The endoplasmic reticulum-gateway of the secretory pathway

The cytosolic nucleoprotein of the plant-infecting bunyavirus tomato spotted wilt recruits endoplasmic reticulum-resident proteins to endoplasmic reticulum export sites

In contrast with animal-infecting viruses, few known plant viruses contain a lipid envelope, and the processes leading to their membrane envelopment remain largely unknown. Plant viruses with lipid envelopes include viruses of the Bunyaviridae, which obtain their envelope from the Golgi complex. The envelopment process is predominantly dictated by two viral glycoproteins (Gn and Gc) and the viral nucleoprotein (N). During maturation of the plant-infecting bunyavirus Tomato spotted wilt, Gc localizes at endoplasmic reticulum (ER) membranes and becomes ER export competent only upon coexpression with Gn. In the presence of cytosolic N, Gc remains arrested in the ER but changes its distribution from reticular into punctate spots. Here, we show that these areas correspond to ER export sites (ERESs), distinct ER domains where glycoprotein cargo concentrates prior to coat protein II vesicle-mediated transport to the Golgi. Gc concentration at ERES is mediated by an interaction between its cytoplasmic tail (CT) and N. Interestingly, an ER-resident calnexin provided with Gc-CT was similarly recruited to ERES when coexpressed with N. Furthermore, disruption of actin filaments caused the appearance of a larger amount of smaller ERES loaded with N-Gc complexes, suggesting that glycoprotein cargo concentration acts as a trigger for de novo synthesis of ERES.

The production and characterization of a new active lipase from Acremonium alcalophilum using a plant bioreactor

BACKGROUND

Microorganisms are the most proficient decomposers in nature, using secreted enzymes in the hydrolysis of lignocellulose. As such, they present the most abundant source for discovery of new enzymes. Acremonium alcalophilum is the only known cellulolytic fungus that thrives in alkaline conditions and can be cultured readily in the laboratory. Its optimal conditions for growth are 30°C and pH 9.0-9.2. The genome sequence of Acremonium alcalophilum has revealed a large number of genes encoding biomass-degrading enzymes. Among these enzymes, lipases are interesting because of several industrial applications including biofuels, detergent, food processing and textile industries.

RESULTS

We identified a lipA gene in the genome sequence of Acremonium alcalophilum, encoding a protein with a predicted lipase domain with weak sequence identity to characterized enzymes. Unusually, the predicted lipase displays ≈ 30% amino acid sequence identity to both feruloyl esterase and lipase of Aspergillus niger. LipA, when transiently produced in Nicotiana benthamiana, accumulated to over 9% of total soluble protein. Plant-produced recombinant LipA is active towards p-nitrophenol esters of various carbon chain lengths with peak activity on medium-chain fatty acid (C8). The enzyme is also highly active on xylose tetra-acetate and oat spelt xylan. These results suggests that LipA is a novel lipolytic enzyme that possesses both lipase and acetylxylan esterase activity. We determined that LipA is a glycoprotein with pH and temperature optima at 8.0 and 40°C, respectively.

CONCLUSION

Besides being the first heterologous expression and characterization of a gene coding for a lipase from A. alcalophilum, this report shows that LipA is very versatile exhibiting both acetylxylan esterase and lipase activities potentially useful for diverse industry sectors, and that tobacco is a suitable bioreactor for producing fungal proteins.

Multiple evolutionarily conserved Di-leucine like motifs in the carboxyl terminus control the anterograde trafficking of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome, a life-threatening kidney disease. Yet the mechanisms underlying the regulation of NKCC2 trafficking in renal cells are scarcely known. We previously showed that naturally occurring mutations depriving NKCC2 of its distal COOH-terminal tail and interfering with the (1081)LLV(1083) motif result in defects in the ER exit of the co-transporter. Here we show that this motif is necessary but not sufficient for anterograde trafficking of NKCC2. Indeed, we have identified two additional hydrophobic motifs, (1038)LL(1039) and (1048)LI(1049), that are required for ER exit and surface expression of the co-transporter. Double mutations of (1038)LL(1039) or (1048)LI(1049) to di-alanines disrupted glycosylation and cell surface expression of NKCC2, independently of the expression system. Pulse-chase analysis demonstrated that the absence of the terminally glycosylated form of NKCC2 was not due to reduced synthesis or increased rates of degradation of mutant co-transporters, but was instead caused by defects in maturation. Co-immunolocalization experiments revealed that (1038)AA(1039) and (1048)AA(1049) were trapped mainly in the ER as indicated by extensive co-localization with the ER marker calnexin. Remarkably, among several analyzed motifs present in the NKCC2 COOH terminus, only those required for ER exit and surface expression of NKCC2 are evolutionarily conserved in all members of the SLC12A family, a group of cation-chloride co-transporters that are targets of therapeutic drugs and mutated in several human diseases. Based upon these data, we propose abnormal anterograde trafficking as a common mechanism associated with mutations depriving NKCC2, and also all other members of the SLC12A family, of their COOH terminus.

Multiple evolutionarily conserved Di-leucine like motifs in the carboxyl terminus control the anterograde trafficking of NKCC2

Mutations in the apical Na-K-2Cl co-transporter, NKCC2, cause type I Bartter syndrome, a life-threatening kidney disease. Yet the mechanisms underlying the regulation of NKCC2 trafficking in renal cells are scarcely known. We previously showed that naturally occurring mutations depriving NKCC2 of its distal COOH-terminal tail and interfering with the (1081)LLV(1083) motif result in defects in the ER exit of the co-transporter. Here we show that this motif is necessary but not sufficient for anterograde trafficking of NKCC2. Indeed, we have identified two additional hydrophobic motifs, (1038)LL(1039) and (1048)LI(1049), that are required for ER exit and surface expression of the co-transporter. Double mutations of (1038)LL(1039) or (1048)LI(1049) to di-alanines disrupted glycosylation and cell surface expression of NKCC2, independently of the expression system. Pulse-chase analysis demonstrated that the absence of the terminally glycosylated form of NKCC2 was not due to reduced synthesis or increased rates of degradation of mutant co-transporters, but was instead caused by defects in maturation. Co-immunolocalization experiments revealed that (1038)AA(1039) and (1048)AA(1049) were trapped mainly in the ER as indicated by extensive co-localization with the ER marker calnexin. Remarkably, among several analyzed motifs present in the NKCC2 COOH terminus, only those required for ER exit and surface expression of NKCC2 are evolutionarily conserved in all members of the SLC12A family, a group of cation-chloride co-transporters that are targets of therapeutic drugs and mutated in several human diseases. Based upon these data, we propose abnormal anterograde trafficking as a common mechanism associated with mutations depriving NKCC2, and also all other members of the SLC12A family, of their COOH terminus.

Ubiquitin initiates sorting of Golgi and plasma membrane proteins into the vacuolar degradation pathway

BACKGROUND

In yeast and mammals, many plasma membrane (PM) proteins destined for degradation are tagged with ubiquitin. These ubiquitinated proteins are internalized into clathrin-coated vesicles and are transported to early endosomal compartments. There, ubiquitinated proteins are sorted by the endosomal sorting complex required for transport (ESCRT) machinery into the intraluminal vesicles of multivesicular endosomes. Degradation of these proteins occurs after endosomes fuse with lysosomes/lytic vacuoles to release their content into the lumen. In plants, some PM proteins, which cycle between the PM and endosomal compartments, have been found to be ubiquitinated, but it is unclear whether ubiquitin is sufficient to mediate internalization and thus acts as a primary sorting signal for the endocytic pathway. To test whether plants use ubiquitin as a signal for the degradation of membrane proteins, we have translationally fused ubiquitin to different fluorescent reporters for the plasma membrane and analyzed their transport.

