Role of propeptide glycan in post-translational processing and transport of barley lectin to vacuoles in transgenic tobacco

N-Linked Glycosylation Modulates Golgi-Independent Vacuolar Sorting Mediated by the Plant Specific Insert

In plant cells, the conventional route to the vacuole involves the endoplasmic reticulum, the Golgi and the prevacuolar compartment. However, over the years, unconventional sorting to the vacuole, bypassing the Golgi, has been described, which is the case of the Plant-Specific Insert (PSI) of the aspartic proteinase cardosin A. Interestingly, this Golgi-bypass ability is not a characteristic shared by all PSIs, since two related PSIs showed to have different sensitivity to ER-to-Golgi blockage. Given the high sequence similarity between the PSI domains, we sought to depict the differences in terms of post-translational modifications. In fact, one feature that draws our attention is that one is N-glycosylated and the other one is not. Using site-directed mutagenesis to obtain mutated versions of the two PSIs, with and without the glycosylation motif, we observed that altering the glycosylation pattern interferes with the trafficking of the protein as the non-glycosylated PSI-B, unlike its native glycosylated form, is able to bypass ER-to-Golgi blockage and accumulate in the vacuole. This is also true when the PSI domain is analyzed in the context of the full-length cardosin. Regardless of opening exciting research gaps, the results obtained so far need a more comprehensive study of the mechanisms behind this unconventional direct sorting to the vacuole.

Delivering of proteins to the plant vacuole--an update

Trafficking of soluble cargo to the vacuole is far from being a closed issue as it can occur by different routes and involve different intermediates. The textbook view of proteins being sorted at the post-Golgi level to the lytic vacuole via the pre-vacuole or to the protein storage vacuole mediated by dense vesicles is now challenged as novel routes are being disclosed and vacuoles with intermediate characteristics described. The identification of Vacuolar Sorting Determinants is a key signature to understand protein trafficking to the vacuole. Despite the long established vacuolar signals, some others have been described in the last few years, with different properties that can be specific for some cells or some types of vacuoles. There are also reports of proteins having two different vacuolar signals and their significance is questionable: a way to increase the efficiency of the sorting or different sorting depending on the protein roles in a specific context? Along came the idea of differential vacuolar sorting, suggesting a possible specialization of the trafficking pathways according to the type of cell and specific needs. In this review, we show the recent advances in the field and focus on different aspects of protein trafficking to the vacuoles.

Cardosin A contains two vacuolar sorting signals using different vacuolar routes in tobacco epidermal cells

Several vacuolar sorting determinants (VSDs) have been described for protein trafficking to the vacuoles in plant cells. Because of the variety in plant models, cell types and experimental approaches used to decipher vacuolar targeting processes, it is not clear whether the three well-known groups of VSDs identified so far exhaust all the targeting mechanisms, nor if they reflect certain protein types or families. The vacuolar targeting mechanisms of the aspartic proteinases family, for instance, are not yet fully understood. In previous studies, cardosin A has proven to be a good reporter for studying the vacuolar sorting of aspartic proteinases. We therefore propose to explore the roles of two different cardosin A domains, common to several aspartic proteinases [i.e. the plant-specific insert (PSI) and the C-terminal peptide VGFAEAA] in vacuolar sorting. Several truncated versions of the protein conjugated with fluorescent protein were made, with and without these putative sorting determinants. These domains were also tested independently, for their ability to sort other proteins, rather than cardosin A, to the vacuole. Fluorescent chimaeras were tracked in vivo, by confocal laser scanning microscopy, in Nicotiana tabacum cells. Results demonstrate that either the PSI or the C terminal was necessary and sufficient to direct fluorescent proteins to the vacuole, confirming that they are indeed vacuolar sorting determinants. Further analysis using blockage experiments of the secretory pathway revealed that these two VSDs mediate two different trafficking pathways.

N-glycosylation does not affect the catalytic activity of ricin a chain but stimulates cytotoxicity by promoting its transport out of the endoplasmic reticulum

Ricin A chain (RTA) depurinates the α-sarcin/ricin loop after it undergoes retrograde trafficking to the cytosol. The structural features of RTA involved in intracellular transport are not known. To explore this, we fused enhanced green fluorescent protein (EGFP) to precursor (preRTA-EGFP), containing a 35-residue leader, and mature RTA (matRTA-EGFP). Both were enzymatically active and toxic in Saccharomyces cerevisiae. PreRTA-EGFP was localized in the endoplasmic reticulum (ER) initially and was subsequently transported to the vacuole, whereas matRTA-EGFP remained in the cytosol, indicating that ER localization is a prerequisite for vacuole transport. When the two glycosylation sites in RTA were mutated, the mature form was fully active and toxic, suggesting that the mutations do not affect catalytic activity. However, nonglycosylated preRTA-EGFP had reduced toxicity, depurination and delayed vacuole transport, indicating that N-glycosylation affects transport of RTA out of the ER. Point mutations in the C-terminal hydrophobic region restricted RTA to the ER and eliminated toxicity and depurination, indicating that this sequence is critical for ER exit. These results demonstrate that N-glycosylation and the C-terminal hydrophobic region stimulate the toxicity of RTA by promoting ER export. The timing of depurination coincided with the timing of vacuole transport, suggesting that RTA may enter the cytosol during vacuole transport.

Expression of a glycosylated GFP as a bivalent reporter in exocytosis

The complex-type N-linked glycans of plants differ markedly in structure from those of animals. Like those of insects and mollusks they lack terminal sialic acid(s) and may contain an alpha-(1,3)-fucose (Fuc) linked to the proximal GlcNAc residue and/or a beta-(1,2)-xylose (Xyl) residue attached to the proximal mannose (Man) of the glycan core. N-glycosylated GFPs were used in previous studies showing their effective use to report on membrane traffic between the ER and the Golgi apparatus in plant cells. In all these cases glycosylated tags were added at the GFP termini. Because of the position of the tag and depending on the sorting and accumulation site of these modified GFP, there is always a risk of processing and degradation, and this protein design cannot be considered ideal. Here, we describe the development of three different GFPs in which the glycosylation site is internally localized at positions 80, 133, or 172 in the internal sequence. The best glycosylation site was at position 133. This glycosylated GFPgl133 appears to be protected from undesired processing of the glycosylation site and represents a bivalent reporter for biochemical and microscopic studies. After experimental validation, we can conclude that amino acid 133 is an effective glycosylation site and that the GFPgl133 is a powerful tool for in vivo investigations in plant cell biology.

