apparatus, Golgi-derived dense vesicles, and the PSV, suggesting that Rab5a participates in the transport of the proglutelin from the Golgi to the PSV. This spatial distribution pattern was dramatically altered in the glup4 mutants. Numerous smaller protein bodies containing glutelin and α-globulin were evident, and the proteins were secreted extracellularly. Moreover, all three independent glup4 allelic lines displayed the novel appearance of a large dilated, structurally complex paramural body containing proglutelins, α-globulins, membrane biomarkers for the Golgi apparatus, prevacuolar compartment, PSV, and the endoplasmic reticulum luminal chaperones BiP and protein disulfide isomerase as well as β-glucan. These results indicate that the formation of the paramural bodies in glup4 endosperm was due to a significant disruption of endocytosis and membrane vesicular transport by Rab5a loss of function. Overall, Rab5a is required not only for the intracellular transport of proglutelins from the Golgi to the PSV in rice endosperm but also in the maintenance of the general structural organization of the endomembrane system in developing rice seeds.

Plant RMR proteins: unique vacuolar sorting receptors that couple ligand sorting with membrane internalization

In receptor-mediated sorting of soluble protein ligands in the endomembrane system of eukaryotic cells, three completely different receptor proteins for mammalian (mannose 6-phosphate receptor), yeast (Vps10p) and plant cells (vacuolar sorting receptor; VSR) have in common the features of pH-dependent ligand binding and receptor recycling. In striking contrast, the plant receptor homology-transmembrane-RING-H2 (RMR) proteins serve as sorting receptors to a separate type of vacuole, the protein storage vacuole, but do not recycle, and their trafficking pathway results in their internalization into the destination vacuole. Even though plant RMR proteins share high sequence similarity with the best-characterized mammalian PA-TM-RING family proteins, these two families of proteins appear to play distinctly different roles in plant and animal cells. Thus, this minireview focuses on this unique sorting mechanism and traffic of RMR proteins via dense vesicles in various plant cell types.

Protein disulfide isomerase like 1-1 participates in the maturation of proglutelin within the endoplasmic reticulum in rice endosperm

The rice esp2 mutation was previously characterized by the abnormal accumulation of elevated levels of proglutelin and the absence of an endosperm-specific protein disulfide isomerase like (PDIL1-1). Here we show that Esp2 is the structural gene for PDIL1-1 and that this lumenal chaperone is asymmetrically distributed within the cortical endoplasmic reticulum (ER) and largely restricted to the cisternal ER. Temporal studies indicate that PDIL1-1 is essential for the maturation of proglutelin only when its rate of synthesis significantly exceeds its export from the ER, a condition resulting in its build up in the ER lumen and the induction of ER quality control processes which lower glutelin levels as well as those of the other storage proteins. As proglutelin is initially synthesized on the cisternal ER, its deposition within prolamine protein bodies in esp2 suggests that PDIL1-1 helps retain proglutelin in the cisternal ER lumen until it attains competence for ER export and, thereby, indirectly preventing heterotypic interactions with prolamine polypeptides.

The changing fate of a secretory glycoprotein in developing maize endosperm

Zeins are the major storage proteins in maize (Zea mays) endosperm, and their accumulation in zein bodies derived from the endoplasmic reticulum is well characterized. In contrast, relatively little is known about post-Golgi compartments or the trafficking of vacuolar proteins in maize endosperm, specifically the presence of globulins in structures resembling protein storage vacuoles that appear in early to mid-stage seed development. We investigated this pathway by expressing and analyzing a recombinant reporter glycoprotein during endosperm maturation, using a combination of microscopy and sensitive glycopeptide analysis. Specific N-glycan acceptor sites on the protein were followed through the stages of grain development, revealing a shift from predominantly paucimannosidic vacuolar glycoforms to predominantly trimmed glycan structures lacking fucose. This was accompanied by a change in the main subcellular localization of the protein from large protein storage vacuole-like post-Golgi organelles to the endoplasmic reticulum and zein bodies. The endogenous storage proteins corn alpha-globulin and corn legumin-1 showed a similar spatiotemporal profile both in transgenic plants expressing the reporter glycoprotein and in wild-type plants. This indicates that the shift of the intracellular trafficking route, as observed with our reporter glycoprotein, may be a common strategy in maize seed development.

