Influence of KDEL on the fate of trimeric or assembly-defective phaseolin: selective use of an alternative route to vacuoles

Two γ-zeins induce the unfolded protein response

The rapid, massive synthesis of storage proteins that occurs during seed development stresses endoplasmic reticulum (ER) homeostasis, which activates the ER unfolded protein response (UPR). However, how different storage proteins contribute to UPR is not clear. We analyzed vegetative tissues of transgenic Arabidopsis (Arabidopsis thaliana) plants constitutively expressing the common bean (Phaseolus vulgaris) soluble vacuolar storage protein PHASEOLIN (PHSL) or maize (Zea mays) prolamins (27-kDa γ-zein or 16-kDa γ-zein) that participate in forming insoluble protein bodies in the ER. We show that 16-kDa γ-zein significantly activates the INOSITOL REQUIRING ENZYME1/BASIC LEUCINE ZIPPER 60 (bZIP60) UPR branch-but not the bZIP28 branch or autophagy-leading to induction of major UPR-controlled genes that encode folding helpers that function inside the ER. Protein blot analysis of IMMUNOGLOBULIN-BINDING PROTEIN (BIP) 1 and 2, BIP3, GLUCOSE REGULATED PROTEIN 94 (GRP94), and ER-localized DNAJ family 3A (ERDJ3A) polypeptides confirmed their higher accumulation in the plant expressing 16-kDa γ-zein. Expression of 27-kDa γ-zein significantly induced only BIP3 and ERDJ3A transcription even though an increase in GRP94 and BIP1/2 polypeptides also occurred in this plant. These results indicate a significant but weaker effect of 27-kDa γ-zein compared to 16-kDa γ-zein, which corresponds with the higher availability of 16-kDa γ-zein for BIP binding, and indicates subtle protein-specific modulations of plant UPR. None of the analyzed genes was significantly induced by PHSL or by a mutated, soluble form of 27-kDa γ-zein that traffics along the secretory pathway. Such variability in UPR induction may have influenced the evolution of storage proteins with different tissue and subcellular localization.

A Model for ERD2 Function in Higher Plants

ER lumenal proteins have a K(H)DEL motif at their C-terminus. This is recognized by the ERD2 receptor (KDEL receptor in animals), which localizes to the Golgi apparatus and serves to capture escaped ER lumenal proteins. ERD2-ligand complexes are then transported back to the ER via COPI coated vesicles. The neutral pH of the ER causes the ligands to dissociate with the receptor being returned to the Golgi. According to this generally accepted scenario, ERD2 cycles between the ER and the Golgi, although it has been found to have a predominant Golgi localization. In this short article, we present a model for the functioning of ERD2 receptors in higher plants that explains why it is difficult to detect fluorescently tagged ERD2 proteins in the ER. The model assumes that the residence time for ERD2 in the ER is very brief and restricted to a specific domain of the ER. This is the small disc of ER immediately subjacent to the first cis-cisterna of the Golgi stack, representing specialized ER export and import sites and therefore constituting part of what is known as the "secretory unit", a mobile aggregate of ER domain plus Golgi stack. ERD2 molecules in the ER domain of the secretory unit may be small in number, transient and optically difficult to differentiate from the larger population of ERD2 molecules in the overlying Golgi stack in the confocal microscope.

Mutation of the OsSAC1 Gene, which Encodes an Endoplasmic Reticulum Protein with an Unknown Function, Causes Sugar Accumulation in Rice Leaves

Sugars are the most abundant organic compounds produced by plants, and can be used to build carbon skeletons and generate energy. The sugar accumulation 1 (OsSAC1) gene encodes a protein with an unknown function that exhibits four N-terminal transmembrane regions and two conserved domains of unknown function, DUF4220 and DUF594. OsSAC1 was found to be poorly and specifically expressed at the bottoms of young leaves and in the developing leaf sheaths. Subcellular location results showed that OsSAC1 was co-localized with ER:mCherry and targeted the endoplasmic reticulum (ER). OsSAC1 has been found to affect sugar partitioning in rice (Oryza sativa). I2/KI starch staining, ultrastructure observations and starch content measurements indicated that more and larger starch granules accumulated in ossac1 source leaves than in wild-type (WT) source leaves. Additionally, higher sucrose and glucose concentrations accumulated in the ossac1 source leaves than in WT source leaves, whereas lower sucrose and glucose concentrations were observed in the ossac1 young leaves and developing leaf sheaths than in those of the WT. Much greater expression of OsAGPL1 and OsAGPS1 (responsible for starch synthesis) and significantly less expression of OscFBP1, OscFBP2, OsSPS1 and OsSPS11 (responsible for sucrose synthesis) and OsSWEET11, OsSWEET14 and OsSUT1 (responsible for sucrose loading) occurred in ossac1 source leaves than in WT source leaves. A greater amount of the rice plasmodesmatal negative regulator OsGSD1 was detected in ossac1 young leaves and developing leaf sheaths than in those of the WT. These results suggest that ER-targeted OsSAC1 may indirectly regulate sugar partitioning in carbon-demanding young leaves and developing leaf sheaths.

The pharmaceutics from the foreign empire: the molecular pharming of the prokaryotic staphylokinase in Arabidopsis thaliana plants

Here, we present the application of microbiology and biotechnology for the production of recombinant pharmaceutical proteins in plant cells. To the best of our knowledge and belief it is one of few examples of the expression of the prokaryotic staphylokinase (SAK) in the eukaryotic system. Despite the tremendous progress made in the plant biotechnology, most of the heterologous proteins still accumulate to low concentrations in plant tissues. Therefore, the composition of expression cassettes to assure economically feasible level of protein production in plants remains crucial. The aim of our research was obtaining a high concentration of the bacterial anticoagulant factor—staphylokinase, in Arabidopsis thaliana seeds. The coding sequence of staphylokinase was placed under control of the β-phaseolin promoter and cloned between the signal sequence of the seed storage protein 2S2 and the carboxy-terminal KDEL signal sequence. The engineered binary vector pATAG-sak was introduced into Arabidopsis thaliana plants via Agrobacterium tumefaciens-mediated transformation. Analysis of the subsequent generations of Arabidopsis seeds revealed both presence of the sak and nptII transgenes, and the SAK protein. Moreover, a plasminogen activator activity of staphylokinase was observed in the protein extracts from seeds, while such a reaction was not observed in the leaf extracts showing seed-specific activity of the β-phaseolin promoter.

Phaseolin expression in tobacco chloroplast reveals an autoregulatory mechanism in heterologous protein translation

Plastid DNA engineering is a well-established research area of plant biotechnology, and plastid transgenes often give high expression levels. However, it is still almost impossible to predict the accumulation rate of heterologous protein in transplastomic plants, and there are many cases of unsuccessful transgene expression. Chloroplasts regulate their proteome at the post-transcriptional level, mainly through translation control. One of the mechanisms to modulate the translation has been described in plant chloroplasts for the chloroplast-encoded subunits of multiprotein complexes, and the autoregulation of the translation initiation of these subunits depends on the availability of their assembly partners [control by epistasy of synthesis (CES)]. In Chlamydomonas reinhardtii, autoregulation of endogenous proteins recruited in the assembly of functional complexes has also been reported. In this study, we revealed a self-regulation mechanism triggered by the accumulation of a soluble recombinant protein, phaseolin, in the stroma of chloroplast-transformed tobacco plants. Immunoblotting experiments showed that phaseolin could avoid this self-regulation mechanism when targeted to the thylakoids in transplastomic plants. To inhibit the thylakoid-targeted phaseolin translation as well, this protein was expressed in the presence of a nuclear version of the phaseolin gene with a transit peptide. Pulse-chase and polysome analysis revealed that phaseolin mRNA translation on plastid ribosomes was repressed due to the accumulation in the stroma of the same soluble polypeptide imported from the cytosol. We suggest that translation autoregulation in chloroplast is not limited to heteromeric protein subunits but also involves at least some of the foreign soluble recombinant proteins, leading to the inhibition of plastome-encoded transgene expression in chloroplast.