Expression of a glycosylated GFP as a bivalent reporter in exocytosis

The complex-type N-linked glycans of plants differ markedly in structure from those of animals. Like those of insects and mollusks they lack terminal sialic acid(s) and may contain an alpha-(1,3)-fucose (Fuc) linked to the proximal GlcNAc residue and/or a beta-(1,2)-xylose (Xyl) residue attached to the proximal mannose (Man) of the glycan core. N-glycosylated GFPs were used in previous studies showing their effective use to report on membrane traffic between the ER and the Golgi apparatus in plant cells. In all these cases glycosylated tags were added at the GFP termini. Because of the position of the tag and depending on the sorting and accumulation site of these modified GFP, there is always a risk of processing and degradation, and this protein design cannot be considered ideal. Here, we describe the development of three different GFPs in which the glycosylation site is internally localized at positions 80, 133, or 172 in the internal sequence. The best glycosylation site was at position 133. This glycosylated GFPgl133 appears to be protected from undesired processing of the glycosylation site and represents a bivalent reporter for biochemical and microscopic studies. After experimental validation, we can conclude that amino acid 133 is an effective glycosylation site and that the GFPgl133 is a powerful tool for in vivo investigations in plant cell biology.

Plant Vacuolar ATP-binding Cassette Transporters That Translocate Folates and Antifolates in Vitro and Contribute to Antifolate Tolerance in Vivo

The vacuoles of pea (Pisum sativum) leaves and red beet (Beta vulgaris) storage root are major sites for the intracellular compartmentation of folates. In the light of these findings and preliminary experiments indicating that some plant multidrug resistance-associated protein (MRP) subfamily ATP-binding cassette transporters are able to transport compounds of this type, the Arabidopsis thaliana vacuolar MRP, AtMRP1 (AtABCC1), and its functional equivalent(s) in vacuolar membrane vesicles purified from red beet storage root were studied. In so doing, it has been determined that heterologously expressed AtMRP1 and its equivalents in red beet vacuolar membranes are not only competent in the transport of glutathione conjugates but also folate monoglutamates and antifolates as exemplified by pteroyl-l-glutamic acid and methotrexate (MTX), respectively. In agreement with the results of these in vitro transport measurements, analyses of atmrp1 T-DNA insertion mutants of Arabidopsis ecotypes Wassilewskia and Columbia disclose an MTX-hypersensitive phenotype. atmrp1 knock-out mutants are more sensitive than wild-type plants to growth retardation by nanomolar concentrations of MTX, and this is associated with impaired vacuolar antifolate sequestration. The vacuoles of protoplasts isolated from the leaves of Wassilewskia atmrp1 mutants accumulate 50% less [(3)H]MTX than the vacuoles of protoplasts from wild-type plants when incubated in media containing nanomolar concentrations of this antifolate, and vacuolar membrane-enriched vesicles purified from the mutant catalyze MgATP-dependent [(3)H]MTX uptake at only 40% of the capacity of the equivalent membrane fraction from wild-type plants. AtMRP1 and its counterparts in other plant species therefore have the potential for participating in the vacuolar accumulation of folates and related compounds.

The shoot meristem identity gene TFL1 is involved in flower development and trafficking to the protein storage vacuole

Plants are unique in their ability to store proteins in specialized protein storage vacuoles (PSVs) within seeds and vegetative tissues. Although plants use PSV proteins during germination, before photosynthesis is fully functional, the roles of PSVs in adult vegetative tissues are not understood. Trafficking pathways to PSVs and lytic vacuoles appear to be distinct. Lytic vacuoles are analogous evolutionarily to yeast and mammalian lysosomes. However, it is unclear whether trafficking to PSVs has any analogy to pathways in yeast or mammals, nor is PSV ultrastructure known in Arabidopsis vegetative tissue. Therefore, alternative approaches are required to identify components of this pathway. Here, we show that an Arabidopsis thaliana mutant that disrupts PSV trafficking identified TERMINAL FLOWER 1 (TFL1), a shoot meristem identity gene. The tfl1-19/mtv5 (for "modified traffic to the vacuole") mutant is specifically defective in trafficking of proteins to the PSV. TFL1 localizes to endomembrane compartments and colocalizes with the putative delta-subunit of the AP-3 adapter complex. Our results suggest a developmental role for the PSV in vegetative tissues.

Electrochemical biosensors for monitoring the recognition of glycoprotein–lectin interactions

Despite the wide applicability and specificity of lectins to carbohydrate moieties, there are few lectin specific biosensors. This is attributed to the difficulty in defining the relevant experimental parameters to measure for sensing. We hereby describe the development of direct and indirect electrochemical sensors to determine the exact trace amounts of probarley lectin (ProBL) and its conversion product wheat germ agglutinin (WGA). In addition to WGA, the antigens (ProBL) employed in this study were over expressed in bacteria, isolated from protein bodies, and purified using immobilized N-acetylglusamine in order to obtain correctly folded active lectins. The amperometric immunosensor uses cell lines producing monoclonal antibody (mAB) to the pro-region of ProBL over expressed from Escherichia coli. The efficacy and sensing characteristics of the lectin were optimized using monoclonal antibody to WGA and the resulting sensor was found to detect only ProBL in the linear range 10(-3)-10(2) microg mL(-1) and a detection limit of 10(-3) microg mL(-1).

Expression of a beta-glucosidase gene results in increased accumulation of salicylic acid in transgenic Nicotiana tabacum cv. Xanthi-nc NN genotype

A beta-glucosidase gene (bglA) from Butyrivibrio fibrisolvens H17c was cloned into the binary vector pGA482 under the control of the 35S Cauliflower Mosaic Virus (CaMV) promoter. A second construct was generated for accumulation of the bglA gene product in the vacuole of transformed tobacco plants. Reverse transcription-polymerase chain reaction analysis demonstrated that the bglA gene was expressed in 71% of cytosol-targeted and 67% of vacuole-targeted transgenic tobacco T(1) plants. T(1) transgenic plants (pGLU100 and pGLU200) exhibited elevated levels of free salicylic acid (SA) with a concomitant significant decrease in the level of glucosylsalicylic acid (GSA) compared to the untransformed tobacco plants and tobacco plants transformed with the empty vector (pGA482). Following inoculation with Tobacco Mosaic Virus (TMV), lesion area was 51% smaller in pGLU100 plants and 60% smaller in pGLU200 plants compared to inoculated untransformed and negative control plants.

Enhancement of essential amino acid contents in crops by genetic engineering and protein design

The importance and urgency of providing humans and animals with quality proteins are reflected in the growing scientific and industrial interest in augmenting the nutritive value of the world's protein sources. Such nutritive value is determined by the protein content in 'essential amino acids', those that cannot be synthesized de novo and that must be supplied from the diet. It is the object of this review to discuss recent advances in the genetic modification of crops that aim to provide enhanced quantities of essential amino acids.