RESULTS

Ubiquitin-tagged PM reporters localized to endosomes and to the lumen of the lytic vacuole in tobacco mesophyll protoplasts and in tobacco epidermal cells. The internalization of these reporters was significantly reduced if clathrin-mediated endocytosis was inhibited by the coexpression of a mutant of the clathrin heavy chain, the clathrin hub. Surprisingly, a ubiquitin-tagged reporter for the Golgi was also transported into the lumen of the vacuole. Vacuolar delivery of the reporters was abolished upon inhibition of the ESCRT machinery, indicating that the vacuolar delivery of these reporters occurs via the endocytic transport route.

CONCLUSIONS

Ubiquitin acts as a sorting signal at different compartments in the endomembrane system to target membrane proteins into the vacuolar degradation pathway: If displayed at the PM, ubiquitin triggers internalization of PM reporters into the endocytic transport route, but it also mediates vacuolar delivery if displayed at the Golgi. In both cases, ubiquitin-tagged proteins travel via early endosomes and multivesicular bodies to the lytic vacuole. This suggests that vacuolar degradation of ubiquitinated proteins is not restricted to PM proteins but might also facilitate the turnover of membrane proteins in the early secretory pathway.

BEX5/RabA1b regulates trans-Golgi network-to-plasma membrane protein trafficking in Arabidopsis

Constitutive endocytic recycling is a crucial mechanism allowing regulation of the activity of proteins at the plasma membrane and for rapid changes in their localization, as demonstrated in plants for PIN-FORMED (PIN) proteins, the auxin transporters. To identify novel molecular components of endocytic recycling, mainly exocytosis, we designed a PIN1-green fluorescent protein fluorescence imaging-based forward genetic screen for Arabidopsis thaliana mutants that showed increased intracellular accumulation of cargos in response to the trafficking inhibitor brefeldin A (BFA). We identified bex5 (for BFA-visualized exocytic trafficking defective), a novel dominant mutant carrying a missense mutation that disrupts a conserved sequence motif of the small GTPase, RAS GENES FROM RAT BRAINA1b. bex5 displays defects such as enhanced protein accumulation in abnormal BFA compartments, aberrant endosomes, and defective exocytosis and transcytosis. BEX5/RabA1b localizes to trans-Golgi network/early endosomes (TGN/EE) and acts on distinct trafficking processes like those regulated by GTP exchange factors on ADP-ribosylation factors GNOM-LIKE1 and HOPM INTERACTOR7/BFA-VISUALIZED ENDOCYTIC TRAFFICKING DEFECTIVE1, which regulate trafficking at the Golgi apparatus and TGN/EE, respectively. All together, this study identifies Arabidopsis BEX5/RabA1b as a novel regulator of protein trafficking from a TGN/EE compartment to the plasma membrane.

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.

The formation, function and fate of protein storage compartments in seeds

Seed storage proteins (SSPs) have been studied for more than 250 years because of their nutritional value and their impact on the use of grain in food processing. More recently, the use of seeds for the production of recombinant proteins has rekindled interest in the behavior of SSPs and the question how they are able to accumulate as stable storage reserves. Seed cells produce vast amounts of SSPs with different subcellular destinations creating an enormous logistic challenge for the endomembrane system. Seed cells contain several different storage organelles including the complex and dynamic protein storage vacuoles (PSVs) and other protein bodies (PBs) derived from the endoplasmic reticulum (ER). Storage proteins destined for the PSV may pass through or bypass the Golgi, using different vesicles that follow different routes through the cell. In addition, trafficking may depend on the plant species, tissue and developmental stage, showing that the endomembrane system is capable of massive reorganization. Some SSPs contain sorting signals or interact with membranes or with other proteins en route in order to reach their destination. The ability of SSPs to form aggregates is particularly important in the formation or ER-derived PBs, a mechanism that occurs naturally in response to overloading with proteins that cannot be transported and that can be used to induce artificial storage bodies in vegetative tissues. In this review, we summarize recent findings that provide insight into the formation, function, and fate of storage organelles and describe tools that can be used to study them.

Is the 6 kDa tobacco etch viral protein a bona fide ERES marker?

The claim that the 6 kDa viral protein (VP) of Tobacco Etch Virus is a marker for ER exit sites (ERES) has been investigated. When transiently expressed as a CFP tagged fusion construct in tobacco mesophyll protoplasts, this integral membrane protein co-localizes with both the COPII coat protein YFP-SEC24 and the Golgi marker Man1-RFP. However, when over-expressed the VP locates to larger spherical structures which co-localize with neither ER nor Golgi markers. Nevertheless, deletion of the COPII interactive N-terminal D(X)E motif causes it to be broadly distributed throughout the ER, supporting the notion that this protein could be an ERES marker. Curiously, whereas brefeldin A (BFA) caused a typical Golgi-stack response (redistribution into the ER) of the VP in leaf epidermal cells, in protoplasts it resulted in the formation of structures identical to those formed by over-expression. However, anomalous results were obtained with protoplasts: when co-expressed with the non-cycling cis-Golgi marker Man1-RFP, a BFA-induced redistribution of the VP-CFP signal into the ER was observed, but, in the presence of the cycling Golgi marker ERD2-YFP, this did not occur. High resolution images of side-on views of Golgi stacks in epidermal cells showed that the 6 kDa VP-CFP signal overlapped considerably more with YFP-SEC24 than with Man1-RFP, indicating that the VP is proportionately more associated with ERES. However, based on a consideration of the structure of its cytoplasmic tail, the scenario that the VP collects at ERES and is transported to the cis-Golgi before being recycled back to the ER, is supported.

KMS1 and KMS2, two plant endoplasmic reticulum proteins involved in the early secretory pathway

We have identified two endoplasmic reticulum (ER)-associated Arabidopsis proteins, KMS1 and KMS2, which are conserved among most species. Fluorescent protein fusions of KMS1 localised to the ER in plant cells, and over-expression induced the formation of a membrane structure, identified as ER whorls by electron microscopy. Hydrophobicity analysis suggested that KMS1 and KMS2 are integral membrane proteins bearing six transmembrane domains. Membrane protein topology was assessed by a redox-based topology assay (ReTA) with redox-sensitive GFP and confirmed by a protease protection assay. A major loop domain between transmembrane domains 2 and 3, plus the N- and C-termini were found on the cytosolic side of the ER. A C-terminal di(tri)-lysine motif is involved in retrieval of KMS1 and deletion led to a reduction of the GFP-KMS1 signal in the ER. Over-expression of KMS1/KMS2 truncations perturbed ER and Golgi morphology and similar effects were also seen when KMS1/KMS2 were knocked-down by RNA interference. Microscopy and biochemical experiments suggested that expression of KMS1/KMS2 truncations inhibited ER to Golgi protein transport.