Crystal structure and functional insights of hemopexin fold protein from grass pea

A regulatory protein from grass pea (Lathyrus sativus), LS-24, a close homolog of albumin 2 from garden pea (Pisum sativum) that is associated with polyamine biosynthesis, was characterized and the structure of a hemopexin-type fold among plant proteins illustrated. Crystal structure of LS-24 determined at 2.2 A resolution by multiple isomorphous replacement phasing showed four-bladed beta-propeller structure having a pseudo 4-fold molecular symmetry along a metal ion-binding central channel. The structure represents typical mammalian hemopexin fold with discernible features correlated with the possible functional variations. The protein was found to exist in the dimeric state. While LS-24 dimer binds to spermine in the crystal structure as well as in solution, binding of heme in solution resulted in the dissociation of the dimer into monomers with concomitant release of bound spermine. Interactions of heme and spermine with LS-24 bear physiological implications. While binding of spermine to LS-24 can be linked with polyamine biosynthesis that of heme correlates with oxidative stress. Mutually exclusive binding of heme and spermine in different oligomeric states suggest a role for LS-24 in sensing oxidative stress through a ligand-regulated monomer-dimer transition switch.

GNOM-LIKE1/ERMO1 and SEC24a/ERMO2 are required for maintenance of endoplasmic reticulum morphology in Arabidopsis thaliana

The endoplasmic reticulum (ER) is composed of tubules, sheets, and three-way junctions, resulting in a highly conserved polygonal network in all eukaryotes. The molecular mechanisms responsible for the organization of these structures are obscure. To identify novel factors responsible for ER morphology, we employed a forward genetic approach using a transgenic Arabidopsis thaliana plant (GFP-h) with fluorescently labeled ER. We isolated two mutants with defects in ER morphology and designated them endoplasmic reticulum morphology1 (ermo1) and ermo2. The cells of both mutants developed a number of ER-derived spherical bodies, approximately 1 microm in diameter, in addition to the typical polygonal network of ER. The spherical bodies were distributed throughout the ermo1 cells, while they formed a large aggregate in ermo2 cells. We identified the responsible gene for ermo1 to be GNOM-LIKE1 (GNL1) and the gene for ermo2 to be SEC24a. Homologs of both GNL1 and SEC24a are involved in membrane trafficking between the ER and Golgi in yeast and animal cells. Our findings, however, suggest that GNL1/ERMO1 and SEC24a/ERMO2 have a novel function in ER morphology in higher plants.

Protein storage vacuoles of Brassica napus zygotic embryos accumulate a BURP domain protein and perturbation of its production distorts the PSV

BNM2is a prototypical member of the enigmatic BURP domain protein family whose members contain the signature FX6-7GX10-28PX25-31CX11-12X2SX45-56CHX10 CHX25-29CHX2TX15-16PX5CH in the C-terminus. This protein family occurs only in plants, and the cognate genes vary very widely in their expression contexts in vegetative and reproductive tissues. None of theBURP family members has been assigned any biochemical function. BNM2 was originally discovered as a gene expressed in microspore derived embryos (MDE) of Brassica napus but we found that MDE do not contain the corresponding protein. We show that BNM2 protein production is confined to the seeds and localized to the protein storage vacuoles (PSV) even though the transcript is found in vegetative parts and floral buds as well. In developing seeds, transcript accumulation precedes protein appearance by more than 18 days. RNA accumulation peaks at approximately 20 days post anthesis (DPA) whereas protein accumulation reaches its maximum at approximately 40 DPA. Transgenic expression of BNM2 does not abrogate this regulation to yield ectopic protein production or to alter the temporal aspect ofBNM2 accumulation. Overexpression ofBNM2 led to spatial distortion of storage protein accumulation within PSV and to some morphological alterations of PSVs. However, the overall storage protein content was not altered.