Fusion with HDEL protects cell wall invertase from early degradation when N-glycosylation is inhibited

Previous data obtained in different suspension-cultured plant cells have clearly illustrated that N-glycans are absolutely required for transport of glycoproteins to the extracellular compartment, regardless of their oligosaccharide structure [see Lerouge et al. (1998) Plant Mol. Biol. 38: 31 for review]. In the present study the role of N-glycosylation in the transport of glycoproteins to the cell surface was studied in BY2 tobacco cells using both endogenous and recombinant cell wall invertases as markers. When synthesized without their N-glycans, both invertases were very rapidly degraded. This degradation did not occur in an acidic compartment and was brefeldin A-insensitive. Therefore, it most probably represents a pre-Golgi event. However, the low efficiency of specific inhibitors did not favor a strong contribution of proteasomes in this proteolysis. In contrast, addition of a C-terminal His-Asp-Glu-Leu (HDEL) extension prevented arrival of these non-glycosylated glycoproteins in the compartment where they are degraded. These results argue for the presence of an endoplasmic reticulum (ER) domain specialized in protein degradation. Consistent with our results and the well-known stabilization of recombinant proteins retained in the ER, the addition of an ER retention signal to a protein would prevent its targeting to an ER domain devoted to degradation.

Expression and processing of a hormonally regulated beta-expansin from soybean

Expansin proteins are essential components of acid-induced cell wall loosening in plants. Beta-expansins, which constitute a subfamily of related expansin proteins, include the group I grass pollen allergens. To provide a better description of beta-expansin expression, we have characterized a cytokinin-inducible beta-expansin from soybean (Glycine max cv Mandarin) called Cim1. Our results demonstrate that the hormones cytokinin and auxin act synergistically to induce the accumulation and proteolytic processing of Cim1. Carboxyl terminal truncation of a 35-kD form of Cim1 is predicted to remove the putative cellulose binding domain from the amino terminal cysteine-rich domain, resulting in a 20-kD form of the protein. Furthermore, the identical amino termini of the 35- and 20-kD forms of Cim1 correspond to a position 11 amino acids downstream of the predicted signal sequence cleavage site, suggesting proteolysis of a short amino terminal propeptide after removal of the signal peptide. This propeptide fragment contains a consensus site for N-glycosylation and our data suggest that it is glycosylated by a tunicamycin-sensitive mechanism in cultured soybean cells. The onset of Cim1 expression correlates with increased growth of soybean cultures. Ultimately, Cim1 is rapidly and specifically proteolyzed as soybean cultures reach stationary phase. These findings are consistent with the hypothesis that beta-expansin proteins are extensively modified by post-translational N-glycosylation and proteolysis.

Sorting of soluble proteins in the secretory pathway of plants

The secretory pathway of plants is a network of organelles that communicate via vesicle transport. This process involves budding on donor membranes followed by their targeting to, recognition by and fusion with the acceptor membrane. Protein sorting through the plant secretory pathway is a process that requires the specific recognition of signals by receptor molecules. For soluble proteins, recognition takes place in the lumen of the secretory pathway. The sorting receptors must mediate signal transduction across the membrane to convey the information about the presence of cargo molecules to cytosolic factors, which regulate the formation of transport vesicles. Recently, a number of key elements in this process have been identified, providing tools to study protein sorting at the molecular level.

The plant vacuolar sorting receptor AtELP is involved in transport of NH(2)-terminal propeptide-containing vacuolar proteins in Arabidopsis thaliana

Many soluble plant vacuolar proteins are sorted away from secreted proteins into small vesicles at the trans-Golgi network by transmembrane cargo receptors. Cleavable vacuolar sorting signals include the NH(2)-terminal propeptide (NTPP) present in sweet potato sporamin (Spo) and the COOH-terminal propeptide (CTPP) present in barley lectin (BL). These two proteins have been found to be transported by different mechanisms to the vacuole. We examined the ability of the vacuolar cargo receptor AtELP to interact with the sorting signals of heterologous and endogenous plant vacuolar proteins in mediating vacuolar transport in Arabidopsis thaliana. AtELP extracted from microsomes was found to interact with the NTPPs of barley aleurain and Spo, but not with the CTPPs of BL or tobacco chitinase, in a pH-dependent and sequence-specific manner. In addition, EM studies revealed the colocalization of AtELP with NTPP-Spo at the Golgi apparatus, but not with BL-CTPP in roots of transgenic Arabidopsis plants. Further, we found that AtELP interacts in a similar manner with the NTPP of the endogenous vacuolar protein AtALEU (Arabidopsis thaliana Aleu), a protein highly homologous to barley aleurain. We hypothesize that AtELP functions as a vacuolar sorting receptor involved in the targeting of NTPP-, but not CTPP-containing proteins in Arabidopsis.

Post-translational maturation of natural and drug-induced missorted phytohemagglutinin

The bean lectin phytohemagglutinin (PHA) was expressed in transgenic suspension-cultured BY-2 tobacco cells simultaneously with another recombinant vacuolar protein, the sweet potato sporamin. In contrast to previous observations in different transgenic plant systems when expressed in BY-2 tobacco cells, phytohemagglutinin is mostly but not exclusively targeted to the vacuole. Indeed, a small amount of recombinant phytohemagglutinin is secreted into the culture medium of tobacco cells. Furthermore part of this extracellular phytohemagglutinin has no lectin activity and presents an abnormal glycosylation consistent with higher accessibility of glycans N-linked to these extracellular phytohemagglutinin forms. Phytohemagglutinin secretion occurs regardless of recombinant protein expression level. Consequently, missorting in this case is due to an abnormal phytohemagglutinin conformation or oligomerization rather than to receptor saturation. The treatment of BY-2 cells with drugs, such as monensin and wortmannin, increases even more the transport of phytohemagglutinin to the cell surface through a general inhibition of the sorting mechanisms of vacuolar proteins. The sensitivity to wortmannin is similar for the sorting of phytohemagglutinin and endogenous tobacco chitinase and beta-1,3-glucanase, suggesting that phytohemagglutinin and COOH-terminal propeptide mediated vacuolar sorting share similar mechanisms. A characterization of glycans N-linked to extracellular phytohemagglutinin secreted by monensin- or wortmannin-treated transgenic tobacco cells illustrates that in contrast with monensin, wortmannin completely inhibits the sorting of vacuolar proteins without having any effect on the efficiency of Golgi processing enzymes.