Gene networks in the synthesis and deposition of protein polymers during grain development of wheat

As the amino acid storing organelle, the protein bodies provide nutrients for embryo development, seed germination and early seedling growth through storage proteolysis in cereal plants, such as wheat and rice. In protein bodies, the monomeric and polymeric prolamins, i.e. gliadins and glutenins, form gluten and play a key role in determining dough functionality and end-product quality of wheat. The formation of intra- and intermolecular bonds, including disulphide and tyrosine bonds, in and between prolamins confers cohesivity, viscosity, elasticity and extensibility to wheat dough during mixing and processing. In this review, we summarize recent progress in wheat gluten research with a focus on the fundamental molecular biological aspects, including transcriptional regulation on genes coding for prolamin components, biosynthesis, deposition and secretion of protein polymers, formation of protein bodies, genetic control of seed storage proteins, the transportation of the protein bodies and key enzymes for determining the formation of disulphide bonds of prolamin polymers.

The ER-localized TWD1 immunophilin is necessary for localization of multidrug resistance-like proteins required for polar auxin transport in Arabidopsis roots

Multidrug resistance ABC transporters in plants are required for polar transport of the hormone auxin (indole-3-acetic acid). They are studied in animals primarily because their overexpression confers resistance to anticancer agents. Immunophilins are studied in both plants and animals for their roles in folding and trafficking of proteins, particularly those with signal transducing functions and susceptibility to immunosuppressant drugs. Previous genetic and molecular studies in Arabidopsis thaliana established a physical and functional interaction between some ABCB transporters and the TWISTED DWARF1 (TWD1) immunophilin. In this work, confocal microscopy of fluorescently tagged TWD1 shows it to reside at the endoplasmic reticulum (ER). Mutations in TWD1 caused mislocalization of ABCB1, ABCB4, and ABCB19 to the ER instead of the plasma membrane as shown by confocal microscopy of fluorescently tagged fusion proteins and transmission electron microscopy of immunogold-labeled samples in the case of ABCB19. Localization of the unrelated PIN-FORMED2 auxin transporter or plasma membrane marker proteins was not affected by loss of TWD1. Abnormal spread of auxin signaling into the elongation zone of twd1 roots, attributable to mislocalized ABCB transporters and suppressed by an auxin transport inhibitor, appeared to cause the twisted cell files characteristic of twd1 roots.

Endoplasmic reticulum protein quality control and its relationship to environmental stress responses in plants

The endoplasmic reticulum (ER) has a sophisticated quality control (QC) system to eliminate improperly folded proteins from the secretory pathway. Given that protein folding is such a fastidious process and subject to adverse environmental conditions, the ER QC system appears to have been usurped to serve as an environmental sensor and responder in plants. Under stressful conditions, the ER protein folding machinery reaches a limit as the demands for protein folding exceed the capacity of the system. Under these conditions, misfolded or unfolded proteins accumulate in the ER, triggering an unfolded protein response (UPR). UPR mitigates ER stress by upregulating the expression of genes encoding components of the protein folding machinery or the ER-associated degradation system. In Arabidopsis thaliana, ER stress is sensed and stress signals are transduced by membrane-bound transcription factors, which are activated and mobilized under environmental stress conditions. Under acute or chronic stress conditions, UPR can also lead to apoptosis or programmed cell death. Despite recent progress in our understanding of plant protein QC, discovering how different environmental conditions are perceived is one of the major challenges in understanding this system. Since the ER QC system is one among many stress response systems in plants, another major challenge is determining the extent to which the ER QC system contributes to various stress responses in plants.

Cytosolic N-terminal arginine-based signals together with a luminal signal target a type II membrane protein to the plant ER

BACKGROUND

In eukaryotic cells, the membrane compartments that constitute the exocytic pathway are traversed by a constant flow of lipids and proteins. This is particularly true for the endoplasmic reticulum (ER), the main "gateway of the secretory pathway", where biosynthesis of sterols, lipids, membrane-bound and soluble proteins, and glycoproteins occurs. Maintenance of the resident proteins in this compartment implies they have to be distinguished from the secretory cargo. To this end, they must possess specific ER localization determinants to prevent their exit from the ER, and/or to interact with receptors responsible for their retrieval from the Golgi apparatus. Very few information is available about the signal(s) involved in the retention of membrane type II protein in the ER but it is generally accepted that sorting of ER type II cargo membrane proteins depends on motifs mainly located in their cytosolic tails.

RESULTS

Here, using Arabidopsis glucosidase I as a model, we have identified two types of signals sufficient for the location of a type II membrane protein in the ER. A first signal is located in the luminal domain, while a second signal corresponds to a short amino acid sequence located in the cytosolic tail of the membrane protein. The cytosolic tail contains at its N-terminal end four arginine residues constitutive of three di-arginine motifs (RR, RXR or RXXR) independently sufficient to confer ER localization. Interestingly, when only one di-arginine motif is present, fusion proteins are located both in the ER and in mobile punctate structures, distinct but close to Golgi bodies. Soluble and membrane ER protein markers are excluded from these punctate structures, which also do not colocalize with an ER-exit-site marker. It is hypothesized they correspond to sites involved in Golgi to ER retrotransport.

CONCLUSION

Altogether, these results clearly show that cytosolic and luminal signals responsible for ER retention could coexist in a same type II membrane protein. These data also suggest that both retrieval and retention mechanisms govern protein residency in the ER membrane. We hypothesized that mobile punctate structures not yet described at the ER/Golgi interface and tentatively named GERES, could be involved in retrieval mechanisms from the Golgi to the ER.

Overexpression of a mutant form of EhRabA, a unique Rab GTPase of Entamoeba histolytica, alters endoplasmic reticulum morphology and localization of the Gal/GalNAc adherence lectin

Entamoeba histolytica is a protozoan parasite that causes amoebic dysentery and liver abscess. Vesicle trafficking events, such as phagocytosis and delivery of plasma membrane proteins, have been implicated in pathogenicity. Rab GTPases are proteins whose primary function is to regulate vesicle trafficking; therefore, understanding the function of Rabs in this organism may provide insight into virulence. E. histolytica possesses a number of unique Rabs that exhibit limited homology to host Rabs. In this study we examined the function of one such Rab, EhRabA, by characterizing a mutant overexpressing a constitutively GTP-bound version of the protein. Overexpression of mutant EhRabA resulted in decreased adhesion to and phagocytosis of human red blood cells and in the appearance of large tubular organelles that could be stained with endoplasmic reticulum (ER)-specific but not Golgi complex-specific antibodies. Consistent with the adhesion defect, two subunits of a cell surface adhesin, the galactose/N-acetylgalactosamine lectin, were mislocalized to the novel organelle. A cysteine protease, EhCP2, was also localized to the ER-like compartment in the mutant; however, the localization of two additional cell surface proteins, Igl and SREHP, remained unchanged in the mutant. The phenotype of the mutant could be recapitulated by treatment with brefeldin A, a cellular toxin that disrupts ER-to-Golgi apparatus vesicle traffic. This suggests that EhRabA influences vesicle trafficking pathways that are also sensitive to brefeldin A. Together, the data indicate that EhRabA directly or indirectly influences the morphology of secretory organelles and regulates trafficking of a subset of secretory proteins in E. histolytica.