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.

Suppression of Arabidopsis vesicle-SNARE expression inhibited fusion of H2O2-containing vesicles with tonoplast and increased salt tolerance

Intracellular vesicle trafficking performs essential functions in eukaryotic cells, such as membrane trafficking and delivery of molecules to their destinations. A major endocytotic route in plants is vesicle trafficking to the vacuole that plays an important role in plant salt tolerance. The final step in this pathway is mediated by the AtVAMP7C family of vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptors (v-SNAREs) that carry out the vesicle fusion with the tonoplast. Exposure to high-salt conditions causes immediate ionic and osmotic stresses, followed by production of reactive oxygen species. Here, we show that the reactive oxygen species are produced intracellularly, in endosomes that were targeted to the central vacuole. Suppression of the AtVAMP7C genes expression by antisense AtVAMP711 gene or in mutants of this family inhibited fusion of H2O2-containing vesicles with the tonoplast, which resulted in formation of H2O2-containing megavesicles that remained in the cytoplasm. The antisense and mutant plants exhibited improved vacuolar functions, such as maintenance of DeltapH, reduced release of calcium from the vacuole, and greatly improved plant salt tolerance. The antisense plants exhibited increased calcium-dependent protein kinase activity upon salt stress. Improved vacuolar ATPase activity during oxidative stress also was observed in a yeast system, in a DeltaVamp7 knockout strain. Interestingly, a microarray-based analysis of the AtVAMP7C genes showed a strong down-regulation of most genes in wild-type roots during salt stress, suggesting an evolutionary molecular adaptation of the vacuolar trafficking.

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 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.

Subcellular localization of Arabidopsis 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Plants produce diverse isoprenoids, which are synthesized in plastids, mitochondria, endoplasmic reticulum (ER), and the nonorganellar cytoplasm. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the synthesis of mevalonate, a rate-limiting step in the cytoplasmic pathway. Several branches of the pathway lead to the synthesis of structurally and functionally varied, yet essential, isoprenoids. Several HMGR isoforms have been identified in all plants examined. Studies based on gene expression and on fractionation of enzyme activity suggested that subcellular compartmentalization of HMGR is an important intracellular channeling mechanism for the production of the specific classes of isoprenoids. Plant HMGR has been shown previously to insert in vitro into the membrane of microsomal vesicles, but the final in vivo subcellular localization(s) remains controversial. To address the latter in Arabidopsis (Arabidopsis thaliana) cells, we conducted a multipronged microscopy and cell fractionation approach that included imaging of chimeric HMGR green fluorescent protein localizations in transiently transformed cell leaves, immunofluorescence confocal microscopy in wild-type and stably transformed seedlings, immunogold electron microscopy examinations of endogenous HMGR in seedling cotyledons, and sucrose density gradient analyses of HMGR-containing organelles. Taken together, the results reveal that endogenous Arabidopsis HMGR is localized at steady state within ER as expected, but surprisingly also predominantly within spherical, vesicular structures that range from 0.2- to 0.6-microm diameter, located in the cytoplasm and within the central vacuole in differentiated cotyledon cells. The N-terminal region, including the transmembrane domain of HMGR, was found to be necessary and sufficient for directing HMGR to ER and the spherical structures. It is believed, although not directly demonstrated, that these vesicle-like structures are derived from segments of HMGR-ER. Nevertheless, they represent a previously undescribed subcellular compartment likely capable of synthesizing mevalonate, which provides new evidence for multiorganelle compartmentalization of the isoprenoid biosynthetic pathways in plants.