Protein body formation in the endoplasmic reticulum as an evolution of storage protein sorting to vacuoles: insights from maize γ-zein

The albumin and globulin seed storage proteins present in all plants accumulate in storage vacuoles. Prolamins, which are the major proteins in cereal seeds and are present only there, instead accumulate within the endoplasmic reticulum (ER) lumen as very large insoluble polymers termed protein bodies. Inter-chain disulfide bonds play a major role in polymerization and insolubility of many prolamins. The N-terminal domain of the maize prolamin 27 kD γ-zein is able to promote protein body formation when fused to other proteins and contains seven cysteine residues involved in inter-chain bonds. We show that progressive substitution of these amino acids with serine residues in full length γ-zein leads to similarly progressive increase in solubility and availability to traffic from the ER along the secretory pathway. Total substitution results in very efficient secretion, whereas the presence of a single cysteine is sufficient to promote partial sorting to the vacuole via a wortmannin-sensitive pathway, similar to the traffic pathway of vacuolar storage proteins. We propose that the mechanism leading to accumulation of prolamins in the ER is a further evolutionary step of the one responsible for accumulation in storage vacuoles.

Unconventional pathways of secretory plant proteins from the endoplasmic reticulum to the vacuole bypassing the Golgi complex

Studies on the basic mechanisms that regulate vacuolar delivering of proteins synthesized in the endoplasmic reticulum (ER) have a great importance in plant cell biology. Indeed, many aspects of plant physiology are affected by this intracellular traffic, for example, germination or reaction to biotic stresses due to the accumulation of storage proteins in seeds or enzymes in vegetative tissues, respectively. Up to now, the Golgi complex has been considered the main hub in the sorting of vacuolar secretory proteins; those polypeptides able to reach their final destination without the aid of this organelle are regarded as exceptions to an established route. This mini-review aims to emphasize the existence of several Golgi-independent pathways involved in the trafficking of different types of vacuolar proteins.

Auxiliary KChIP4a suppresses A-type K+ current through endoplasmic reticulum (ER) retention and promoting closed-state inactivation of Kv4 channels

In the brain and heart, auxiliary Kv channel-interacting proteins (KChIPs) co-assemble with pore-forming Kv4 α-subunits to form a native K(+) channel complex and regulate the expression and gating properties of Kv4 currents. Among the KChIP1-4 members, KChIP4a exhibits a unique N terminus that is known to suppress Kv4 function, but the underlying mechanism of Kv4 inhibition remains unknown. Using a combination of confocal imaging, surface biotinylation, and electrophysiological recordings, we identified a novel endoplasmic reticulum (ER) retention motif, consisting of six hydrophobic and aliphatic residues, 12-17 (LIVIVL), within the KChIP4a N-terminal KID, that functions to reduce surface expression of Kv4-KChIP complexes. This ER retention capacity is transferable and depends on its flanking location. In addition, adjacent to the ER retention motif, the residues 19-21 (VKL motif) directly promote closed-state inactivation of Kv4.3, thus leading to an inhibition of channel current. Taken together, our findings demonstrate that KChIP4a suppresses A-type Kv4 current via ER retention and enhancement of Kv4 closed-state inactivation.

BINDING PROTEIN is a master regulator of the endoplasmic reticulum stress sensor/transducer bZIP28 in Arabidopsis

BINDING PROTEIN (BiP) is a major chaperone in the endoplasmic reticulum (ER) lumen, and this study shows that BiP binds to the C-terminal tail of the stress sensor/transducer bZIP28, a membrane-associated transcription factor, retaining it in the ER under unstressed conditions. In response to ER stress, BiP dissociates from bZIP28, allowing it to be mobilized from the ER to the Golgi where it is proteolytically processed and released to enter the nucleus. Under unstressed conditions, BiP binds to bZIP28 as it binds to other client proteins, through its substrate binding domain. BiP dissociates from bZIP28 even when bZIP28's exit from the ER or its release from the Golgi is blocked. Both BiP1 and BiP3 bind bZIP28, and overexpression of either BiP detains bZIP28 in the ER under stress conditions. A C-terminally truncated mutant of bZIP28 eliminating most of the lumenal domain does not bind BiP and is not retained in the ER under unstressed conditions. BiP binding sites in the C-terminal tail of bZIP28 were identified in a phage display system. BiP was found to bind to intrinsically disordered regions on bZIP28's lumen-facing tail. Thus, the dissociation of BiP from the C-terminal tail of bZIP28 is a major switch that activates one arm of the unfolded protein response signaling pathway in plants.

Traffic of human α-mannosidase in plant cells suggests the presence of a new endoplasmic reticulum-to-vacuole pathway without involving the Golgi complex

The transport of secretory proteins from the endoplasmic reticulum to the vacuole requires sorting signals as well as specific transport mechanisms. This work is focused on the transport in transgenic tobacco (Nicotiana tabacum) plants of a human α-mannosidase, MAN2B1, which is a lysosomal enzyme involved in the turnover of N-linked glycoproteins and can be used in enzyme replacement therapy. Although ubiquitously expressed, α-mannosidases are targeted to lysosomes or vacuoles through different mechanisms according to the organisms in which these proteins are produced. In tobacco cells, MAN2B1 reaches the vacuole even in the absence of mannose-6-phosphate receptors, which are responsible for its transport in animal cells. We report that MAN2B1 is targeted to the vacuole without passing through the Golgi complex. In addition, a vacuolar targeting signal that is recognized in plant cells is located in the MAN2B1 amino-terminal region. Indeed, when this amino-terminal domain is removed, the protein is retained in the endoplasmic reticulum. Moreover, when this domain is added to a plant-secreted protein, the resulting fusion protein is partially redirected to the vacuole. These results strongly suggest the existence in plants of a new type of vacuolar traffic that can be used by leaf cells to transport vacuolar proteins.

Molecular and immunohistochemical characterization of the chitinase gene from Pieris rapae granulovirus

The chitinase gene of baculoviruses is expressed in the late phase of virus replication in insects and possesses high exo- and endochitinase activity, which can hydrolyze chitin in the body of the insect, thus promoting terminal host liquefaction. Alphabaculovirus viral chitinases (vChitA) have been well analyzed, but information regarding viral chitinases from betabaculoviruses is limited. Whole-genome sequencing of a Korean isolate of Pieris rapae GV (PiraGV-K) predicted a putative chitinase gene corresponding to ORF10. The PiraGV-K chitinase gene had a coding sequence of 1,761 bp, encoding a protein of 586 amino acid (aa) residues, including an 18-aa putative signal peptide. Time course induction pattern observed by SDS-PAGE and subsequent Western blot with anti-PiraGV-K chitinase antibody revealed the cleavage of the signal peptide from the intact chitinase. Edman sequencing analysis was further conducted to confirm the exact nature of the mature chitinase, and the N-terminal amino acid sequence (KPGAP) exactly matched the sequence following the signal peptide sequence. The transcriptomics of PiraGV-K chitinase in infected P. rapae larvae, examined by real-time PCR, revealed a significant 75-fold increase after four days of feeding with PiraGV-K-treated leaves, with a subsequent decline at the later stages of infection. Confocal microscopic analysis showed that PiraGV-K chitinase possibly exists as a secreted protein, with strong chitinase-specific signals in fat body cells and integument at four days postinfection. Furthermore, immunogold labeling and electron microscopy studies localized the PiraGV-K chitinase in the cytoplasm and sparsely within vacuolar structures in the fat body apart from the extensive aggregation in the cuticular lining of the integument.

Comparative study of wheat low-molecular-weight glutenin and α-gliadin trafficking in tobacco cells

KEY MESSAGE : Wheat low-molecular-weight-glutenin and α-gliadin were accumulated in the endoplasmic reticulum and formed protein body-like structures in tobacco cells, with the participation of BiP chaperone. Possible interactions between these prolamins were investigated. Wheat prolamins are the major proteins that accumulate in endosperm cells and are largely responsible for the unique biochemical properties of wheat products. They are accumulated in the endoplasmic reticulum (ER) where they form protein bodies (PBs) and are then transported to the storage vacuole where they form a protein matrix in the ripe seeds. Whereas previous studies have been carried out to determine the atypical trafficking pathway of prolamins, the mechanisms leading to ER retention and PB formation are still not clear. In this study, we examined the trafficking of a low-molecular-weight glutenin subunit (LMW-glutenin) and α-gliadin fused to fluorescent proteins expressed in tobacco cells. Through transient transformation in epidermal tobacco leaves, we demonstrated that both LMW-glutenin and α-gliadin were retained in the ER and formed mobile protein body-like structures (PBLS) that generally do not co-localise with Golgi bodies. An increased expression level of BiP in tobacco cells transformed with α-gliadin or LMW-glutenin was observed, suggesting the participation of this chaperone protein in the accumulation of wheat prolamins in tobacco cells. When stably expressed in BY-2 cells, LMW-glutenin fusion was retained longer in the ER before being exported to and degraded in the vacuole, compared with α-gliadin fusion, suggesting the involvement of intermolecular disulphide bonds in ER retention, but not in PBLS formation. Co-localisation experiments showed that gliadins and LMW-glutenin were found in the same PBLS with no particular distribution, which could be due to their ability to interact with each other as indicated by yeast two-hybrid assays.