Deposition of a recombinant peptide in ER-derived protein bodies by retention with cysteine-rich prolamins in transgenic rice seed

A 7Crp peptide composed of seven major human T cell epitopes derived from the Japanese cedar pollen allergens Cry j 1 and Cry j 2 is an ideal tolerogen for peptide immunotherapy against Japanese cedar pollinosis. To maximize the accumulation level of the 7Crp peptide in transgenic rice seed, we tested endosperm specific promoters and intracellular localizations suitable for stable accumulation. A 7Crp peptide carrying the KDEL ER retention signal directed by the 2.3-kb promoter of the glutelin GluB-1, which contains a signal peptide, accumulated at the highest level of about 60 microg/grain. Notably, the 7Crp peptide predominantly accumulated in ER-derived protein bodies irrespective of the presence of various sorting signals or expression as a fusion protein with glutelin. We attribute this abnormal pattern of accumulation to the formation of disulfide bonds between the 7Crp peptide and cysteine-rich (Cys-rich) prolamin storage proteins. Furthermore, the formation of these aggregates induced the chaperone proteins BiP and PDI as an ER stress response.

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.

A green fluorescent protein fused to rice prolamin forms protein body-like structures in transgenic rice

Prolamins, a group of rice (Oryza sativa) seed storage proteins, are synthesized on the rough endoplasmic reticulum (ER) and deposited in ER-derived type I protein bodies (PB-Is) in rice endosperm cells. The accumulation mechanism of prolamins, which do not possess the well-known ER retention signal, remains unclear. In order to elucidate whether the accumulation of prolamin in the ER requires seed-specific factors, the subcellular localization of the constitutively expressed green fluorescent protein fused to prolamin (prolamin-GFP) was examined in seeds, leaves, and roots of transgenic rice plants. The prolamin-GFP fusion proteins accumulated not only in the seeds but also in the leaves and roots. Microscopic observation of GFP fluorescence and immunocytochemical analysis revealed that prolamin-GFP fusion proteins specifically accumulated in PB-Is in the endosperm, whereas they were deposited in the electron-dense structures in the leaves and roots. The ER chaperone BiP was detected in the structures in the leaves and roots. The results show that the aggregation of prolamin-GFP fusion proteins does not depend on the tissues, suggesting that the prolamin-GFP fusion proteins accumulate in the ER by forming into aggregates. The findings bear out the importance of the assembly of prolamin molecules and the interaction of prolamin with BiP in the formation of ER-derived PBs.

The secretory system of Arabidopsis

Over the past few years, a vast amount of research has illuminated the workings of the secretory system of eukaryotic cells. The bulk of this work has been focused on the yeast Saccharomyces cerevisiae, or on mammalian cells. At a superficial level, plants are typical eukaryotes with respect to the operation of the secretory system; however, important differences emerge in the function and appearance of endomembrane organelles. In particular, the plant secretory system has specialized in several ways to support the synthesis of many components of the complex cell wall, and specialized kinds of vacuole have taken on a protein storage role-a role that is intended to support the growing seedling, but has been co-opted to support human life in the seeds of many crop plants. In the past, most research on the plant secretory system has been guided by results in mammalian or fungal systems but recently plants have begun to stand on their own as models for understanding complex trafficking events within the eukaryotic endomembrane system.

Analysis of grape berry cell wall proteome: a comparative evaluation of extraction methods

Different methods were tested for the extraction of proteins from the cell wall-enriched fraction (CWEf) obtained from a sample formed by skin and seeds of ripe berries of Vitis vinifera L. cv. Cabernet Sauvignon. The CWEf was isolated using a disruptive approach that involves tissue homogenization and precipitation by centrifugation. To extract proteins, the CWEf was treated with CaCl(2) and LiCl in two successive steps or, alternatively, with phenol. The efficiency of the protocols was evaluated by measuring protein yield and by analyzing two-dimensional gel electrophoresis (2-DE) gels for the highest detectable spot number and the greatest spot resolution. The phenol method was also adopted for the extraction of proteins from the cytosolic fraction (CYf). The comparison of 2-DE reference maps of protein extracts from CWEf and CYf indicated the presence of both common traits and unique characteristics. To survey this aspect some spots detected in both fractions or present in only one fraction were analyzed by liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Of the 47 spots identified, some were found to be cell wall proteins, while others were proteins not traditionally considered as localized in the apoplastic space. The data presented here provide initial information regarding the apoplastic proteome of grape berry tissues, but also raise the issue of the technical problems that characterize the isolation of cell wall proteins from these very hardy tissues.

The Chitinase A from the baculovirus AcMNPV enhances resistance to both fungi and herbivorous pests in tobacco

Biotechnology has allowed the development of novel strategies to obtain plants that are more resistant to pests, fungal pathogens and other agents of biotic stress. The obvious advantages of having genotypes with multiple beneficial traits have recently fostered the development of gene pyramiding strategies, but less attention has been given to the study of genes that can increase resistance to different types of harmful organisms. Here we report that a recombinant Chitinase A protein of the Autographa californica nuclear polyhedrosis virus (AcMNPV) has both antifungal and insecticide properties in vitro. Transgenic tobacco plants expressing an active ChiA protein showed reduced damages caused by fungal pathogens and lepidopteran larvae, while did not have an effect on aphid populations. To our knowledge, this is the first report on the characterisation and expression in plants of a single gene that increases resistance against herbivorous pests and fungal pathogens and not affecting non-target insects. The implications and the potential of the ChiA gene for plant molecular breeding and biotechnology are discussed.

Expression of Dm-AMP1 in rice confers resistance to Magnaporthe oryzae and Rhizoctonia solani

Magnaporthe oryzae and Rhizoctonia solani, are among the most important pathogens of rice, severely limiting its productivity. Dm-AMP1, an antifungal plant defensin from Dahlia merckii, was expressed in rice (Oryza sativa L. sp. indica cv. Pusa basmati 1) using Agrobacterium tumefaciens-mediated transformation. Expression levels of Dm-AMP1 ranged from 0.43% to 0.57% of total soluble protein in transgenic plants. It was observed that constitutive expression of Dm-AMP1 suppresses the growth of M. oryzae and R. solani by 84% and 72%, respectively. Transgenic expression of Dm-AMP1 was not accompanied by an induction of pathogenesis-related (PR) gene expression, indicating that the expression of DmAMP1 directly inhibits the pathogen. The results of in vitro, in planta and microscopic analyses suggest that Dm-AMP1 expression has the potential to provide broad-spectrum disease resistance in rice.

Studying protein export from the endoplasmic reticulum in plants

Understanding the mechanisms of protein sorting and targeting through the plant secretory pathway has become the focus of many research laboratories. The development of a model system whereby recombinant genes can be transiently expressed in protoplasts has facilitated the study of protein transport signals. Experimental strategies combining a protoplast expression system with endoglycosidase H, vacuole purification, and pulse-chase analyses are used to investigate aspects of specific proteins as they pass through the secretory system. This chapter provides details of protoplast preparation and electroporation as well as techniques to study protein trafficking from the endoplasmic reticulum to the Golgi apparatus or vacuolar compartments. Recommendations as to how to troubleshoot problems that can arise while following these protocols are also discussed in this chapter.

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.