Processing, targeting, and antifungal activity of stinging nettle agglutinin in transgenic tobacco

The gene encoding the precursor to stinging nettle (Urtica dioica L. ) isolectin I was introduced into tobacco (Nicotiana tabacum). In transgenic plants this precursor was processed to mature-sized lectin. The mature isolectin is deposited intracellularly, most likely in the vacuoles. A gene construct lacking the C-terminal 25 amino acids was also introduced in tobacco to study the role of the C terminus in subcellular trafficking. In tobacco plants that expressed this construct, the mutant precursor was correctly processed and the mature isolectin was targeted to the intercellular space. These results indicate the presence of a C-terminal signal for intracellular retention of stinging nettle lectin and most likely for sorting of the lectin to the vacuoles. In addition, correct processing of this lectin did not depend on vacuolar deposition. Isolectin I purified from tobacco displayed identical biological activities as isolectin I isolated from stinging nettle. In vitro antifungal assays on germinated spores of the fungi Botrytis cinerea, Trichoderma viride, and Colletotrichum lindemuthianum revealed that growth inhibition by stinging nettle isolectin I occurs at a specific phase of fungal growth and is temporal, suggesting that the fungi had an adaptation mechanism.

Processing of preproricin in transgenic tobacco

The plant protein toxin ricin has found widespread application as a potential therapeutic agent for many human diseases and in disease-model systems such as those involving apoptosis. Genetic engineering and expression of the complete two-polypeptide chain toxin have only been possible in plants, specifically in transgenic tobacco carrying the preproricin gene under the control the cauliflower mosaic virus 35S promoter. Production of modified ricin for altered controllable activity and/or fusion therapeutics to target delivery requires knowledge of the heterologous processing that occurs when preproricin is expressed in tobacco. Here, recombinant ricin from transgenic tobacco was purified using lectin affinity chromatography and characterized using various biochemical and biophysical techniques. Coomassie blue staining of an SDS-PAGE gel of lactose-agarose purified material identified predominant proteins of 30 and 35 kDa molecular weight. Western analysis using anti-ricin a- and b-chain antibodies confirmed the expression and purification of recombinant ricin, with identical protein banding profiles to that of authentic castor-bean-derived ricin. High-resolution gel filtration chromatography characterized the lactose binding complex as a 66-kDa native molecular weight protein which could be separated into 30- and 35-kDa proteins upon incubation with the reducing agent dithiothreitol. N-terminal sequencing of the recombinant ricin a-chain revealed that an equimolar ratio of two alternately processed peptides was present, which varied by an additional amino acid derived from the signal peptide. Similar analysis of ricin b-chain again identified two forms of this polypeptide as well; however, full-length ricin b-chain and b-chain missing the first alanine residue were present at 11:1 molar ratios. Transgenic tobacco plants expressing ricin were used to develop a stable cell suspension culture system from callus induced with the growth regulators 2,4-dichlorophenoxyacetic acid and 6-benzylaminopurine. Double sandwich enzyme-linked immunosorbent assay using anti-ricin b-chain antibodies and Western analysis identified soluble ricin in the media of the cultures, indicating that cell cultures provide a safe and simple means to produce properly processed recombinant ricin.

Sorting of proteins to vacuoles in plant cells

An individual plant cell may contain at least two functionally and structurally distinct types of vacuoles: protein storage vacuoles and lytic vacuoles. Presumably a cell that stores proteins in vacuoles must maintain these separate compartments to prevent exposure of the storage proteins to an acidified environment with active hydrolytic enzymes where they would be degraded. Thus, the organization of the secretory pathway in plant cells, which includes the vacuoles, has a fascinating complexity not anticipated from the extensive genetic and biochemical studies of the secretory pathway in yeast. Plant cells must generate the membranes to form two separate types of tonoplast, maintain them as separate organelles, and direct soluble proteins from the secretory flow specifically to one or the other via separate vesicular pathways. Individual soluble and membrane proteins must be recognized and sorted into one or the other pathway by distinct, specific mechanisms. Here we review the emerging picture of how separate plant vacuoles are organized structurally and how proteins are recognized and sorted to each type.

The vacuolar targeting signal of the 2S albumin from Brazil nut resides at the C terminus and involves the C-terminal propeptide as an essential element

Genetic constructs in which different N- and C-terminal segments of Brazil nut (Bertholletia excelsa H.B.K.) 2S albumin were fused to secretory yeast invertase were transformed into tobacco (Nicotiana tabacum) plants to investigate the vacuolar targeting signal of the 2S albumin. None of the N-terminal segments, including the complete precursor containing all propeptides, was able to direct the invertase to the vacuoles. However, a short C-terminal segment comprising the last 20 amino acids of the precursor was sufficient for efficient targeting of yeast invertase to the vacuoles of the transformed tobacco plants. Further analyses showed that peptides of 16 and 13 amino acids of the C-terminal segment were still sufficient, although they had slightly lower efficiency. When segments of 9 amino acids or shorter were analyzed, a decrease to approximately 30% was observed. These segments included the C-terminal propeptide of four amino acids (Ile-Ala-Gly-Phe). When the 2S albumin was expressed in tobacco, it was also localized to the vacuoles of mesophyll cells. If the C-terminal propeptide was deleted from the 2S albumin precursor, all of this truncated 2S albumin was secreted from the tobacco cells. These results indicate that the C-terminal propeptide is necessary but not sufficient for vacuolar targeting. In addition, an adjacent segment of at least 12 amino acids of the mature protein is needed to form the complete signal for efficient targeting.

Antimicrobial peptides from Mirabilis jalapa and Amaranthus caudatus: expression, processing, localization and biological activity in transgenic tobacco

The cDNAs encoding the seed antimicrobial peptides (AMPs) from Mirabilis jalapa (Mj-AMP2) and Amaranthus caudatus (Ac-AMP2) have previously been characterized and it was found that Mj-AMP2 and Ac-AMP2 are processed from a precursor preprotein and preproprotein, respectively [De Bolle et al., Plant Mol Biol 28:713-721 (1995) and 22:1187-1190 (1993), respectively]. In order to study the processing, sorting and biological activity of these antimicrobial peptides in transgenic tobacco, four different gene constructs were made: a Mj-AMP2 wild-type gene construct, a Mj-AMP2 mutant gene construct which was extended by a sequence encoding the barley lectin carboxyl-terminal propeptide, a known vacuolar targeting signal [Bednarek and Raikhel, Plant Cell 3: 1195-1206 (1991)]; an Ac-AMP2 wild-type gene construct; and finally, an Ac-AMP2 mutant gene construct which was truncated in order to delete the sequence encoding the genuine carboxyl-terminal propeptide. Processing and localization analysis indicated that an isoform of Ac-AMP2 with a cleaved-off carboxyl-terminal arginine was localized in the intercellular fluid fraction of plants expressing either wild-type or mutant gene constructs. Mj-AMP2 was recovered extracellularly in plants transformed with Mj-AMP2 wild-type gene construct, whereas an Mj-AMP2 isoform with a cleaved-off carboxyl-terminal arginine accumulated intracellularly in plants expressing the mutant precursor protein with the barley lectin propeptide. The in vitro antifungal activity of the AMPs purified from transgenic tobacco expressing any of the four different precursor proteins was similar to that of the authentic proteins. However, none of the transgenic plants showed enhanced resistance against infection with either Botrytis cinerea or Alternaria longipes.