Differential subcellular targeting of recombinant human α₁-proteinase inhibitor influences yield, biological activity and in planta stability of the protein in transgenic tomato plants

The response of protein accumulation site on yield, biological activity and in planta stability of therapeutic recombinant human proteinase inhibitor (α₁-PI) was analyzed via targeting to different subcellular locations, like endoplasmic reticulum (ER), apoplast, vacuole and cytosol in leaves of transgenic tomato plants. In situ localization of the recombinant α₁-PI protein in transgenic plant cells was monitored by immunohistochemical staining. Maximum accumulation of recombinant α₁-PI in T₀ and T₁ transgenic tomato plants was achieved from 1.5 to 3.2% of total soluble protein (TSP) by retention in ER lumen, followed by vacuole and apoplast, whereas cytosolic targeting resulted into degradation of the protein. The plant-derived recombinant α₁-PI showed biological activity for elastase inhibition, as monitored by residual porcine pancreatic elastase (PPE) activity assay and band-shift assay. Recombinant α₁-PI was purified from transgenic tomato plants with high yield, homogeneity and biological activity. Purified protein appeared as a single band of ∼48-50 kDa on SDS-PAGE with pI value ranging between 5.1 and 5.3. Results of mass spectrometry and optical spectroscopy of purified recombinant α₁-PI revealed the structural integrity of the recombinant protein comparable to native serum α₁-PI. Enzymatic deglycosylation and lectin-binding assays with the purified recombinant α₁-PI showed compartment-specific N-glycosylation of the protein targeted to ER, apoplast and vacuole. Conformational studies based on urea-induced denaturation and circular dichroism (CD) spectroscopy revealed relatively lower stability of the recombinant α₁-PI protein, compared to its serum counterpart. Pharmacokinetic evaluation of plant derived recombinant and human plasma-purified α₁-PI in rat, by intravenous route, revealed significantly faster plasma clearance and lower area under curve (AUC) of recombinant protein. Our data suggested significance of protein sorting sequences and feasibility to use transgenic plants for the production of stable, glycosylated and biologically active recombinant α₁-PI for further therapeutic applications.

Localization of rabies virus glycoprotein into the endoplasmic reticulum produces immunoprotective antigen

Rabies virus surface glycoprotein (rabies G-protein) with (G+RS) and without (G-RS) endoplasmic reticulum retrieval signal was expressed and characterized in tobacco plants. Transgenically expressed rabies G-protein was estimated at 0.015-0.38 % of total leaf protein. The relative migration of the rabies G-protein on SDS-PAGE was at the position, as anticipated for the viral coat protein (~66 kDa). Immunolocalization by confocal microscopy established that immunoprotective G+RS expressed in tobacco was primarily confined to ER. G+RS showed binding to Con A lectin and was susceptible to N-glycosidase F activity similar to native rabies G-protein. However, the G-RS transgenically expressed in tobacco leaves was glycosylated differently and was resitant to N-glycosidase F. Immunological studies and Rapid Fluorescent Foci Inhibition Test (RFFIT) showed that G+RS was immunogenic and immunoprotective, whereas G-RS was moderately immunogenic but non-protective against live virus challenge. Hence, plants can express the antigenic component of rabies virus with suitable glycosylation, which is important to give protection against rabies virus infection.

Influence of an ER-retention signal on the N-glycosylation of recombinant human α-L-iduronidase generated in seeds of Arabidopsis

Processes associated with late events of N-glycosylation within the plant Golgi complex are a major limitation to the use of plant-based systems to produce recombinant pharmaceutical proteins for parenteral administration. Specifically, sugars added to the N-glycans of a recombinant protein during glycan maturation to complex forms (e.g. β1,2 xylose and α1,3 fucose) can render the product immunogenic. In order to avoid these sugars, the human enzyme α-L-iduronidase (IDUA, EC 3.2.1.76), with a C-terminal ER-retention sequence SEKDEL, was expressed in seeds of complex-glycan-deficient (cgl) mutant and wild-type (Col-0) Arabidopsis thaliana, under the control of regulatory (5'-, signal-peptide-encoding-, and 3'-) sequences from the arcelin 5-I gene of Phaseolus vulgaris (cgl-IDUA-SEKDEL and Col-IDUA-SEKDEL, respectively). The SEKDEL motif had no adverse effect on the specific activity of the purified enzyme. Surprisingly, the majority of the N-glycans of Col-IDUA-SEKDEL were complex N-glycans (i.e. contained xylose and/or fucose) (88 %), whereas complex N-glycans comprised a much lower proportion of the N-glycans of cgl-IDUA-SEKDEL (26 %), in which high-mannose forms were predominant. In contrast to the non-chimeric IDUA of cgl seeds, which is mainly secreted into the extracellular spaces, the addition of the SEKDEL sequence to human recombinant IDUA expressed in the same background led to retention of the protein in ER-derived vesicles/compartments and its partial localization in protein storage vacuoles. Our data support the contention that the use of a C-terminal ER retention motif as an effective strategy to prevent or reduce complex N-glycan formation, is protein specific.

Secretory pathway research: the more experimental systems the better

Transient gene expression, in plant protoplasts or specific plant tissues, is a key technique in plant molecular cell biology, aimed at exploring gene products and their modifications to examine functional subdomains, their interactions with other biomolecules, and their subcellular localization. Here, we highlight some of the major advantages and potential pitfalls of the most commonly used transient gene expression models and illustrate how ectopic expression and the use of dominant mutants can provide insights into protein function.

A new type of compartment, defined by plant-specific Atg8-interacting proteins, is induced upon exposure of Arabidopsis plants to carbon starvation

Atg8 is a central protein in bulk starvation-induced autophagy, but it is also specifically associated with multiple protein targets under various physiological conditions to regulate their selective turnover by the autophagy machinery. Here, we describe two new closely related Arabidopsis thaliana Atg8-interacting proteins (ATI1 and ATI2) that are unique to plants. We show that under favorable growth conditions, ATI1 and ATI2 are partially associated with the endoplasmic reticulum (ER) membrane network, whereas upon exposure to carbon starvation, they become mainly associated with newly identified spherical compartments that dynamically move along the ER network. These compartments are morphologically distinct from previously reported spindle-shaped ER bodies and, in contrast to them, do not contain ER-lumenal markers possessing a C-terminal HDEL sequence. Organelle and autophagosome-specific markers show that the bodies containing ATI1 are distinct from Golgi, mitochondria, peroxisomes, and classical autophagosomes. The final destination of the ATI1 bodies is the central vacuole, indicating that they may operate in selective turnover of specific proteins. ATI1 and ATI2 gene expression is elevated during late seed maturation and desiccation. We further demonstrate that ATI1 overexpression or suppression of both ATI1 and ATI2, respectively, stimulate or inhibit seed germination in the presence of the germination-inhibiting hormone abscisic acid.