Isolation and characterization of multiple abundant lipid transfer protein isoforms in developing sesame (Sesamum indicum L.) seeds

Sesame (Sesamum indicum) is an important oilseed crop; approximately 50% of the seed dry weight is storage oil. In a previous report, developing sesame seed expressed sequence tags (ESTs) revealed that ESTs encoding lipid transfer protein (LTPs) were one of the most abundant groups of sesame ESTs. LTP functions in the transfer of wax or cutin monomers and in the defense response against pathogen attack. To study the biological role of the abundant LTP isoforms in developing seeds, 122 ESTs out of 3328 sesame ESTs were analyzed against Arabidopsis and rice proteome databases. LTP fraction, which was partially purified from developing sesame seeds, actively transferred fluorescent phospholipids and bound to fatty acids. Full-length cDNAs of five out of 21 LTP isoforms were isolated and named SiLTP1-SiLTP5. The predicted amino acid sequences of the five SiLTPs harbor typical characteristics of LTPs, including conserved arrangement of cysteine residues. Northern blot analysis revealed that the five SiLTP isoforms were most abundantly expressed in developing seeds, but were also detected in flower tissues. Also, SiLTP3 and SiLTP4 transcripts were expressed in leaves and seed-pot walls, respectively. In addition, SiLTP2 and SiLTP4 transcripts were significantly induced in 6-day-old sesame seedlings by application of NaCl, mannitol, and abscisic acid (ABA). Transient expression of green fluorescent protein (GFP)-fusion constructs in Arabidopsis protoplasts revealed that SiLTP1 and SiLTP2 were secreted by different pathways. Taken together, the abundant LTPs in developing sesame seeds are involved in lipid transfer into the extracellular matrix. Possible biological roles of SiLTPs related to organ-specific expression and abiotic stresses are discussed.

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.

Promiscuous, non-catalytic, tandem carbohydrate-binding modules modulate the cell-wall structure and development of transgenic tobacco (Nicotiana tabacum) plants

We have compared heterologous expression of two types of carbohydrate binding module (CBM) in tobacco cell walls. These are the promiscuous CBM29 modules (a tandem CBM29-1-2 and its single derivative CBM29-2), derived from a non-catalytic protein1, NCP1, of the Piromyces equi cellulase/hemicellulase complex, and the less promiscuous tandem CBM2b-1-2 from the Cellulomonas fimi xylanase 11A. CBM-labelling studies revealed that CBM29-1-2 binds indiscriminately to every tissue of the wild-type tobacco stem whereas binding of CBM2b-1-2 was restricted to vascular tissue. The promiscuous CBM29-1-2 had much more pronounced effects on transgenic tobacco plants than the less promiscuous CBM2b-1-2. Reduced stem elongation and prolonged juvenility, resulting in delayed flower development, were observed in transformants expressing CBM29-1-2 whereas such growth phenotypes were not observed for CBM2b-1-2 plants. Histological examination and electron microscopy revealed layers of collapsed cortical cells in the stems of CBM29-1-2 plants whereas cellular deformation in the stem cortical cells of CBM2b-1-2 transformants was less severe. Altered cell expansion was also observed in most parts of the CBM29-1-2 stem whereas for the CBM2b-1-2 stem this was observed in the xylem cells only. The cellulose content of the transgenic plants was not altered. These results support the hypothesis that CBMs can modify cell wall structure leading to modulation of wall loosening and plant growth.

Coated vesicles in plant cells

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

Two short sequences from amaranth 11S globulin are sufficient to target green fluorescent protein and beta-glucuronidase to vacuoles in Arabidopsis cells

Vacuolar sorting of seed storage proteins is a very complex process since several sorting pathways and interactions among proteins of different classes have been reported. In addition, although the C-terminus of several 7S proteins is important for vacuolar delivery, other signals seem also to be involved in this process. In this work, the ability of two sequences of the Amaranthus hypochondriacus 11S globulin (amaranthin) to target reporter proteins to vacuoles was studied. We show that the C-terminal pentapeptide (KISIA) and the GNIFRGF internal sequence fused at the C terminal region of genes encoding secretory versions of green fluorescent protein (GFP) and GFP-beta-glucuronidase (GFP-GUS) were sufficient to redirect these reporter proteins to the vacuole of Arabidopsis cells. According to the three-dimensional structure of 7S and 11S storage globulins, this internal vacuolar sorting sequence corresponds to the alpha helical region involved in trimer formation, and is conserved within these families. In addition, these sequences were able to interact in vitro, in a calcium dependent manner, with the sunflower vacuolar sorting receptor homolog to pea BP-80/AtVSR1/pumpkin PV72. This work shows for the first time the role of a short internal sequence conserved among 7S and 11S proteins in vacuolar sorting.

Secretion marker proteins and cell-wall polysaccharides move through different secretory pathways

The building up of the cell wall is tightly dependent on the functionality of the secretory pathway. Syntaxins as well as other SNARE proteins play important roles during vesicle secretion and fusion. We have compared the secretion of newly synthesised cell-wall polysaccharides to that of secretory marker proteins such as secreted green-fluorescent protein (sec-GFP) and secreted rat preputial beta-glucuronidase (secRGUS) in leaf protoplasts and roots of wild-type and transgenic Nicotiana tabacum plants, overexpressing a syntaxin homologue NtSyr1 (Sp1) and its soluble variant Sp2 that interferes specifically with Sp1 function, affecting post-Golgi transport. In protoplasts transiently transformed with secGFP and Sp1, no variation was observed in the pattern of fluorescence with respect to control; on the contrary, GFP fluorescence accumulate within the cells in protoplasts co-transformed with secGFP and Sp2. Sp2 reduced the percentage of marker protein secretion to 53% as quantified with secRGUS. In protoplasts obtained from leaves of wild-type and transformed tobacco plants expressing Sp1, Sp2 and Sp1 plus Sp2, no remarkable differences in the percentage of newly synthesised polysaccharides incorporated into the regenerating cell walls were observed. The same results were confirmed in roots of whole transformed seedlings. Tests with cytochalasin D (CD) showed a marked decrease in the amount of newly synthesised polysaccharides into the wall and a simultaneous sharp increase in membrane-associated polysaccharides. SecRGUS secretion was also inhibited by CD. The data indicate that marker proteins and matrix polysaccharides, as well as cellulose synthase complexes, are secreted through the involvement of different secretory machineries.

Secretion marker proteins and cell-wall polysaccharides move through different secretory pathways

The building up of the cell wall is tightly dependent on the functionality of the secretory pathway. Syntaxins as well as other SNARE proteins play important roles during vesicle secretion and fusion. We have compared the secretion of newly synthesised cell-wall polysaccharides to that of secretory marker proteins such as secreted green-fluorescent protein (sec-GFP) and secreted rat preputial beta-glucuronidase (secRGUS) in leaf protoplasts and roots of wild-type and transgenic Nicotiana tabacum plants, overexpressing a syntaxin homologue NtSyr1 (Sp1) and its soluble variant Sp2 that interferes specifically with Sp1 function, affecting post-Golgi transport. In protoplasts transiently transformed with secGFP and Sp1, no variation was observed in the pattern of fluorescence with respect to control; on the contrary, GFP fluorescence accumulate within the cells in protoplasts co-transformed with secGFP and Sp2. Sp2 reduced the percentage of marker protein secretion to 53% as quantified with secRGUS. In protoplasts obtained from leaves of wild-type and transformed tobacco plants expressing Sp1, Sp2 and Sp1 plus Sp2, no remarkable differences in the percentage of newly synthesised polysaccharides incorporated into the regenerating cell walls were observed. The same results were confirmed in roots of whole transformed seedlings. Tests with cytochalasin D (CD) showed a marked decrease in the amount of newly synthesised polysaccharides into the wall and a simultaneous sharp increase in membrane-associated polysaccharides. SecRGUS secretion was also inhibited by CD. The data indicate that marker proteins and matrix polysaccharides, as well as cellulose synthase complexes, are secreted through the involvement of different secretory machineries.