Expression of cDNA for a bark lectin of Robinia in transgenic tobacco plants

A cDNA encoding a bark lectin of Robinia pseudoacacia was introduced into tobacco plants. The expression of the lectin cDNA under control of the 35S promoter was confirmed by Western blot analysis and a hemagglutination assay of extracts of transgenic plants. Western blot analysis revealed that the subunit of the lectin from tobacco had a molecular mass of 29 kDa. The sequence of nine amino acids from the N-terminus of the lectin from transgenic tobacco plants was identical to that of the bark lectin from Robinia, indicating that the lectin had been processed correctly at its N-terminus in tobacco. The molecular mass of the purified native lectin produced by tobacco plants was estimated to be 112 kDa by gel filtration on a column of Superdex 200. It is suggested that the lectin subunits assembled to form tetramers in transgenic tobacco plants.

Different sensitivity to wortmannin of two vacuolar sorting signals indicates the presence of distinct sorting machineries in tobacco cells

Vacuolar matrix proteins in plant cells are sorted from the secretory pathway to the vacuoles at the Golgi apparatus. Previously, we reported that the NH2-terminal propeptide (NTPP) of the sporamin precursor and the COOH-terminal propeptide (CTPP) of the barley lectin precursor contain information for vacuolar sorting. To analyze whether these propeptides are interchangeable, we expressed constructs consisting of wild-type or mutated NTPP with the mature part of barley lectin and sporamin with CTPP and mutated NTPP in tobacco BY-2 cells. The vacuolar localization of these constructs indicated that the signals were interchangeable. We next analyzed the effect of wortmannin, a specific inhibitor of mammalian phosphatidylinositol (PI) 3-kinase on vacuolar delivery by NTPP and CTPP in tobacco cells. Pulse-chase analysis indicated that 33 microM wortmannin caused almost complete inhibition of CTPP-mediated transport to the vacuoles, while NTPP-mediated transport displayed almost no sensitivity to wortmannin at this concentration. This indicates that there are at least two different mechanisms for vacuolar sorting in tobacco cells, and the CTPP-mediated pathway is sensitive to wortmannin. We compared the dose dependencies of wortmannin on the inhibition of CTPP-mediated vacuolar delivery of proteins and on the inhibition of the synthesis of phospholipids in tobacco cells. Wortmannin inhibited PI 3- and PI 4-kinase activities and phospholipid synthesis. Missorting caused by wortmannin displays a dose dependency that is similar to the dose dependency for the inhibition of synthesis of PI 4-phosphate and major phospholipids. This is different, however, than the inhibition of synthesis of PI 3-phosphate. Thus, the synthesis of phospholipids could be involved in CTPP-mediated vacuolar transport.

cDNA structure and regulatory properties of a family of starvation-induced ribonucleases from tomato

In previous work we have determined the primary structure of two of the five ribonucleases which are induced by phosphate starvation in cultured tomato cells. Here, we present the isolation and characterization of the cDNAs for the extracellular ribonuclease LE and the intracellular, but extravacuolar ribonuclease LX. Structural analysis of these cDNAs together with partial protein-sequencing of vacuolar ribonucleases LV1, LV2 and LV3 revealed a family of very similar ribonucleases. From these data we assume identify between ribonucleases LE and LV3 for which the targeting mechanism has to be shown. Furthermore, RNase LV1 and RNase LV2 might be posttranslational processing products of RNase LX which travel to the vacuoles after splitting off the putative ER retention signal present at RNase LX. Additionally, we show by northern blot analysis that phosphate starvation in plant cells leads to an increase in the steady-state level of this type of enzymes revealing close similarities of the plant response to a limited supply of inorganic phosphate with the PHO regulation in bacteria and fungi.

Accumulation of 15-Kilodalton Zein in Novel Protein Bodies in Transgenic Tobacco

Zeins, the seed storage proteins of maize, are a group of alcohol-soluble polypeptides of different molecular masses that share a similar amino acid composition but vary in their sulfur amino acid composition. They are synthesized on the rough endoplasmic reticulum (ER) in the endosperm and are stored in ER-derived protein bodies. Our goal is to balance the amino acid composition of the methionine-deficient forage legumes by expressing the sulfur amino acid-rich 15-kD zeins in their leaves. However, it is crucial to know whether this protein would be stable in nonseed tissues of transgenic plants. The major focus of this paper is to compare the accumulation pattern of the 15-kD zein protein with a vacuolar targeted seed protein, [beta]-phaseolin, in nonseed tissues and to determine the basis for its stability/instability. We have introduced the 15-kD zein and bean [beta]-phaseolin-coding sequences behind the 35S cauliflower mosaic virus promoter into tobacco (Nicotiana tabacum) and analyzed the protein's accumulation pattern in different tissues. Our results demonstrate that the 15-kD seed protein is stable not only in seeds but in all nonseed tissues tested, whereas the [beta]-phaseolin protein accumulated only in mid- and postmaturation seeds. Interestingly, zein accumulates in novel protein bodies both in the seeds and in nonseed tissues. We attribute the instability of the [beta]-phaseolin protein in nonseed tissues to the fact that it is targeted to protease-rich vacuoles. The stability of the 15-kD zein could be attributed to its retention in the ER or to the protease-resistant nature of the protein.

Do de-N-glycosylation enzymes have an important role in plant cells?