High-coverage profiling analysis of genes expressed during rice seed development, using an improved amplified fragment length polymorphism technique

A novel method, based on quantitative PCR and amplified fragment length polymorphism was applied to the analysis of high-coverage gene expression profiles during the development of rice seeds. This represents the first report of the application of this method to plants, which permitted the detection and analysis of approximately 70% of all the genes that are expressed in rice. The method was used to compare gene expression at different developmental stages, subspecies or cultivars, and phyletic lines to identify genes of interest through differences in their level of expression. Using this approach, even novel anonymous genes could be detected. Examples of these include the soluble starch synthase (SS) II-I and the rice branching enzyme 4 (rbe4) genes in the starch synthesis pathway. A profiling database was compiled and the results compared with public data on full-length cDNA sequences of rice. The method enables candidate novel genes to be immediately identified among the large numbers of genes that are expressed during the development of rice seeds. Our results will contribute to a better understanding of comparative transcriptomics in all plant species.

Identification of multivesicular bodies as prevacuolar compartments in Nicotiana tabacum BY-2 cells

Little is known about the dynamics and molecular components of plant prevacuolar compartments (PVCs). We have demonstrated recently that vacuolar sorting receptor (VSR) proteins are concentrated on PVCs. In this study, we generated transgenic Nicotiana tabacum (tobacco) BY-2 cell lines expressing two yellow fluorescent protein (YFP)-fusion reporters that mark PVC and Golgi organelles. Both transgenic cell lines exhibited typical punctate YFP signals corresponding to distinct PVC and Golgi organelles because the PVC reporter colocalized with VSR proteins, whereas the Golgi marker colocalized with mannosidase I in confocal immunofluorescence. Brefeldin A induced the YFP-labeled Golgi stacks but not the YFP-marked PVCs to form typical enlarged structures. By contrast, wortmannin caused YFP-labeled PVCs but not YFP-labeled Golgi stacks to vacuolate. VSR antibodies labeled multivesicular bodies (MVBs) on thin sections prepared from high-pressure frozen/freeze substituted samples, and the enlarged PVCs also were indentified as MVBs. MVBs were further purified from BY-2 cells and found to contain VSR proteins via immunogold negative staining. Similar to YFP-labeled Golgi stacks, YFP-labeled PVCs are mobile organelles in BY-2 cells. Thus, we have unequivocally identified MVBs as PVCs in N. tabacum BY-2 cells. Uptake studies with the styryl dye FM4-64 strongly indicate that PVCs also lie on the endocytic pathway of BY-2 cells.

An ER-localized form of PV72, a seed-specific vacuolar sorting receptor, interferes the transport of an NPIR-containing proteinase in Arabidopsis leaves

Putative vacuolar sorting receptors that bind to the vacuolar targeting signals have been found in various plants; pumpkin PV72, pea BP-80 and Arabidopsis AtELP. PV72 is a seed-specific receptor that is predicted to sort seed storage proteins to protein storage vacuoles. Analysis by surface plasmon resonance showed that the lumenal domain of PV72 bound to an NPIR (a typical vacuolar targeting signal)-containing peptide of the precursor of a cysteine proteinase, AtALEU, in the presence of Ca(2+) (K(D) = 0.1 micro M). To elucidate the receptor-dependent transport of vacuolar proteins in plant cells, we produced transgenic Arabidopsis plants that expressed a fusion protein (PV72-HDEL) composed of the lumenal domain of PV72 and an endoplasmic reticulum (ER)-retention signal, HDEL. The expression of PV72-HDEL induced the accumulation of the AtALEU precursor. The accumulation level of the AtALEU precursor was dependent on that of PV72-HDEL. In contrast, it did not induce the accumulation of a precursor of another cysteine proteinase, RD21, which contains no NPIR. Detailed subcellular localization revealed that both the AtALEU precursor and PV72-HDEL accumulated in the ER fraction. We found that most of the AtALEU precursor molecules formed a complex with PV72-HDEL. The AtALEU precursor might be trapped by PV72-HDEL in the ER and not transported to the vacuoles. This in-planta analysis supports the hypothesis that an Arabidopsis homolog of PV72 functions as a sorting receptor for the NPIR-containing proteinase. The overall results suggest that vacuolar sorting receptors for the protein storage vacuoles and the lytic vacuoles share the similar recognition mechanism for a vacuolar targeting signal.