Roles of the ubiquitin/proteasome pathway in pollen tube growth with emphasis on MG132-induced alterations in ultrastructure, cytoskeleton, and cell wall components

The ubiquitin/proteasome pathway represents one of the most important proteolytic systems in eukaryotes and has been proposed as being involved in pollen tube growth, but the mechanism of this involvement is still unclear. Here, we report that proteasome inhibitors MG132 and epoxomicin significantly prevented Picea wilsonii pollen tube development and markedly altered tube morphology in a dose- and time-dependent manner, while hardly similar effects were detected when cysteine-protease inhibitor E-64 was used. Fluorogenic kinetic assays using fluorogenic substrate sLLVY-AMC confirmed MG132-induced inhibition of proteasome activity. The inhibitor-induced accumulation of ubiquitinated proteins (UbPs) was also observed using immunoblotting. Transmission electron microscopy revealed that MG132 induces endoplasmic reticulum (ER)-derived cytoplasmic vacuolization. Immunogold-labeling analysis demonstrated a significant accumulation of UbPs in degraded cytosol and dilated ER in MG132-treated pollen tubes. Fluorescence labeling with fluorescein isothiocyanate-phalloidin and beta-tubulin antibody revealed that MG132 disrupts the organization of F-actin and microtubules and consequently affects cytoplasmic streaming in pollen tubes. However, tip-focused Ca2+ gradient, albeit reduced, seemingly persists after MG132 treatment. Finally, fluorescence labeling with antipectin antibodies and calcofluor indicated that MG132 treatment induces a sharp decline in pectins and cellulose. This result was confirmed by Fourier transform infrared analysis, thus demonstrating for the first time the inhibitor-induced weakening of tube walls. Taken together, these findings suggest that MG132 treatment promotes the accumulation of UbPs in pollen tubes, which induces ER-derived cytoplasmic vacuolization and depolymerization of cytoskeleton and consequently strongly affects the deposition of cell wall components, providing a mechanistic framework for the functions of proteasome in the tip growth of pollen tubes.

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.

Golgi-mediated vacuolar sorting of the endoplasmic reticulum chaperone BiP may play an active role in quality control within the secretory pathway

Quality control in the endoplasmic reticulum (ER) prevents the arrival of incorrectly or incompletely folded proteins at their final destinations and targets permanently misfolded proteins for degradation. Such proteins have a high affinity for the ER chaperone BiP and are finally degraded via retrograde translocation from the ER lumen back to the cytosol. This ER-associated protein degradation (ERAD) is currently thought to constitute the main disposal route, but there is growing evidence for a vacuolar role in quality control. We show that BiP is transported to the vacuole in a wortmannin-sensitive manner in tobacco (Nicotiana tabacum) and that it could play an active role in this second disposal route. ER export of BiP occurs via COPII-dependent transport to the Golgi apparatus, where it competes with other HDEL receptor ligands. When HDEL-mediated retrieval from the Golgi fails, BiP is transported to the lytic vacuole via multivesicular bodies, which represent the plant prevacuolar compartment. We also demonstrate that a subset of BiP-ligand complexes is destined to the vacuole and differs from those likely to be disposed of via the ERAD pathway. Vacuolar disposal could act in addition to ERAD to maximize the efficiency of quality control in the secretory pathway.

High-yield production of authentic human growth hormone using a plant virus-based expression system

We describe here a high-yield transient expression system for the production of human growth hormone (hGH, or somatotropin) in transfected Nicotiana benthamiana leaves. The system is based on a recently described plant virus-based modular expression vector [Gleba, Y., Marillonnet, S. and Klimyuk, V. (2004) Engineering viral expression vectors for plants: the 'full virus' and the 'deconstructed virus' strategies. Curr. Opin. Plant Biol. 7, 182-188; Marillonnet, S., Giritch, A., Gils, M., Kandzia, R., Klimyuk, V. and Gleba, Y. (2004) In planta engineering of viral RNA replicons: efficient assembly by recombination of DNA modules delivered by Agrobacterium. Proc. Natl. Acad. Sci. USA, 101, 6852-6857], and represents a simple and fast alternative to stable transformation. By using various combinations of provector modules, hGH was produced in three compartments of the cell: the apoplast, the chloroplast and the cytosol. We found that targeting to the apoplast provided the highest amount of correctly processed and biologically active hGH, with a yield of up to 10% of total soluble protein or 1 mg per gram of fresh weight leaf biomass. These results indicate that the use of viral vectors for high-yield production of human therapeutic proteins in plants by transient expression provides an attractive alternative to production protocols using standard expression vectors in transgenic or transplastomic plants.

Optimization of Acidothermus cellulolyticus endoglucanase (E1) production in transgenic tobacco plants by transcriptional, post-transcription and post-translational modification

An attempt was made to obtain a high-level production of intact Acidothermus cellulolyticus endoglucanase (E1) in transgenic tobacco plants. The E1 expression was examined under the control of the constitutive and strong Mac promoter or light-inducible tomato Rubisco small sub-unit (RbcS-3C) promoter with its original or Alfalfa Mosaic Virus (AMV) RNA4 5'-untranslated leader (UTL) and targeted to different sub-cellular compartments via transit peptides. The transit peptides included native E1, endoplasmic reticulum, vacuole, apoplast, and chloroplast. E1 expression and its stability in transgenic plants were determined via E1 activity, protein immunoblotting, and RNA gel-blotting analyses. Effects of sub-cellular compartments on E1 production and its stability were determined in transgenic tobacco plants carrying one of six transgene expression vectors, where the E1 was under the control of Mac promoter, mannopine synthase transcription terminator, and one of the five transit peptides. Transgenic tobacco plants with an apoplastic transit peptide had the highest average E1 activity and protein accumulation, which was about 0.25% of total leaf soluble proteins estimated via E1 specific activity and protein gel blots. Intercellular fluid analyses confirmed that E1 signal peptide functioned properly in tobacco cells to secret E1 protein into the apoplast. By replacing RbcS-3C UTL with AMV RNA4 UTL E1 production was enhanced more than twofold, while it was less effective than the mannopine synthase UTL. It was observed that RbcS-3C promoter was more favorable for E1 expression in transgenic plants than the Mac promoter. E1 activity in dried tobacco seeds stored one year at room temperature was 45% higher than that observed immediately after harvesting, suggesting that E1 protein can be stored at room temperature for a long period. E1 stability in different sub-cellular compartments and the optimal combination of promoter, 5'-UTL, and sub-cellular compartmentation for heterologous protein production in transgenic plants are discussed.

KATAMARI1/MURUS3 Is a novel golgi membrane protein that is required for endomembrane organization in Arabidopsis

In plant cells, unlike animal and yeast cells, endomembrane dynamics appear to depend more on actin filaments than on microtubules. However, the molecular mechanisms of endomembrane-actin filament interactions are unknown. In this study, we isolated and characterized an Arabidopsis thaliana mutant, katamari1 (kam1), which has a defect in the organization of endomembranes and actin filaments. The kam1 plants form abnormally large aggregates that consist of endoplasmic reticulum with actin filaments in the perinuclear region within the cells and are defective in normal cell elongation. Map-based cloning revealed that the KAM1 gene is allelic to the MUR3 gene. We demonstrate that the KAM1/MUR3 protein is a type II membrane protein composed of a short cytosolic N-terminal domain and a transmembrane domain followed by a large lumenal domain and is localized specifically on Golgi membranes. We further show that actin filaments interact with Golgi stacks via KAM1/MUR3 to maintain the proper organization of endomembranes. Our results provide functional evidence that KAM1/MUR3 is a novel component of the Golgi-mediated organization of actin functioning in proper endomembrane organization and cell elongation.