Human α-mannosidase produced in transgenic tobacco plants is processed in human α-mannosidosis cell lines

Deficiency in human lysosomal α-mannosidase (MAN2B1) results in α-mannosidosis, a lysosomal storage disorder; patients present a wide range of neurological, immunological, and skeletal symptoms caused by a multisystemic accumulation of mannose-containing oligosaccharides. Here, we describe the expression of recombinant MAN2B1 both transiently in Nicotiana benthamiana leaves and in the leaves and seeds of stably transformed N. tabacum plants. After purification from tobacco leaves, the recombinant enzyme was found to be N-glycosylated and localized in vacuolar compartments. In the fresh leaves of tobacco transformants, MAN2B1 was measured at 10,200 units/kg, and the purified enzyme from these leaves had a specific activity of 32-45 U/mg. Furthermore, tobacco-produced MAN2B1 was biochemically similar to the enzyme purified from human tissues, and it was internalized and processed by α-mannosidosis fibroblast cells. These results strongly indicate that plants can be considered a promising expression system for the production of recombinant MAN2B1 for use in enzyme replacement therapy.

A plant secretory signal peptide targets plastome-encoded recombinant proteins to the thylakoid membrane

Plastids are considered promising bioreactors for the production of recombinant proteins, but the knowledge of the mechanisms regulating foreign protein folding, targeting, and accumulation in these organelles is still incomplete. Here we demonstrate that a plant secretory signal peptide is able to target a plastome-encoded recombinant protein to the thylakoid membrane. The fusion protein zeolin with its native signal peptide expressed by tobacco (Nicotiana tabacum) transplastomic plants was directed into the chloroplast thylakoid membranes, whereas the zeolin mutant devoid of the signal peptide, Δzeolin, is instead accumulated in the stroma. We also show that zeolin folds in the thylakoid membrane where it accumulates as trimers able to form disulphide bonds. Disulphide bonds contribute to protein accumulation since zeolin shows a higher accumulation level with respect to stromal Δzeolin, whose folding is hampered as the protein accumulates at low amounts in a monomeric form and it is not oxidized. Thus, post-transcriptional processes seem to regulate the stability and accumulation of plastid-synthesized zeolin. The most plausible zeolin targeting mechanism to thylakoid is discussed herein.

Expression of a new chimeric protein with a highly repeated sequence in tobacco cells

In wheat, the high-molecular weight (HMW) glutenin subunits are known to contribute to gluten viscoelasticity, and show some similarities to elastomeric animal proteins as elastin. When combining the sequence of a glutenin with that of elastin is a way to create new chimeric functional proteins, which could be expressed in plants. The sequence of a glutenin subunit was modified by the insertion of several hydrophobic and elastic motifs derived from elastin (elastin-like peptide, ELP) into the hydrophilic repetitive domain of the glutenin subunit to create a triblock protein, the objective being to improve the mechanical (elastomeric) properties of this wheat storage protein. In this study, we investigated an expression model system to analyze the expression and trafficking of the wild-type HMW glutenin subunit (GS(W)) and an HMW glutenin subunit mutated by the insertion of elastin motifs (GS(M)-ELP). For this purpose, a series of constructs was made to express wild-type subunits and subunits mutated by insertion of elastin motifs in fusion with green fluorescent protein (GFP) in tobacco BY-2 cells. Our results showed for the first time the expression of HMW glutenin fused with GFP in tobacco protoplasts. We also expressed and localized the chimeric protein composed of plant glutenin and animal elastin-like peptides (ELP) in BY-2 protoplasts, and demonstrated its presence in protein body-like structures in the endoplasmic reticulum. This work, therefore, provides a basis for heterologous production of the glutenin-ELP triblock protein to characterize its mechanical properties.

Recombinant human GAD65 accumulates to high levels in transgenic tobacco plants when expressed as an enzymatically inactive mutant

The 65-kDa isoform of glutamic acid decarboxylase (GAD65) is the major autoantigen implicated in the development of type 1 diabetes mellitus (T1DM). The bulk manufacture of GAD65 is a potential issue in the fight against T1DM but current production platforms are expensive. We show that a catalytically inactive form of GAD65 (GAD65mut) accumulates at up to 2.2% total soluble protein in transgenic tobacco leaves, which is more than 10-fold the levels achieved with active GAD65, yet the protein retains the immunogenic properties required to treat T1DM. This higher yield was found to be a result of a higher rate of protein synthesis and not transcript availability or protein stability. We found that targeting GAD65 to the endoplasmic reticulum, a strategy that increases the accumulation of many recombinant proteins expressed in plants, did not improve production of GAD65mut. The production of a catalytically inactive autoantigen that retains its immunogenic properties could be a useful strategy to provide high-quality therapeutic protein for treatment of autoimmune T1DM.

Higher accumulation of F1-V fusion recombinant protein in plants after induction of protein body formation

Improving foreign protein accumulation is crucial for enhancing the commercial success of plant-based production systems since product yields have a major influence on process economics. Cereal grain evolved to store large amounts of proteins in tightly organized aggregates. In maize, gamma-Zein is the major storage protein synthesized by the rough endoplasmic reticulum (ER) and stored in specialized organelles called protein bodies (PB). Zera (gamma-Zein ER-accumulating domain) is the N-terminal proline-rich domain of gamma-zein that is sufficient to induce the assembly of PB formation. Fusion of the Zera domain to proteins of interest results in assembly of dense PB-like, ER-derived organelles, containing high concentration of recombinant protein. Our main goal was to increase recombinant protein accumulation in plants in order to enhance the efficiency of orally-delivered plant-made vaccines. It is well known that oral vaccination requires substantially higher doses than parental formulations. As a part of a project to develop a plant-made plague vaccine, we expressed our model antigen, the Yersinia pestis F1-V antigen fusion protein, with and without a fused Zera domain. We demonstrated that Zera-F1-V protein accumulation was at least 3x higher than F1-V alone when expressed in three different host plant systems: Ncotiana benthamiana, Medicago sativa (alfalfa) and Nicotiana tabacum NT1 cells. We confirmed the feasibility of using Zera technology to induce protein body formation in non-seed tissues. Zera expression and accumulation did not affect plant development and growth. These results confirmed the potential exploitation of Zera technology to substantially increase the accumulation of value-added proteins in plants.

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.

Induction of protein body formation in plant leaves by elastin-like polypeptide fusions

BACKGROUND

Elastin-like polypeptides are synthetic biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that are valuable for the simple non-chromatographic purification of recombinant proteins. In addition, elastin-like polypeptide fusions have been shown to enhance the accumulation of a range of different recombinant proteins in plants, thus addressing the major limitation of plant-based expression systems, which is a low production yield. This study's main objectives were to determine the general utility of elastin-like polypeptide protein fusions in various intracellular compartments and to elucidate elastin-like polypeptide's mechanism of action for increasing recombinant protein accumulation in the endoplasmic reticulum of plants.

RESULTS

The effect of elastin-like polypeptide fusions on the accumulation of green fluorescent protein targeted to the cytoplasm, chloroplasts, apoplast, and endoplasmic reticulum was evaluated. The endoplasmic reticulum was the only intracellular compartment in which an elastin-like polypeptide tag was shown to significantly enhance recombinant protein accumulation. Interestingly, endoplasmic reticulum-targeted elastin-like polypeptide fusions induced the formation of a novel type of protein body, which may be responsible for elastin-like polypeptide's positive effect on recombinant protein accumulation by excluding the heterologous protein from normal physiological turnover. Although expressed in the leaves of plants, these novel protein bodies appeared similar in size and morphology to the prolamin-based protein bodies naturally found in plant seeds. The elastin-like polypeptide-induced protein bodies were highly mobile organelles, exhibiting various dynamic patterns of movement throughout the cells, which were dependent on intact actin microfilaments and a functional actomyosin motility system.

CONCLUSION

An endoplasmic reticulum-targeted elastin-like polypeptide fusion approach provides an effective strategy for depositing large amounts of concentrated heterologous protein within the limited space of the cell via storage in stable protein bodies. Furthermore, encapsulation of recombinant proteins into physiologically inert organelles can function to insulate the protein from normal cellular mechanisms, thus limiting unnecessary stress to the host cell. Since elastin-like polypeptide is a mammalian-derived protein, this study demonstrates that plant seed-specific factors are not required for the formation of protein bodies in vegetative plant tissues, suggesting that the endoplasmic reticulum possesses an intrinsic ability to form protein body-like accretions in eukaryotic cells when overexpressing particular proteins.