Vacuolar sorting receptor for seed storage proteins in Arabidopsis thaliana

The seeds of higher plants accumulate large quantities of storage protein. During seed maturation, storage protein precursors synthesized on rough endoplasmic reticulum are sorted to protein storage vacuoles, where they are converted into the mature forms and accumulated. Previous attempts to determine the sorting machinery for storage proteins have not been successful. Here we show that a type I membrane protein, AtVSR1/AtELP, of Arabidopsis functions as a sorting receptor for storage proteins. The atvsr1 mutant missorts storage proteins by secreting them from cells, resulting in an enlarged and electron-dense extracellular space in the seeds. The atvsr1 seeds have distorted cells and smaller protein storage vacuoles than do WT seeds, and atvsr1 seeds abnormally accumulate the precursors of two major storage proteins, 12S globulin and 2S albumin, together with the mature forms of these proteins. AtVSR1 was found to bind to the C-terminal peptide of 12S globulin in a Ca2+-dependent manner. These findings demonstrate a receptor-mediated transport of seed storage proteins to protein storage vacuoles in higher plants.

A wound-inducible organelle derived from endoplasmic reticulum: a plant strategy against environmental stresses?

Endoplasmic reticulum (ER) is the most multitalented and adaptable compartment in plant cells. Recently, a wound-inducible organelle, which is derived from ER and designated the ER body, was found in Arabidopsis. Wounding and methyl jasmonate induce many ER bodies in rosette leaves, which have no ER bodies under normal conditions. In contrast, tender seedlings have a wide distribution of the ER bodies especially in all the epidermal cells, which are easily stressed by the external environment. The ER bodies play a role in a novel and unique type of endomembrane system that is involved in the response of plant cells to environmental stress and wounding.

Foreign gene products can be enhanced by introduction into low storage protein mutants

Transgenic rice expressing soybean glycinin in its endosperm was crossed with two types of low-glutelin mutants to determine how much storage the protein mutants can contribute to increases in glycinin accumulation. The glycinin level (102 microg/100 mg seed) in the parental transgenic line was enhanced to approximately 224-237 microg/100 mg seed within a genetic background deficient in glutelin (i.e. of low glutelins). The enrichment of this foreign gene product was compensated by a decrease in the expression of other endogenous prolamine and globulin storage proteins, resulting in an almost equivalent total amount of seed storage proteins. These results show that low storage protein mutants can provide potentially useful hosts for the expression of foreign genes, allowing a higher-level accumulation, because they can provide wider space for the accumulation of foreign gene products than in the normal host plant.

The ER body, a novel endoplasmic reticulum-derived structure in Arabidopsis

Plant cells develop various endoplasmic reticulum (ER)-derived structures with specific functions. The ER body, a novel ER-derived compartment in Arabidopsis, is a spindle-shaped structure (approximately 10 microm long and approximately 1 microm wide) that is surrounded by ribosomes. Similar structures were found in many Brassicaceae plants in the 1960s and 1970s, but their main components and biological functions have remained unknown. ER bodies can be visualized in transgenic Arabidopsis expressing the green fluorescent protein with an ER-retention signal. A large number of ER bodies are observed in cotyledons, hypocotyls and roots of seedlings, but very few are observed in rosette leaves. Recently nai1, a mutant that does not develop ER bodies in whole seedlings, was isolated. Analysis of the nai1 mutant reveals that a beta-glucosidase, called PYK10, is the main component of ER bodies. The putative biological function of PYK10 and the inducibility of ER bodies in rosette leaves by wound stress suggest that the ER body functions in the defense against herbivores.