In this review de-N-glycosylation was defined as the removal of the glycan(s) from a N-glycosylprotein, by means of enzymes acting on the di-N-acetylchitobiosyl part of the invariant pentasaccharide inner-core of N-glycosylproteins. Peptide-N4-(N-acetyl-beta-D-glucosaminyl) asparagine amidases (PNGase) and endo-N-acetyl-beta-D-glucosaminidases (ENGase) were both considered as de-N-glycosylation enzymes. A detailed description of the characterization and the function of plant PNGases and ENGases is presented, together with a brief presentation on the occurrence and the current knowledge on the function of microbial and animal enzymes. De-N-glycosylation of plant glycoproteins was proposed as a possible mechanism for the release of oligosaccharides displaying biological activities and the removal of N-glycans could also explain the regulation of protein activity. Each enzyme seems to have a specific function during germination and post-germinative development. All the arguments concur that de-N-glycosylation enzymes have an important role in plant cells and confirm that the N-glycosylation/de-N-glycosylation system should occur more commonly than presently recognized in living organisms.

The aspartic proteinase of barley is a vacuolar enzyme that processes probarley lectin in vitro

Previous work suggested that the aspartic proteinase from Hordeum vulgare (HvAP) would be a vacuolar protein in plant cells. Based on N-terminal sequencing we show that the in vitro-translated protein was translocated into the lumen of microsomal membranes, causing a concomitant removal of 25 amino acid residues from the protein. Vacuoles were purified from barley leaf protoplasts and were shown to contain all of the aspartic proteinase activity found in the protoplasts. This vacuolar localization of HvAP was confirmed with immunocytochemical electron microscopy using antibodies to HvAP in both barley leaf and root cells. In an attempt to discern a function for this protease, we investigated the ability of HvAP to process the C-terminal proregion of barley lectin (BL) in vitro. Prolectin (proBL), expressed in bacteria, was processed rapidly when HvAP was added. Using several means, we were able to determine that 13 amino acid residues at the C terminus of proBL were cleaved off, whereas the N terminus stayed intact during this incubation. Immunohistochemical electron microscopy showed that HvAP and BL are co-localized in the root cells of developing embryos and germinating seedlings. Thus, we propose that the vacuolar HvAP participates in processing the C terminus of BL.

Determination of the functional elements within the vacuolar targeting signal of barley lectin

We have previously demonstrated that the carboxyl-terminal propeptide of barley lectin is both necessary and sufficient for protein sorting to the plant vacuole. Specific mutations were constructed to determine which amino acid residues or secondary structural determinants of the carboxyl-terminal propeptide affect proper protein sorting. We have found that no consensus sequence or common structural determinants are required for proper sorting of barley lectin to the vacuole. However, our analysis demonstrated the importance of hydrophobic residues in vacuolar targeting. In addition, at least three exposed amino acid residues are necessary for efficient sorting. Sorting was disrupted by the addition of two glycine residues at the carboxyl-terminal end of the targeting signal or by the translocation of the glycan to the carboxy terminus of the propeptide. These results suggest that some components of the sorting apparatus interact with the carboxy terminus of the propeptide.

Extracellular targeting of the vacuolar tobacco proteins AP24, chitinase and beta-1,3-glucanase in transgenic plants

The Nicotiana tabacum ap24 gene encoding a protein with antifungal activity toward Phytophthora infestans has been characterized. Analysis of cDNA clones revealed that at least three ap24-like genes are induced in tobacco upon infection with tobacco mosaic virus. Amino acid sequencing of the purified protein showed that AP24 is synthesized as a preproprotein from which an amino-terminal signal peptide and a carboxyl-terminal propeptide (CTPP) are cleaved off during post-translational processing. The functional role of the CTPP was investigated by expressing chimeric genes encoding either wild-type AP24 or a mutant protein lacking the CTPP. Plants expressing the wild-type construct resulted in proteins properly sorted to the vacuole. In contrast, the proteins produced in plants expressing the mutant construct were secreted extracellularly, indicating that the CTPP is necessary for targeting of AP24 to the vacuoles. Similar results were obtained for vacuolar chitinases and beta-1,3-glucanases of tobacco. The extracellularly targeted mutant proteins were shown to have retained their biological activity. Together, these results suggest that within all vacuolar pathogenesis-related proteins the targeting information resides in a short carboxyl-terminal propeptide which is removed during or after transport to the plant vacuole.

Expression and secretion of wheat germ agglutinin by Saccharomyces cerevisiae

Genes encoding pre-protein and prepro-protein of wheat germ agglutinin isolectin 2 (WGA2) were chemically synthesized and expressed in the yeast Saccharomyces cerevisiae under the control of the ENO1 promoter. Yeast harboring either a pre-WGA2 or a prepro-WGA2 gene expression plasmid secreted a mature form of WGA2 into the culture medium. The amount of WGA2 secreted by the strain KS58-2Ddel, which has a ssl1 mutation causing a supersecretion of human lysozyme [Suzuki, K., Ichikawa, K. & Jigami, Y. (1989) Mol. Gen. Genet. 219, 58-64], was 20-fold greater than that secreted by the wild-type strain KK4. The recombinant WGA2 from the cells containing the prepro-WGA2 gene expression plasmid was purified to homogeneity by a three-step ion-exchange chromatography scheme. As in wheat, the N-terminal signal peptide of recombinant WGA2 purified from yeast culture was processed to form an N-terminal 5-oxoprolyl (pyroglutamyl) residue. Likewise, we found that the C-terminal pro-region of recombinant WGA2 had also been processed in yeast. Using electrospray ionization mass spectrometry, we found the processed C-terminus to be heterogeneous in both recombinant WGA2 purified from yeast and in authentic WGA2. The major component of the recombinant WGA2 contained two additional amino acids at its C-terminus compared to that of authentic WGA2. In spite of this difference in the C-terminus, the recombinant WGA2 exhibited a sugar binding activity that was indistinguishable from that of authentic WGA2.

Isolation and characterization of pro-barley lectin expressed in Escherichia coli

Lectins are a class of proteins with specific carbohydrate-binding properties found in a wide variety of plants and animals. Gramineae lectins are presumably defense-related proteins in plants that exert their effect by binding to N-acetylglucosamine. Barley lectin is a vacuolar protein synthesized with an amino-terminal signal sequence for entering the secretory pathway and a carboxyl-terminal propeptide necessary for proper targeting to the vacuole. To analyze the three-dimensional structure of barley lectin with the carboxyl-terminal extension and to investigate whether the conversion of the prolectin into the mature molecule leads to a conformational change, the precursor and the mature forms of barley lectin were expressed in Escherichia coli. Both proteins accumulated in denatured form in inclusion bodies were solubilized in 8 M urea and renatured in a redox buffer system. Active pro- and mature barley lectins were purified to homogeneity by affinity chromatography.