{alpha}' Subunit of soybean {beta}-conglycinin forms complex with rice glutelin via a disulphide bond in transgenic rice seeds

The alpha' and beta subunits of soybean beta-conglycinin were expressed in rice seeds in order to improve the nutritional and physiological properties of rice as a food. The alpha' subunit accumulated in rice seeds at a higher level than the beta subunit, but no detectable difference in mRNA transcription level between subunits was observed. Sequential extraction results indicate that the alpha' subunit formed one or more disulphide bonds with glutelin. Electron microscopic analysis showed that the alpha' subunit and the beta subunit were transported to PB-II together with glutelin. In mature transgenic seeds, the beta subunit accumulated in low electron density regions in the periphery of PB-II, whereas the alpha' subunit accumulated together with glutelin in high-density regions of the periphery. The subcellular localization of mutated alpha' subunits lacking one cysteine residue in the N-terminal mature region (alpha'DeltaCys1) or five cysteine residues in the pro and N-terminal mature regions (alpha'DeltaCys5) were also examined. Low-density regions were formed in PB-II in mature seeds of transgenic rice expressing alpha'DeltaCys 5 and alpha'DeltaCys1. alpha'DeltaCys5 was localized only in the low-density regions, whereas alpha'DeltaCys1 was found in both low- and high-density regions. These results suggest that the alpha' subunit could make a complex via one or more disulphide bonds with glutelin and accumulate together in PB-II of transgenic rice seeds.

Transgenic chloroplasts are efficient sites for high-yield production of the vaccinia virus envelope protein A27L in plant cellsdagger

Orthopoxviruses (OPVs) have recently received increasing attention because of their potential use in bioterrorism and the occurrence of zoonotic OPV outbreaks, highlighting the need for the development of safe and cost-effective vaccines against smallpox and related viruses. In this respect, the production of subunit protein-based vaccines in transgenic plants is an attractive approach. For this purpose, the A27L immunogenic protein of vaccinia virus was expressed in tobacco using stable transformation of the nuclear or plastid genome. The vaccinia virus protein was expressed in the stroma of transplastomic plants in soluble form and accumulated to about 18% of total soluble protein (equivalent to approximately 1.7 mg/g fresh weight). This level of A27L accumulation was 500-fold higher than that in nuclear transformed plants, and did not decline during leaf development. Transplastomic plants showed a partial reduction in growth and were chlorotic, but reached maturity and set fertile seeds. Analysis by immunofluorescence microscopy indicated altered chlorophyll distribution. Chloroplast-synthesized A27L formed oligomers, suggesting correct folding and quaternary structure, and was recognized by serum from a patient recently infected by a zoonotic OPV. Taken together, these results demonstrate that chloroplasts are an attractive production vehicle for the expression of OPV subunit vaccines.

Viral and murine interleukin-10 are correctly processed and retain their biological activity when produced in tobacco

BACKGROUND

Interleukin-10 (IL-10) is a potent anti-inflammatory cytokine, with therapeutic applications in several autoimmune and inflammatory diseases. Oral administration of this cytokine alone, or in combination with disease-associated autoantigens could confer protection form the onset of a specific autoimmune disease through the induction of oral tolerance. Transgenic plants are attractive systems for production of therapeutic proteins because of the ability to do large scale-up at low cost, and the low maintenance requirements. They are highly amenable to oral administration and could become effective delivery systems without extensive protein purification. We investigated the ability of tobacco plants to produce high levels of biologically-active viral and murine IL-10.

RESULTS

Three different subcellular targeting strategies were assessed in transient expression experiments, and stable transgenic tobacco plants were generated with the constructs that yielded the highest accumulation levels by targeting the recombinant proteins to the endoplasmic reticulum. The best yields using this strategy in T1 plants were 10.8 and 37.0 microg/g fresh leaf weight for viral and murine IL-10, respectively. The recombinant proteins were purified from transgenic leaf material and characterized in terms of their N-glycan composition, dimerization and biological activity in in vitro assays. Both molecules formed stable dimers, were able to activate the IL-10 signaling pathway and to induce specific anti-inflammatory responses in mouse J774 macrophage cells.

CONCLUSION

Tobacco plants are able to correctly process viral and murine IL-10 into biologically active dimers, therefore representing a suitable platform for the production for these cytokines. The accumulation levels obtained are high enough to allow delivery of an immunologically relevant dose of IL-10 in a reasonable amount of leaf material, without extensive purification. This study paves the way to performing feeding studies in mouse models of autoimmune diseases, that will allow the evaluation the immunomodulatory properties and effectiveness of the viral IL-10 in inducing oral tolerance compared to the murine protein.

Plant-based strategies aimed at expressing HIV antigens and neutralizing antibodies at high levels. Nef as a case study

The first evidence that plants represent a valid, safe and cost-effective alternative to traditional expression systems for large-scale production of antigens and antibodies was described more than 10 years ago. Since then, considerable improvements have been made to increase the yield of plant-produced proteins. These include the use of signal sequences to target proteins to different cellular compartments, plastid transformation to achieve high transgene dosage, codon usage optimization to boost gene expression, and protein fusions to improve recombinant protein stability and accumulation. Thus, several HIV/SIV antigens and neutralizing anti-HIV antibodies have recently been successfully expressed in plants by stable nuclear or plastid transformation, and by transient expression systems based on plant virus vectors or Agrobacterium-mediated infection. The current article gives an overview of plant expressed HIV antigens and antibodies and provides an account of the use of different strategies aimed at increasing the expression of the accessory multifunctional HIV-1 Nef protein in transgenic plants.

Protein body induction: a new tool to produce and recover recombinant proteins in plants

Stable accumulation of storage proteins, lipids and carbohydrates is a hallmark of the plant seed, and is a characteristic that is typically deficient in existing platforms for recombinant protein manufacture. One of the biological sequestration mechanisms that facilitate the folding, assembly and stabilization of plant seed storage proteins involve the de novo formation of unique intracellular organelles, the endoplasmic reticulum (ER)-derived protein bodies (PBs). In cereals, such as maize, PBs are formed directly in the lumen of the ER of endosperm cells and contain zeins, a group of polypeptides, which account for more than half of the total seed protein mass. The 27 kD gamma zein protein localizes to the periphery of the PBs surrounding aggregates of other zeins (including a zein and delta zein). Heterologous expression of gamma zein has been shown to result in the formation of PB-like structures, and the N-terminal proline-rich domain of gamma zein (Zera), containing eight PPPVHL repeats and a Pro-X sequence is by itself capable of directing ER retention and PB formation in non-seed tissues. We present a novel approach to produce recombinant proteins in plants based on the ability of gamma zein-Zera domain to store recombinant proteins inside PBs. Zera domain fused to several proteins, including a enhanced cyan fluorescent protein (ECFP), calcitonin (Ct) and epidermal growth factor (EGF), were cloned into vectors for transient or stable transformation of tobacco plants. In tobacco leaves, we observed the formation of dense, ER-localized structures containing high concentrations of the respective target proteins. The intact synthetic organelles containing Zera fusions were readily isolated from cellular material using density-based separation methods.

Ricin B chain targeted to the endoplasmic reticulum of tobacco protoplasts is degraded by a CDC48- and vacuole-independent mechanism

The B chain of ricin was expressed and delivered to the endoplasmic reticulum of tobacco protoplasts where it disappeared with time in a manner consistent with degradation. This turnover did not occur in the vacuoles or upon secretion. Indeed, several lines of evidence indicate that, in contrast to the turnover of endoplasmic reticulum-targeted ricin A chain in the cytosol, the bulk of expressed ricin B chain was degraded in the secretory pathway.

Protein domains involved in assembly in the endoplasmic reticulum promote vacuolar delivery when fused to secretory GFP, indicating a protein quality control pathway for degradation in the plant vacuole

The correct folding and assembly of newly synthesized secretory proteins are monitored by the protein quality control system of the endoplasmic reticulum (ER). Through interactions with chaperones such as the binding protein (BiP) and other folding helpers, quality control favors productive folding and sorts for degradation defective proteins. A major route for quality control degradation identified in yeast, plants, and animals is constituted by retrotranslocation from the ER to the cytosol and subsequent disposal by the ubiquitin/proteasome system, but alternative routes involving the vacuole have been identified in yeast. In this study, we have studied the destiny of sGFP418, a fusion between a secretory form of GFP and a domain of the vacuolar protein phaseolin that is involved in the correct assembly of phaseolin and in BiP recognition of unassembled subunits. We show that sGFP418, despite lacking the phaseolin vacuolar sorting signal, is delivered to the vacuole and fragmented, in a process that is inhibited by the secretory traffic inhibitor brefeldin A. Moreover, a fusion between GFP and a domain of the maize storage protein gamma-zein involved in zein polymerization also undergoes post-translational fragmentation similar to that of sGFP418. These results show that defective secretory proteins with permanently exposed sequences normally involved in oligomerization can be delivered to the vacuole by secretory traffic. This strongly suggests the existence of a plant vacuolar sorting mechanism devoted to the disposal of defective secretory proteins.