Protein transport into secondary plastids and the evolution of primary and secondary plastids

Chloroplasts are key organelles in algae and plants due to their photosynthetic abilities. They are thought to have evolved from prokaryotic cyanobacteria taken up by a eukaryotic host cell in a process termed primary endocytobiosis. In addition, a variety of organisms have evolved by subsequent secondary endocytobioses, in which a heterotrophic host cell engulfed a eukaryotic alga. Both processes dramatically enhanced the complexity of the resulting cells. Since the first version of the endosymbiotic theory was proposed more than 100 years ago, morphological, physiological, biochemical, and molecular data have been collected substantiating the emerging picture about the origin and the relationship of individual organisms with different primary or secondary chloroplast types. Depending on their origin, plastids in different lineages may have two, three, or four envelope membranes. The evolutionary success of endocytobioses depends, among other factors, on the specific exchange of molecules between the host and endosymbiont. This raises questions concerning how targeting of nucleus-encoded proteins into the different plastid types occurs and how these processes may have developed. Most studies of protein translocation into plastids have been performed on primary plastids, but in recent years more complex protein-translocation systems of secondary plastids have been investigated. Analyses of transport systems in different algal lineages with secondary plastids reveal that during evolution existing translocation machineries were recycled or recombined rather than being developed de novo. This review deals with current knowledge about the evolution and function of primary and secondary plastids and the respective protein-targeting systems.

Uncleaved legumin in developing maize endosperm: identification, accumulation and putative subcellular localization

While identifying proteins present in the cytoskeleton and protein body fractions from maize (Zea mays L.) endosperm, a 51 kDa protein was discovered in a fraction containing small (approximately 200 nm in diameter) protein bodies. Based on partial amino acid sequences of V8 protease fragments, degenerate primers were made and fragments of cDNA encoding these partial sequences were cloned. Using 3' and 5' PCR, a full-length cDNA encoding this 51 kDa protein was obtained, which was identified as legumin-1. In other plants, this protein is generally cleaved into 20 and 35 kDa subunits after synthesis. However, SDS-PAGE of both the native and denatured protein indicates that cleavage does not occur in corn endosperm, even though the cleavage site (asparagine) is conserved. The lack of cleavage is presumably because the canonical cleavage sequence downstream from the cleavage site is almost totally absent. levels of transcript and encoded protein were compared in all three varieties and it was shown that both are more abundant in wild-type maize than in opaque-2 or sweet corn. Finally, using TEM, it was shown that the protein apparently occurs in morphologically distinct protein bodies, very similar to the protein bodies in legumes.

A novel ER-derived compartment, the ER body, selectively accumulates a beta-glucosidase with an ER-retention signal in Arabidopsis

The ER body is a novel compartment that is derived from endoplasmic reticulum (ER) in Arabidopsis. In contrast to whole seedlings which have a wide distribution of the ER bodies, rosette leaves have no ER bodies. Recently, we reported that wound stress induces the formation of many ER bodies in rosette leaves, suggesting that the ER body plays a role in the defense system of plants. ER bodies were visualized in transgenic plants (GFP-h) expressing green fluorescent protein (GFP) with an ER-retention signal, HDEL. These were concentrated in a 1000-g pellet (P1) of GFP-h plants. We isolated an Arabidopsis mutant, nai1, in which fluorescent ER bodies were hardly detected in whole plants. We found that a 65-kDa protein was specifically accumulated in the P1 fraction of GFP-h plants, but not in the P1 fraction of nai1 plants. N-terminal peptide sequencing revealed that the 65-kDa protein was a beta-glucosidase, PYK10, with an ER-retention signal, KDEL. Immunocytochemistry showed that PYK10 was localized in the ER bodies. Compared with the accumulation of GFP-HDEL, which was associated with both cisternal ER and ER bodies, the accumulation of PYK10 was much more specific to ER bodies. PYK10 was one of the major proteins in cotyledons, hypocotyls and roots of Arabidopsis seedlings, while PYK10 was not detected in rosette leaves that have no ER bodies. These findings indicated that PYK10 is the main component of ER bodies. It is possible that PYK10 produces defense compounds when plants are damaged by insects or wounding.

mRNA localization in plants: targeting to the cell's cortical region and beyond

mRNA localization efficiently targets gene products to specific subcellular compartments, thereby controlling cell fate and polar cell growth. Plants also localize RNAs to target proteins in the endomembrane system, and to different tissues by long-distance transport. Recent developments support the existence of multiple pathways that transport RNA to the cortical region of the cell and to different tissues of the plant.