Intracellular transport and processing of a tobacco vacuolar β-1,3-glucanase

The class I β-1,3-glucanases are basic, vacuolar enzymes implicated in the defense of plants against pathogen infection. The tobacco (Nicotiana tabacum L.) enzyme is synthesized as a preproprotein with an N-terminal signal peptide for targeting to the lumen of the endoplasmic reticulum and an N-glycosylated C-terminal extension which is lost during protein maturation. The transport and processing of β-1,3-glucanase in cellsuspension cultures of the tobacco cultivar Havana 425 was investigated by pulse-chase labelling and cell fractionation. We verified that mature β-1,3-glucanase is localized in the vacuole of the suspension-cultured cells. Comparison of the time course of processing in homogenates, the soluble fraction, and membrane fractions indicates that proglucanase is transported from the endoplasmic reticulum via the Golgi compartment to the vacuole. Processing to the mature form occurs in the vacuole. Treatment of cells with tunicamycin, which inhibits N-glycosylation, and digestion of the (35)S-labelled processing intermediates with endoglycosidase H indicate that β-1,3-glucanase has a single N-glycan attached to the C-terminal extension. Glycosylation is not required for proteolytic processing or correct targeting to the vacuole.

Nuclear localization signal(s) required for nuclear targeting of the maize regulatory protein Opaque-2

The maize regulatory protein Opaque-2 (O2) localizes to the nucleus in both maize and tobacco cells. Here we show that in-frame carboxy- and amino-terminal fusions of O2 to reporter protein beta-glucuronidase (GUS) were sufficient to direct GUS to the nucleus in transgenic tobacco plants and in transiently transformed onion cells. Two independent regions of O2 containing 135 and 149 amino acids were identified that were able to redirect GUS to the nucleus in both systems. A quantitative biochemical analysis of GUS in nuclei isolated from transgenic tobacco plants revealed that the second region was more efficient than the first one. The precise location of nuclear localization signals (NLSs) was determined using an onion transformation system. The first NLS was located between residues 101 and 135 and had the structure of a simian virus 40 NLS. The second NLS was located in the basic, DNA binding domain (between residues 223 and 254) and had a bipartite structure. The presence of one of the O2 NLSs in the basic domain is in complete agreement with similar findings of NLSs in the basic domain of three other basic/leucine zipper proteins, suggesting that this domain may be bifunctional. The effect of amino- versus carboxy-terminal GUS fusions is discussed.

Proaleurain vacuolar targeting is mediated by short contiguous peptide interactions

Targeting of soluble proteins to the plant vacuole is mediated by determinants that reside in the polypeptide. We identified the vacuolar targeting determinant of aleurain, a plant vacuolar thiol protease, by incorporating different sequences from proaleurain into the secreted thiol protease, proendoproteinase B (proEP-B), and vice versa. The targeting fates of the chimeric proteins were analyzed by transient expression in electroporated tobacco protoplasts. The targeting determinant SSSSFADSNPIR is positioned at the N terminus of the aleurain propeptide, and its substitution into the propeptide of EP-B caused vacuolar targeting of the resulting chimeric protein. This determinant can be divided into two smaller determinants, SSSSFADS and SNPIR, each of which is sufficient to target proEP-B chimeras to the vacuole, but with lower efficiency. These smaller determinants interact in a positive manner because the combined determinant SSSSFADSNPIR targeted proEP-B with an efficiency greater than each of the smaller determinants alone. Accordingly, the efficiency of aleurain targeting was decreased when either of the smaller determinants was disrupted by replacement with similarly positioned proEP-B sequences. Further experiments on proaleurain identified an additional determinant, VTDRAAST, adjacent to the SSSSFADSNPIR determinant that is also necessary for efficient vacuolar targeting. Our results provide evidence that efficient vacuolar targeting of this thiol protease in plant cells is mediated by the combined action of smaller contiguous determinants; two of these alone are sufficient for vacuolar targeting.

Sorting of proteins to the vacuoles of plant cells

The secretory system of plant cells sorts a large number of soluble proteins that either are secreted or accumulate in vacuoles. Secretion is a bulk-flow process that requires no information beyond the presence of a signal peptide necessary to enter the endoplasmic reticulum. Many vacuolar proteins are glycoproteins and the glycans are often modified as the proteins pass through the Golgi complex. Vacuolar targeting information is not contained in glycans as it is in animal cells; rather, targeting information is in polypeptide domains as it is in yeast cells. Several such domains have now been identified, but these show little or no amino acid sequence homology. We discuss the possibilities that targeting of protein to plant vacuoles may involve receptors as well as aggregation of protein at low pH.

Biosynthesis, processing and targeting of the G-protein of vesicular stomatitis virus in tobacco protoplasts

Leaf protoplasts of tobacco (Nicotlana tabacum L.) were employed for transfection of chimeric transcriptional gene fusions comprising the 35S promoter from cauliflower mosaic virus, the coding sequence of the G-protein from vesicular stomatitis virus (VSVG) and the transcriptional terminator from the Agrobacterium tumefaciens nopaline-synthetase gene. Transient expression of the chimeric gene was monitored through Northern analysis of total protoplast RNA using a labeled VSV cDNA probe, and through Western-blot analysis of protoplast proteins using a polyclonal and-VSV antiserum. Although a single species of mRNA was detected in the transfected protoplasts, two glycoproteins differing in mass by approx. 9 kDa were detected by the antiserum. Biosynthesis of the VSVG isoforms was not impeded by chemical inhibitors of cell-wall production or of proline hydroxylation. Transfection using mutant forms of the VSVG coding sequence in which either one or both consensus glycosylation sites were removed resulted in the production of progressively smaller VSVG proteins. Those proteins produced from the double mutant had mobilities on sodium dodecyl sulfate-polyacrylamide gel electrophoresis that were very similar to those produced from the wild-type construct in the presence of tunicamycin. Analysis of protoplast homogenates by differential centrifugation showed that the two VSVG isoforms were exclusively associated with cellular membranes. The larger protein co-localized with the plasma membrane and with the organelles of the endomembrane-secretory pathway leading to the plasma membrane. The smaller protein was associated with membranes of lower isopycnic densities which were not identical to the endoplasmic reticulum. The larger protein displayed greater sensitivity than did the smaller to degradation in vivo by exogenously added protease. Immunofluorescence microscopy demonstrated that the VSVG isoforms were present both within the protoplasts and at the surface of the plasma membrane. The intracellular distribution was either punctate or reticulate. These results are consistent with the progressive and accurate glycosylation of the newly synthesized VSVG polypeptide during its passage through the endomembrane-secretory pathway, the access of the larger isoform to the cell surface, and the conversion of the larger to the small isoform by selective proteolysis.