Anchorage to the cytosolic face of the endoplasmic reticulum membrane: a new strategy to stabilize a cytosolic recombinant antigen in plants

The levels of accumulation of recombinant vaccines in transgenic plants are protein specific and strongly influenced by the subcellular compartment of destination. The human immunodeficiency virus protein Nef (negative factor), a promising target for the development of an antiviral vaccine, is a cytosolic protein that accumulates to low levels in transgenic tobacco and is even more unstable when introduced into the secretory pathway, probably because of folding defects in the non-cytosolic environment. To improve Nef accumulation, a new strategy was developed to anchor the molecule to the cytosolic face of the endoplasmic reticulum (ER) membrane. For this purpose, the Nef sequence was fused to the C-terminal domain of mammalian ER cytochrome b5, a long-lived, tail-anchored (TA) protein. This consistently increased Nef accumulation by more than threefold in many independent transgenic tobacco plants. Real-time polymerase chain reaction of mRNA levels and protein pulse-chase analysis indicated that the increase was not caused by higher transcript levels but by enhanced protein stability. Subcellular fractionation and immunocytochemistry indicated that Nef-TA accumulated on the ER membrane. Over-expression of mammalian or plant ER cytochrome b5 caused the formation of stacked membrane structures, as observed previously in similar experiments performed in mammalian cells; however, Nef-TA did not alter membrane organization in tobacco cells. Finally, Nef could be removed in vitro by its tail-anchor, taking advantage of an engineered thrombin cleavage site. These results open up the way to use tail-anchors to improve foreign protein stability in the plant cytosol without perturbing cellular functions.

The human immunodeficiency virus antigen Nef forms protein bodies in leaves of transgenic tobacco when fused to zeolin

Protein bodies (PB) are stable polymers naturally formed by certain seed storage proteins within the endoplasmic reticulum (ER). The human immunodeficiency virus negative factor (Nef) protein, a potential antigen for the development of an anti-viral vaccine, is highly unstable when introduced into the plant secretory pathway, probably because of folding defects in the ER environment. The aim of this study was to promote the formation of Nef-containing PB in tobacco (Nicotiana tabacum) leaves by fusing the Nef sequence to the N-terminal domains of the maize storage protein gamma-zein or to the chimeric protein zeolin (which efficiently forms PB and is composed of the vacuolar storage protein phaseolin fused to the N-terminal domains of gamma-zein). Protein blots and pulse-chase indicate that fusions between Nef and the same gamma-zein domains present in zeolin are degraded by ER quality control. Consistently, a mutated zeolin, in which wild-type phaseolin was substituted with a defective version known to be degraded by ER quality control, is unstable in plant cells. Fusion of Nef to the entire zeolin sequence instead allows the formation of PB detectable by electron microscopy and subcellular fractionation, leading to zeolin-Nef accumulation higher than 1% of total soluble protein, consistently reproduced in independent transgenic plants. It is concluded that zeolin, but not its gamma-zein portion, has a positive dominant effect over ER quality control degradation. These results provide insights into the requirements for PB formation and avoidance of quality-control degradation, and indicate a strategy for enhancing foreign protein accumulation in plants.

A novel platform for biologically active recombinant human interleukin-13 production

Interleukin-13 (IL-13) is a pleiotropic regulatory cytokine with the potential for treating several human diseases, including type-1 diabetes. Thus far, conventional expression systems for recombinant IL-13 production have proven difficult and are limited by efficiency. In this study, transgenic plants were used as a novel expression platform for the production of human IL-13 (hIL-13). DNA constructs containing hIL-13 cDNA were introduced into tobacco plants. Transcriptional expression of the hIL-13 gene in transgenic plants was confirmed by reverse transcriptase-polymerase chain reaction and Northern blotting. Western blot analysis showed that the hIL-13 protein was efficiently accumulated in transgenic plants and present in multiple molecular forms, with an expression level as high as 0.15% of total soluble protein in leaves. The multiple forms of plant-derived recombinant hIL-13 (rhIL-13) are a result of differential N-linked glycosylation, as revealed by enzymatic and chemical deglycosylation, but not of disulphide-linked oligomerization. In vitro trypsin digestion indicated that plant rhIL-13 was more resistant than unglycosylated control rhIL-13 to proteolysis. The stability of plant rhIL-13 to digestion was further supported with simulated gastric and intestinal fluid digestion. In vitro bioassays using a factor-dependent human erythroleukaemic cell line (TF-1 cells) showed that plant rhIL-13 retained the biological functions of the authentic hIL-13 protein. These results demonstrate that transgenic plants are superior to conventional cell-based expression systems for the production of rhIL-13. Moreover, transgenic plants synthesizing high levels of rhIL-13 may prove to be an attractive delivery system for direct oral administration of IL-13 in the treatment of clinical diseases such as type-1 diabetes.

Identification of a novel adenine nucleotide transporter in the endoplasmic reticulum of Arabidopsis

Many metabolic reactions in the endoplasmic reticulum (ER) require high levels of energy in the form of ATP, which is important for cell viability. Here, we report on an adenine nucleotide transporter residing in the ER membranes of Arabidopsis thaliana (ER-ANT1). Functional integration of ER-ANT1 in the cytoplasmic membrane of intact Escherichia coli cells reveals a high specificity for an ATP/ADP antiport. Immunodetection in transgenic ER-ANT1-C-MYC-tag Arabidopsis plants and immunogold labeling of wild-type pollen grain tissue using a peptide-specific antiserum reveal the localization of this carrier in ER membranes. Transgenic ER-ANT1-promoter-beta-glucuronidase Arabidopsis lines show high expression in ER-active tissues (i.e., pollen, seeds, root tips, apical meristems, or vascular bundles). Two independent ER-ANT1 Arabidopsis knockout lines indicate a high physiological relevance of ER-ANT1 for ATP transport into the plant ER (e.g., disruption of ER-ANT1 results in a drastic retardation of plant growth and impaired root and seed development). In these ER-ANT1 knockout lines, the expression levels of several genes encoding ER proteins that are dependent on a sufficient ATP supply (i.e., BiP [for luminal binding protein] chaperones, calreticulin chaperones, Ca2+-dependent protein kinase, and SEC61) are substantially decreased.

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.

Fluorescent reporter proteins for the tonoplast and the vacuolar lumen identify a single vacuolar compartment in Arabidopsis cells

We generated fusions between three Arabidopsis (Arabidopsis thaliana) tonoplast intrinsic proteins (TIPs; alpha-, gamma-, and delta-TIP) and yellow fluorescent protein (YFP). We also produced soluble reporters consisting of the monomeric red fluorescent protein (RFP) and either the C-terminal vacuolar sorting signal of phaseolin or the sequence-specific sorting signal of proricin. In transgenic Arabidopsis leaves, mature roots, and root tips, all TIP fusions localized to the tonoplast of the central vacuole and both of the lumenal RFP reporters were found within TIP-delimited vacuoles. In embryos from developing, mature, and germinating seeds, all three TIPs localized to the tonoplast of protein storage vacuoles. To determine the temporal TIP expression patterns and to rule out mistargeting due to overexpression, we generated plants expressing YFP fused to the complete genomic sequences of the three TIP isoforms. In transgenic Arabidopsis, gamma-TIP expression was limited to vegetative tissues, but specifically excluded from root tips, whereas alpha-TIP was exclusively expressed during seed maturation. delta-TIP was expressed in vegetative tissues, but not root tips, at a later stage than gamma-TIP. Our findings indicate that, in the Arabidopsis tissues analyzed, two different vacuolar sorting signals target soluble proteins to a single vacuolar location. Moreover, TIP isoform distribution is tissue and development specific, rather than organelle specific.