The rice mutant esp2 greatly accumulates the glutelin precursor and deletes the protein disulfide isomerase

Rice (Oryza sativa) accumulates prolamins and glutelins as storage proteins. The latter storage protein is synthesized on the endoplasmic reticulum (ER) as a 57-kD proglutelin precursor, which is then processed into acidic and basic subunits in the protein storage vacuole. Three esp2 mutants, CM1787, EM44, and EM747, contain larger amounts of the 57-kD polypeptide and corresponding lower levels of acidic and basic glutelin subunits than normal. Electron microscopic observation revealed that esp2 contained normal-appearing glutelin-containing protein bodies (PB-II), but lacked the normal prolamin-containing PB (PB-I). Instead, numerous small ER-derived PBs of uniform size (0.5 microm in diameter) and low electron density were readily observed. Immunoblot analysis of purified subcellular fractions and immunocytochemistry at the electron microscopy level showed that these new PBs contained the 57-kD proglutelin precursor and prolamin polypeptides. The 57-kD proglutelin was extracted with 1% (v/v) lactic acid solution only after removal of cysteine-rich prolamin polypeptides, suggesting that these proteins form glutelin-prolamin aggregates via interchain disulfide bonds within the ER lumen. The endosperm of esp2 mutants contains the lumenal chaperones, binding protein and calnexin, but lacks protein disulfide isomerase (PDI) at the protein and RNA levels. The transcript of PDI was expressed in the seed only during the early stage of seed development in the wild type. These results suggest that PDI plays an essential role in the segregation of proglutelin and prolamin polypeptides within the ER lumen.

Membrane anchors for vacuolar targeting: application in plant bioreactors

Transgenic plants are attractive expression systems for producing recombinant proteins. Plant cells compartmentalize and store metabolites and proteins in vacuoles, but foreign proteins need to be targeted to the correct compartments if they are to accumulate in a stable fashion. Here we present a general strategy in which unique transmembrane and cytoplasmic tail sequences are used as anchors for delivering recombinant proteins via distinct vesicular transport pathways to specific vacuolar compartments where stable accumulation can occur.

A complex and mobile structure forms a distinct subregion within the continuous vacuolar membrane in young cotyledons of Arabidopsis

The plant vacuole is a multifunctional organelle which is essential for growth and development. To visualize the dynamics of plant vacuolar membranes, gamma-TIP (tonoplast intrinsic protein) was fused to GFP and expressed in Arabidopsis thaliana. The marker molecule was targeted to the vacuolar membranes in most tissues, as expected. In rapidly expanding cells, some additional spherical structures were often observed within the lumen of vacuoles, which emitted strong fluorescence. To confirm their normal presence, we examined wild-type Arabidopsis cotyledons by transmission electron microscopy. The metal-contact rapid-freezing method revealed that the vacuolar lumen of epidermal cells contained many cytoplasmic projections, which often formed spherical structures (1-3 microm diameter) consisting of double membranes. Thus we concluded that these structures are authentic and named them 'bulbs'. Three-dimensional reconstruction from serial electron microscopic images demonstrates that bulbs are very intricately folded, but are continuous with the limiting vacuolar membrane. The fluorescence intensity of bulbs is about threefold higher than that of vacuolar membrane. GFP-AtRab75c, another marker of the vacuole, did not give fluorescent signals of bulbs in transgenic plants, but the existence of bulbs was still confirmed by electron microscopy. These results suggest that bulbs define a subregion in the continuous vacuolar membrane, where some proteins are concentrated and others segregated.