Ethylene signal transduction

BACKGROUND

The phytohormone ethylene is a key regulator of plant growth and development. Components of the pathway for ethylene signal transduction were identified by genetic approaches in Arabidopsis and have now been shown to function in agronomically important plants as well.

SCOPE

This review focuses on recent advances in our knowledge on ethylene signal transduction, in particular on recently proposed components of the pathway, on the interaction between the pathway components and on the roles of transcriptional and post-transcriptional regulation in ethylene signalling.

CONCLUSIONS

Data indicate that the site of ethylene perception is at the endoplasmic reticulum and point to the importance of protein complexes in mediating the initial steps in ethylene signal transduction. The expression level of pathway components is regulated by both transcriptional and post-transcriptional mechanisms, degradation of the transcription factor EIN3 being a primary means by which the sensitivity of plants to ethylene is regulated. EIN3 also represents a control point for cross-talk with other signalling pathways, as exemplified by the effects of glucose upon its expression level. Amplification of the initial ethylene signal is likely to play a significant role in signal transduction and several mechanisms exist by which this may occur based on properties of known pathway components. Signal output from the pathway is mediated in part by carefully orchestrated changes in gene expression, the breadth of these changes now becoming clear through expression analysis using microarrays.

Conserved ERAD-like quality control of a plant polytopic membrane protein

The endoplasmic reticulum (ER) of eukaryotic cells serves as a checkpoint tightly monitoring protein integrity and channeling malformed proteins into different rescue and degradation routes. The degradation of several ER lumenal and membrane-localized proteins is mediated by ER-associated protein degradation (ERAD) in yeast (Saccharomyces cerevisiae) and mammalian cells. To date, evidence for the existence of ERAD-like mechanisms in plants is indirect and based on heterologous or artificial substrate proteins. Here, we show that an allelic series of single amino acid substitution mutants of the plant-specific barley (Hordeum vulgare) seven-transmembrane domain mildew resistance o (MLO) protein generates substrates for a postinsertional quality control process in plant, yeast, and human cells, suggesting conservation of the underlying mechanism across kingdoms. Specific stabilization of mutant MLO proteins in yeast strains carrying defined defects in protein quality control demonstrates that MLO degradation is mediated by HRD pathway-dependent ERAD. In plants, individual aberrant MLO proteins exhibit markedly reduced half-lives, are polyubiquitinated, and can be stabilized through inhibition of proteasome activity. This and a dependence on homologs of the AAA ATPase CDC48/p97 to eliminate the aberrant variants strongly suggest that MLO proteins are endogenous substrates of an ERAD-related plant quality control mechanism.

MEMBRANE TRAFFICKING IN PLANTS

Plant membrane trafficking shares many features with other eukaryotic organisms, including the machinery for vesicle formation and fusion. However, the plant endomembrane system lacks an ER-Golgi intermediate compartment, has numerous Golgi stacks and several types of vacuoles, and forms a transient compartment during cell division. ER-Golgi trafficking involves bulk flow and efficient recycling of H/KDEL-bearing proteins. Sorting in the Golgi stacks separates bulk flow to the plasma membrane from receptor-mediated trafficking to the lytic vacuole. Cargo for the protein storage vacuole is delivered from the endoplasmic reticulum (ER), cis-Golgi, and trans-Golgi. Endocytosis includes recycling of plasma membrane proteins from early endosomes. Late endosomes appear identical with the multivesiculate prevacuolar compartment that lies on the Golgi-vacuole trafficking pathway. In dividing cells, homotypic fusion of Golgi-derived vesicles forms the cell plate, which expands laterally by targeted vesicle fusion at its margin, eventually fusing with the plasma membrane.

A novel C-terminal sequence from barley polyamine oxidase is a vacuolar sorting signal

Barley contains two different isoforms of flavin-containing polyamine oxidase (BPAO1 and BPAO2). We have previously demonstrated that BPAO2 is a symplastic protein in barley leaves. On the contrary, maize polyamine oxidase (MPAO), the best characterized member of this enzyme class, is apoplastic. Comparison of the derived amino-acid sequences of BPAO2 and MPAO has revealed that both precursor proteins include a cleavable N-terminal signal peptide of 25 amino acid residues, but the barley enzyme shows an extra C-terminal extension of eight amino acids. By means of MPAO engineering with BPAO2 C-terminal tail (MPAO-T) and exploiting transient expression in Nicotiana tabacum protoplasts, we demonstrate that this oligopeptide is a signal for protein sorting to the plant vacuole. The vacuolar sorting of MPAO-T was saturable. Specific mutations of the C-terminal tail were constructed to determine which amino acid residues of this novel propeptide affect proper protein sorting. No consensus sequence or common structural determinant is required for the intracellular retention of the MPAO-T protein, but a gradual lowering of the efficiency was observed as a result of progressive deletion of the C-terminus.

Zeolin. A new recombinant storage protein constructed using maize gamma-zein and bean phaseolin

The major seed storage proteins of maize (Zea mays) and bean (Phaseolus vulgaris), zein and phaseolin, accumulate in the endoplasmic reticulum (ER) and in storage vacuoles, respectively. We show here that a chimeric protein composed of phaseolin and 89 amino acids of gamma-zein, including the repeated and the Pro-rich domains, maintains the main characteristics of wild-type gamma-zein: It is insoluble unless its disulfide bonds are reduced and forms ER-located protein bodies. Unlike wild-type phaseolin, the protein, which we called zeolin, accumulates to very high amounts in leaves of transgenic tobacco (Nicotiana tabacum). A relevant proportion of the ER chaperone BiP is associated with zeolin protein bodies in an ATP-sensitive fashion. Pulse-chase labeling confirms the high affinity of BiP to insoluble zeolin but indicates that, unlike structurally defective proteins that also extensively interact with BiP, zeolin is highly stable. We conclude that the gamma-zein portion is sufficient to induce the formation of protein bodies also when fused to another protein. Because the storage proteins of cereals and legumes nutritionally complement each other, zeolin can be used as a starting point to produce nutritionally balanced and highly stable chimeric storage proteins.

Targeting of a Nicotiana plumbaginifolia H+ -ATPase to the plasma membrane is not by default and requires cytosolic structural determinants

The structural determinants involved in the targeting of multitransmembrane-span proteins to the plasma membrane (PM) remain poorly understood. The plasma membrane H+ -ATPase (PMA) from Nicotiana plumbaginifolia, a well-characterized 10 transmembrane-span enzyme, was used as a model to identify structural elements essential for targeting to the PM. When PMA2 and PMA4, representatives of the two main PMA subfamilies, were fused to green fluorescent protein (GFP), the chimeras were shown to be still functional and to be correctly and rapidly targeted to the PM in transgenic tobacco. By contrast, chimeric proteins containing various combinations of PMA transmembrane spanning domains accumulated in the Golgi apparatus and not in the PM and displayed slow traffic properties through the secretory pathway. Individual deletion of three of the four cytosolic domains did not prevent PM targeting, but deletion of the large loop or of its nucleotide binding domain resulted in GFP fluorescence accumulating exclusively in the endoplasmic reticulum. The results show that, at least for this polytopic protein, the PM is not the default pathway and that, in contrast with single-pass membrane proteins, cytosolic structural determinants are required for correct targeting.