Zeolin is a recombinant storage protein with different solubility and stability properties according to its localization in the endoplasmic reticulum or in the chloroplast

Several strategies have been exploited to maximize heterologous protein accumulation in the plant cell. Recently, it has been shown that a portion of a maize prolamin storage protein, gamma-zein, can be used for the high accumulation of a recombinant protein in novel endoplasmic reticulum (ER)-derived protein bodies of vegetative tissues. In this study, we investigate whether this protein can be expressed in the chloroplast. Our long-term purpose is to use zeolin to produce value-added proteins by fusing these polypeptides with its gamma-zein portion and targeting the recombinant proteins to the ER or to the chloroplast. We show here that zeolin accumulates in the chloroplast to lower levels than in the ER and its stability is compromised by chloroplast proteolytic activity. Co-localization of zeolin and the ER chaperone BiP in the chloroplast does not have a beneficial effect on zeolin accumulation. In this organelle, zeolin is not stored in protein bodies, nor do zeolin polypeptides seem to be linked by inter-chain disulfide bonds, which are usually formed by the six cysteine of the gamma-zein portion, indicating abnormal folding of the recombinant protein. Therefore, it is concluded that to accumulate zeolin in the chloroplast it is necessary to facilitate inter-chain disulfide bond formation.

Overexpression of Arabidopsis AGD7 causes relocation of Golgi-localized proteins to the endoplasmic reticulum and inhibits protein trafficking in plant cells

ADP ribosylation factor (Arf) GTPase-activating proteins (GAPs) promote the hydrolysis of GTP bound to Arfs to GDP, which plays a pivotal role in regulating Arfs by converting the active GTP-bound forms of these proteins into their inactive GDP-bound forms. Here, we investigated the biological role of AGD7, an Arf GAP homolog, in Arabidopsis (Arabidopsis thaliana). We show that AGD7 bears a highly conserved N-terminal region and a unique C-terminal region, interacts with Arf1 both in vitro and in vivo, and stimulates Arf1 GTPase activity in a phosphatidic acid-dependent manner in vitro. In plant cells, AGD7 localized to the Golgi complex, where its overexpression was found to inhibit the Golgi localization of gamma-subunit of coat proteins and promote the relocation of Golgi proteins into the endoplasmic reticulum in both protoplasts and transgenic plants. Furthermore, overexpression of AGD7 inhibited anterograde trafficking of proteins from the endoplasmic reticulum. We propose that AGD7 functions as a GAP for Arf1 in the Golgi complex and plays a critical role in protein trafficking by controlling Arf1 activity.

Expression of the human hepatitis B virus large surface antigen gene in transgenic tomato plants

The original hepatitis B virus (HBV) large surface antigen gene was synthesized. In order to optimize the expression of this gene in tomato plants, the tobacco pathogenesis-related protein S signal peptide was fused to the 5' end of the modified gene and the sequence encoding amino acids S, E, K, D, E, and L was placed at the 3' end. The gene encoding the modified HBV large surface antigen under the control of a fruit-specific promoter was constructed and expressed in transgenic tomato plants. The expression of the antigen from transgenic plants was confirmed by PCR and reverse transcriptase PCR. Enzyme-linked immunoassays using a monoclonal antibody directed against human serum-derived HBsAg revealed that the maximal level of HBsAg was about 0.02% of the soluble protein in transgenic tomato fruit. The amount of HBsAg in mature fruits was found to be 65- to 171-fold larger than in small or medium fruits and leaf tissues. Examination of transgenic plant samples by transmission electron microscopy proved that HBsAg had been expressed and had accumulated. The HBsAg protein was capable of assembling into capsomers and virus-like particles. To our knowledge, this is the first time the HBV large surface antigen has been expressed in plants. This work suggests the possibility of producing a new alternative vaccine for human HBV.

Arabidopsis vacuolar sorting mutants (green fluorescent seed) can be identified efficiently by secretion of vacuole-targeted green fluorescent protein in their seeds

Two Arabidopsis thaliana genes have been shown to function in vacuolar sorting of seed storage proteins: a vacuolar sorting receptor, VSR1/ATELP1, and a retromer component, MAIGO1 (MAG1)/VPS29. Here, we show an efficient and simple method for isolating vacuolar sorting mutants of Arabidopsis. The method was based on two findings in this study. First, VSR1 functioned as a sorting receptor for beta-conglycinin by recognizing the vacuolar targeting signal. Second, when green fluorescent protein (GFP) fusion with the signal (GFP-CT24) was expressed in vsr1, mag1/vps29, and wild-type seeds, both vsr1and mag1/vps29 gave strongly fluorescent seeds but the wild type did not, suggesting that a defect in vacuolar sorting provided fluorescent seeds by the secretion of GFP-CT24 out of the cells. We mutagenized transformant seeds expressing GFP-CT24. From approximately 3,000,000 lines of M2 seeds, we obtained >100 fluorescent seeds and designated them green fluorescent seed (gfs) mutants. We report 10 gfs mutants, all of which caused missorting of storage proteins. We mapped gfs1 to VSR1, gfs2 to KAM2/GRV2, gfs10 to the At4g35870 gene encoding a novel membrane protein, and the others to different loci. This method should provide valuable insights into the complex molecular mechanisms underlying vacuolar sorting of storage proteins.

Plant endoplasmin supports the protein secretory pathway and has a role in proliferating tissues

Endoplasmin is a molecular chaperone of the heat-shock protein 90 class located in the endoplasmic reticulum and its activity is poorly characterized in plants. We assessed the ability of endoplasmin to alleviate stress via its transient overexpression in tobacco protoplasts treated with tunicamycin, an inhibitor of glycosylation and inducer of the unfolded protein response (UPR). Endoplasmin supported the secretion of a model secretory protein but was less effective than BiP, the endoplasmic reticulum member of the heat-shock protein 70 family. Consistently, immunoprecipitation experiments with in vivo radioactively labelled proteins using an antiserum prepared against Arabidopsis endoplasmin showed that a much smaller number of newly synthesized polypeptides associated with endoplasmin than with BiP. Synthesis of endoplasmin was enhanced by UPR inducers in tobacco seedlings but not protoplasts. As BiP synthesis was induced in both systems, we conclude that the UPR acts differently, at least in part, on the expression of the two chaperones. Endoplasmin was not detectable in extracts of leaves and stems of the Arabidopsis endoplasmin T-DNA insertion mutant shepherd. However, the chaperone is present, albeit at low levels, in shepherd mutant callus, mature roots and tunicamycin-treated seedlings, demonstrating that the mutation is leaky. Reduced endoplasmin in the shepherd mutant has no effect on BiP protein levels in callus or mature roots, leaves and stems, but is compensated by increased BiP in seedlings. This increase occurs in proliferating rather than expanding leaf cells, indicating an important role for endoplasmin in proliferating plant tissues.

A KDEL-tagged monoclonal antibody is efficiently retained in the endoplasmic reticulum in leaves, but is both partially secreted and sorted to protein storage vacuoles in seeds

Transgenic plants are attractive biological systems for the large-scale production of pharmaceutical proteins. In particular, seeds offer special advantages, such as ease of handling and long-term stable storage. Nevertheless, most of the studies of the expression of antibodies in plants have been performed in leaves. We report the expression of a secreted (sec-Ab) or KDEL-tagged (Ab-KDEL) mutant of the 14D9 monoclonal antibody in transgenic tobacco leaves and seeds. Although the KDEL sequence has little effect on the accumulation of the antibody in leaves, it leads to a higher antibody yield in seeds. sec-Ab(Leaf) purified from leaf contains complex N-glycans, including Lewis(a) epitopes, as typically found in extracellular glycoproteins. In contrast, Ab-KDEL(Leaf) bears only high-mannose-type oligosaccharides (mostly Man 7 and 8) consistent with an efficient endoplasmic reticulum (ER) retention/cis-Golgi retrieval of the antibody. sec-Ab and Ab-KDEL gamma chains purified from seeds are cleaved by proteases and contain complex N-glycans indicating maturation in the late Golgi compartments. Consistent with glycosylation of the protein, Ab-KDEL(Seed) was partially secreted and sorted to protein storage vacuoles (PSVs) in seeds and not found in the ER. This dual targeting may be due to KDEL-mediated targeting to the PSV and to a partial saturation of the vacuolar sorting machinery. Taken together, our results reveal important differences in the ER retention and vacuolar sorting machinery between leaves and seeds. In addition, we demonstrate that a plant-made antibody with triantennary high-mannose-type N-glycans has similar Fab functionality to its counterpart with biantennary complex N-glycans, but the former antibody interacts with protein A in a stronger manner and is more immunogenic than the latter. Such differences could be related to a variable immunoglobulin G (IgG)-Fc folding that would depend on the size of the N-glycan.