Where, oh where has my protein gone?

A recent publication by R. Chikwamba and colleagues highlights interesting issues in recombinant protein expression in transgenic plants. In the study they expressed a bacterial antigen in maize seed and obtained aberrant localization data. This work is of great importance to the biotechnology industry and raises fascinating questions in plant cell biology that require creative thinking.

A signal peptide secretion screen in Fucus distichus embryos reveals expression of glucanase, EGF domain-containing, and LRR receptor kinase-like polypeptides during asymmetric cell growth

Zygotes of the brown alga Fucus distichus (L.) Powell develop polarity prior to the first embryonic cell division and retain a pattern of asymmetric growth during early embryogenesis. In order to identify F. distichus polypeptides secreted during asymmetric cell growth, we used a functional assay in Saccharomyces cerevisiae to screen a cDNA library generated from asymmetrically growing Fucus embryos for sequences encoding polypeptides that function as signal peptides for secretion. We isolated and sequenced 222 plasmids containing Fucus cDNAs encoding signal peptide activity. The cDNA inserts from these plasmids were translated in silico into 244 potential polypeptide sequences, 169 of which are predicted to contain signal peptides. BlastP analysis of the Fucus sequences revealed similarity between many Fucus proteins and cell surface proteins that function in development in other eukaryotes, including epidermal growth factor (EGF)-like repeat-containing proteins, plant leucine-rich repeat (LRR)-receptor kinases, and algal beta-1, 3-exoglucanase. However, most of the isolated Fucus polypeptides lack similarity to known proteins. The isolation of cDNAs encoding secreted Fucus proteins provides an important step toward characterizing cell surface proteins important for asymmetric organization and growth in fucoid embryos.

A phaseolin domain involved directly in trimer assembly is a determinant for binding by the chaperone BiP

The binding protein (BiP; a member of the heat-shock 70 family) is a major chaperone of the endoplasmic reticulum (ER). Interactions with BiP are believed to inhibit unproductive aggregation of newly synthesized secretory proteins during folding and assembly. In vitro, BiP has a preference for peptide sequences enriched in hydrophobic amino acids, which are expected to be exposed only in folding and assembly intermediates or in defective proteins. However, direct information regarding sequences recognized in vivo by BiP on real proteins is very limited. We have shown previously that newly synthesized monomers of the homotrimeric storage protein phaseolin associate with BiP and that phaseolin trimerization in the ER abolishes such interactions. Using different phaseolin constructs and green fluorescent protein (GFP) fusion proteins, we show here that one of the two alpha-helical regions of polypeptide contact in phaseolin trimers (35 amino acids located close to the C terminus and containing three potential BiP binding sites) effectively promotes BiP association with phaseolin and with secretory GFP fusions expressed in transgenic tobacco or in transfected protoplasts. We also show that overexpressed BiP transiently sequesters phaseolin polypeptides. We conclude that one of the regions of monomer contact is a BiP binding determinant and suggest that during the synthesis of phaseolin, the association with BiP and trimer formation are competing events. Finally, we show that the other, internal region of contact between monomers is necessary for phaseolin assembly in vivo and contains one potential BiP binding site.

Protein transport in plant cells: in and out of the Golgi

In plant cells, the Golgi apparatus is the key organelle for polysaccharide and glycolipid synthesis, protein glycosylation and protein sorting towards various cellular compartments. Protein import from the endoplasmic reticulum (ER) is a highly dynamic process, and new data suggest that transport, at least of soluble proteins, occurs via bulk flow. In this Botanical Briefing, we review the latest data on ER/Golgi inter-relations and the models for transport between the two organelles. Whether vesicles are involved in this transport event or if direct ER-Golgi connections exist are questions that are open to discussion. Whereas the majority of proteins pass through the Golgi on their way to other cell destinations, either by vesicular shuttles or through maturation of cisternae from the cis- to the trans-face, a number of membrane proteins reside in the different Golgi cisternae. Experimental evidence suggests that the length of the transmembrane domain is of crucial importance for the retention of proteins within the Golgi. In non-dividing cells, protein transport out of the Golgi is either directed towards the plasma membrane/cell wall (secretion) or to the vacuolar system. The latter comprises the lytic vacuole and protein storage vacuoles. In general, transport to either of these from the Golgi depends on different sorting signals and receptors and is mediated by clathrin-coated and dense vesicles, respectively. Being at the heart of the secretory pathway, the Golgi (transiently) accommodates regulatory proteins of secretion (e.g. SNAREs and small GTPases), of which many have been cloned in plants over the last decade. In this context, we present a list of regulatory proteins, along with structural and processing proteins, that have been located to the Golgi and the 'trans-Golgi network' by microscopy.

Molecular cloning and characterization of hazel pollen protein (70 kD) as a luminal binding protein (BiP): a novel cross-reactive plant allergen

BACKGROUND

Tree pollen contains many allergens showing cross-reactivity to proteins from pollen, seeds, and fruits of different plant species. Amongst Fagales, responsible for several allergenic responses, hazel provides the best material to study pollen as well as food allergens in one species. The aim of this study was to identify and characterize the physiological function of an allergen from hazel pollen and to determine possible cross-reactivity to proteins from hazelnut.

METHODS

Monoclonal antibodies (mAbs) against hazel pollen crude extract were produced. On the basis of IgE binding, demonstrated by sera from patients allergic to hazel pollen, one mAb indicating the best correlation has been selected, and the putative allergen was purified by preparative gel electrophoresis. Isoforms were investigated by two-dimensional PAGE, and for molecular identification a hazel pollen cDNA library was constructed. In situ localization of the allergen during pollen development was performed by immunofluorescence labelling.

RESULTS

Immunological staining of crude hazel pollen extract with specific IgE and mAb revealed a 70-kD protein. Immunoblot studies with mAb showed cross-reactive proteins of 70-72 kD in different plant tissues and species. After protein purification, the IgE-binding reactivity of the allergen has been reconfirmed, and two isoforms were detected. Molecular cloning identified the allergen as a luminal binding protein (BiP) of the Hsp70 family with 88-92% sequence identity in various plants. Further immunocytological studies indicated involvement of BiP during pollen development.

CONCLUSIONS

Chaperons like BiP play an important role in protein synthesis and in the protection of cellular structures during stress-related processes. Because of their highly conserved protein sequences, we propose that such allergens could be responsible for at least a part of the allergenic cross-reactivity between proteins from different pollens and plant foods.

The C-terminal extension of a hybrid immunoglobulin A/G heavy chain is responsible for its Golgi-mediated sorting to the vacuole

We have assessed the ability of the plant secretory pathway to handle the expression of complex heterologous proteins by investigating the fate of a hybrid immunoglobulin A/G in tobacco cells. Although plant cells can express large amounts of the antibody, a relevant proportion is normally lost to vacuolar sorting and degradation. Here we show that the synthesis of high amounts of IgA/G does not impose stress on the plant secretory pathway. Plant cells can assemble antibody chains with high efficiency and vacuolar transport occurs only after the assembled immunoglobulins have traveled through the Golgi complex. We prove that vacuolar delivery of IgA/G depends on the presence of a cryptic sorting signal in the tailpiece of the IgA/G heavy chain. We also show that unassembled light chains are efficiently secreted as monomers by the plant secretory pathway.