The Quest to Understand the Basis and Mechanisms that Control Expression of Introduced Transgenes in Crop Plants

We discuss mechanisms and factors that influence levels and stability of expressed heterologous proteins in crop plants. We have seen substantial progress in this field over the past two decades in model experimental organisms such as Arabidopsis and tobacco. There is no question such studies have resulted in furthering our understanding of key processes in the plant cell and the elaboration of sophisticated models to explain underlying mechanisms that might influence the fate, levels and stability of expression of recombinant heterologous proteins in plants. However, very often, such information is not applicable outside these laboratory experimental models. In order to generate a knowledge basis that can be used to achieve high levels and stability of heterologous proteins in relevant crop plants it is imperative to perform such studies on the target crops. With this in mind, we discuss key elements of the process at the DNA, RNA and protein levels. We believe it is essential to discuss recombinant protein production in crops in a holistic manner in order to develop a comprehensive knowledge base that will in turn serve plant biotechnology applications well.

Codon-modifications and an endoplasmic reticulum-targeting sequence additively enhance expression of an Aspergillus phytase gene in transgenic canola

Transgenic plants offer advantages for biomolecule production because plants can be grown on a large scale and the recombinant macromolecules can be easily harvested and extracted. We introduced an Aspergillus phytase gene into canola (Brassica napus) (line 9412 with low erucic acid and low glucosinolates) by Agrobacterium-mediated transformation. Phytase expression in transgenic plant was enhanced with a synthetic phytase gene according to the Brassica codon usage and an endoplasmic reticulum (ER) retention signal KDEL that confers an ER accumulation of the recombinant phytase. Secretion of the phytase to the extracellular fluid was also established by the use of the tobacco PR-S signal peptide. Phytase accumulation in mature seed accounted for 2.6% of the total soluble proteins. The enzyme can be glycosylated in the seeds of transgenic plants and retain a high stability during storage. These results suggest a commercial feasibility of producing a stable recombinant phytase in canola at a high level for animal feed supplement and for reducing phosphorus eutrophication problems.

AtRMR1 functions as a cargo receptor for protein trafficking to the protein storage vacuole

Organellar proteins are sorted by cargo receptors on the way to their final destination. However, receptors for proteins that are destined for the protein storage vacuole (PSV) are largely unknown. In this study, we investigated the biological role that Arabidopsis thaliana receptor homology region transmembrane domain ring H2 motif protein (AtRMR) 1 plays in protein trafficking to the PSV. AtRMR1 mainly colocalized to the prevacuolar compartment of the PSV, but a minor portion also localized to the Golgi complex. The coexpression of AtRMR1 mutants that were localized to the Golgi complex strongly inhibited the trafficking of phaseolin to the PSV and caused accumulation of phaseolin in the Golgi complex or its secretion. Coimmunoprecipitation and in vitro binding assays revealed that the lumenal domain of AtRMR1 interacts with the COOH-terminal sorting signal of phaseolin at acidic pH. Furthermore, phaseolin colocalized with AtRMR1 on its way to the PSV. Based on these results, we propose that AtRMR1 functions as the sorting receptor of phaseolin for its trafficking to the PSV.

Plant-derived mouse IgG monoclonal antibody fused to KDEL endoplasmic reticulum-retention signal is N-glycosylated homogeneously throughout the plant with mostly high-mannose-type N-glycans

Plants are potential hosts for the expression of recombinant glycoproteins intended for therapeutic purposes. However, N-glycans of mammalian glycoproteins produced in transgenic plants differ from their natural counterparts. The use of the endoplasmic reticulum (ER)-retention signal has been proposed to restrict glycosylation of plantibodies to only high-mannose-type N-glycans. Furthermore, little is known about the influence of plant development and growth conditions on N-linked glycosylation. Here, we report a detailed N-glycosylation profiling study of CB.Hep1, a mouse IgG2b monoclonal antibody (mAb) against hepatitis B surface antigen (HBsAg) currently expressed in tobacco plants (Nicotiana tabacum L.). The KDEL ER-retention signal was fused to the C-terminal of both light and heavy chains. The structures of the N-linked glycans of this mAb produced in transgenic tobacco plants at various growth stages were analysed by high-performance liquid chromatography (HPLC) profiling techniques and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and compared with those of murine origin. The high-mannose-type oligosaccharides accounted for more than 80% of the total N-glycans, with Man7GlcNAc2 being the most abundant species. Some complex N-glycans bearing xylose and small amounts of oligosaccharides with both xylose and fucose were identified. No appreciable differences were detected when comparing glycosylation at different leaf ages, e.g. from seedling leaves up to 8 weeks old and top or basal leaves of mature plants, or between leaves, stems and whole plants. A strict retention of glycoproteins to ER by the use of the tetrapeptide KDEL was not sufficient, even though the majority of the resulting N-glycosylation was of the high-mannose type. It is highly likely to be dependent on other factors, which are most probably protein specific.

The phaseolin vacuolar sorting signal promotes transient, strong membrane association and aggregation of the bean storage protein in transgenic tobacco

Vacuolar storage proteins of the 7S class are co-translationally introduced into the endoplasmic reticulum and reach storage vacuoles via the Golgi complex and dense vesicles. The signal for vacuolar sorting of one of these proteins, phaseolin of Phaseolus vulgaris, consists of a four-amino acid hydrophobic propeptide at the C-terminus. When this sequence is deleted, phaseolin is secreted instead of being sorted to vacuoles. It is shown here that in transgenic tobacco plants newly-synthesized phaseolin has unusual affinity to membranes and forms SDS-resistant aggregates, but mutated phaseolin polypeptides that are either secreted or defective in assembly do not have these characteristics. Association to membranes and aggregation are transient events: phaseolin accumulated in vacuoles is soluble in the absence of detergents and is not aggregated. Association to membranes starts before the phaseolin glycan acquires a complex structure and therefore before the protein reaches the medial or trans-cisternae of the Golgi complex. These results support the hypothesis of a relationship between aggregation and vacuolar sorting of phaseolin and indicate that sorting may start in early compartments of the secretory pathway.

Cell biology of the plant Golgi apparatus

The higher plant Golgi apparatus, comprising many individual stacks of membrane bounded cisternae, is one of the most enigmatic of the cytoplasmic organelles. Not only can the stacks receive material from the endoplasmic reticulum, process it and target it to the correct cellular destination, but they can also synthesise and export complex carbohydrates and lipids and most likely act as one end point of the endocytic pathway. In many cells such processing and sorting can take place while the stacks are moving within the cytoplasm and, remarkably, the organelle manages to retain its structural integrity. This review considers some of the latest data and views on transport both to and from the Golgi and the mechanisms by which such activity is regulated.

A recombinant multimeric immunoglobulin expressed in rice shows assembly-dependent subcellular localization in endosperm cells

To investigate the role of subunit assembly in the intracellular deposition of multimeric recombinant proteins, we expressed a partially humanized secretory immunoglobulin in rice endosperm cells and determined the subcellular locations of the assembled protein and its individual components. Transgenic rice plants expressing either individual subunits or all the subunits of the antibody were generated by particle bombardment, and protein localization was determined by immunoelectron microscopy. Assembly of the antibody was confirmed by immunoassay and coimmunoprecipitation. Immunolocalization experiments showed no evidence for secretion of the antibody or any of its components to the apoplast. Rather, the nonassembled light chain, heavy chain and secretory component accumulated predominantly within endoplasmic reticulum-derived protein bodies, while the assembled antibody, with antigen-binding function, accumulated specifically in protein storage vacuoles. These results show that the destination of a complex recombinant protein within the plant cell is influenced by its state of assembly.