A short C-terminal sequence is necessary and sufficient for the targeting of chitinases to the plant vacuole

Tobacco contains different isoforms of chitinase (EC 3.2.1.14), a hydrolase thought to be involved in the defense against pathogens. Deduced amino acid sequences for putatively vacuolar, basic chitinases differ from the homologous extracellular, acidic isoforms by the presence of a C-terminal extension. To examine the role of this C-terminal extension in protein sorting, Nicotiana silvestris plants were stably transformed with chimeric genes coding for tobacco basic chitinase A with and without the seven C-terminal amino acids. In plants expressing unmodified chitinase A, the enzyme activity was low in the intercellular wash fluid but high in protoplasts and isolated vacuoles. In contrast, in plants expressing mutant chitinase lacking the C terminus, the activity was high in the intercellular wash fluid but low in protoplasts. N. silvestris plants were also transformed with similar constructions coding for a structurally unrelated, extracellular cucumber chitinase. In plants expressing unmodified cucumber chitinase, its activity was present in the intercellular wash fluid and absent from protoplasts. In plants expressing cucumber chitinase with the C-terminal extension from tobacco chitinase A, activity was low in intercellular wash fluids but high in protoplasts and vacuoles. These results demonstrate that the C-terminal extension of tobacco chitinase A is necessary and sufficient for the vacuolar localization of chitinases and, therefore, that it comprises a targeting signal for plant vacuoles.

A Saporin-6 cDNA containing a precursor sequence coding for a carboxyl-terminal extension

Saporin-6 is a single-chain ribosome inactivating protein (RIP) from the seeds and the leaves of Saponaria officinalis (Caryophyllaceae). Here we have identified the COOH-terminal end of mature Saporin-6 and, by cDNA sequencing, the predicted carboxyl-terminal sequence of a leaf Saporin-6 primary translation product. Our data indicate that the characterized cDNA codes for a precursor containing a 22 amino acid carboxyl-terminal extension, not present in mature Saporin-6, that shows similarity to carboxyl-terminal propeptides of vacuolar proteins, suggesting that it may be involved in protein trafficking.

Heterogeneity of the complex N-linked oligosaccharides at specific glycosylation sites of two secreted carrot glycoproteins

The N-linked glycans from the 52/54-kDa medium protein and cell wall beta-fructosidase, two glycoproteins secreted by carrot suspension culture cells, were characterized. Carrot cells were labelled with [3H]glucosamine or [3H]fucose. The 52/54-kDa medium protein was isolated from the culture medium and beta-fructosidase from cell walls. The purified proteins were digested with trypsin and glycopeptides were isolated and sequenced. Glycans obtained from individual glycopeptides were separated by gel filtration chromatography and characterized by concanavalin A chromatography, by treatments with exoglycosidases and by sugar composition analysis. The 52/54-kDa medium protein and cell wall beta-fructosidase have one high-mannose-type glycan similar to those from yeast and animal glycoproteins. In addition, the 52/54-kDa medium protein has three complex-type and cell wall beta-fructosidase two complex-type glycans per polypeptide. The complex-type glycans isolated from individual glycosylation sites are fairly large and very heterogeneous. The smallest of these glycans has the structure [Xyl](Man)3[Fuc](GlcNAc]2Asn (square brackets indicating branching) whereas the larger ones carry additional sugars like terminal N-acetylglucosamine and possibly rhamnose and arabinose in the case of the 52/54-kDa medium protein and only arabinose in the case of cell wall beta-fructosidase. These terminal sugars are linked to the alpha-mannose residues of the glycan cores. The 52/54-kDa medium protein is secreted with large and homogeneous complex glycans, their heterogeneity originates from slow processing after secretion. The complex glycans from cell wall beta-fructosidase are processed before the enzyme is integrated into the cell wall.

The protein-body proteins phytohemagglutinin and tonoplast intrinsic protein are targeted to vacuoles in leaves of transgenic tobacco

To demonstrate the relationship between protein-bodies in seeds and vacuoles in other tissues, we expressed the coding sequences of two bean (Phaseolus vulgaris L.) protein-body proteins in tobacco (Nicotiana tabacum L.). We chose phytohemagglutinin-L (PHA-L) and tonoplast intrinsic protein (TIP) as representatives of the protein-body contents and protein-body membrane, respectively. The location of the two proteins in the leaves of transgenic tobacco was examined by immunocytochemistry and in preparations of isolated vacuoles. Tonoplast intrinsic protein accumulates primarily in tonoplasts in tobacco leaves, whereas PHA is found exclusively in the vacuolar sap, showing that the signals that target proteins to protein-bodies and their limiting membranes in seeds are correctly recognized in leaves. This observation provides further evidence that proteinbodies of dicotyledonous seeds should be considered as protein-storage vacuoles.

Differential accumulation of four phaseolin glycoforms in transgenic tobacco

An intron-less phaseolin gene was used to express phaseolin polypeptides in transgenic tobacco plants. The corresponding amounts of phaseolin immunoreactive polypeptides and mRNA were similar to those found in plants transformed with a bean genomic DNA sequence that encodes an identical beta-phaseolin subunit. These results justified the use of the intron-less gene for engineering of the phaseolin protein by oligonucleotide-directed mutagenesis. Each and both of the two Asn residues that serve as glycan acceptors in wild-type phaseolin were modified to prevent N-linked glycosylation. Wild-type (beta wti-) and mutant phaseolin glycoforms (beta dgly1, beta dgly2 and beta dgly1,2) were localized to the protein body matrix by immunogold microscopy. Although quantitative slot-blot hybridization analysis showed similar levels of phaseolin mRNA in transgenic seed derived from all constructs, seed from the beta dgly1 and beta dgly2 mutations contained only 41% and 73% of that expressed from the wild-type control; even less (23%) was present in seed of plants transformed with the phaseolin beta dgly1,2 gene. Additionally, the profile of 25-29 kDa processed peptides was different for each of the glycoforms, indicating that processing of the full-length phaseolin polypeptides was modified. Thus, although targeting of phaseolin to the protein body was not eliminated by removal of the glycan side-chains, decreased accumulation and stability of the full-length phaseolin protein in transgenic tobacco seed were evident.

Propeptide of a precursor to a plant vacuolar protein required for vacuolar targeting

Sporamin is a protein without glycans that accumulates in large quantities in the vacuoles of the tuberous root of the sweet potato. It is synthesized as a prepro precursor with an N-terminal extension composed of a 21-amino-acid signal peptide and a 16-amino-acid propeptide. A total of 48 base pairs, corresponding to the nucleotide sequence that encodes the propeptide, was deleted from a cDNA clone for sporamin. This delta pro mutant sequence, as well as the sequence of the wild-type sporamin cDNA, was placed downstream from the promoter of the 35S transcript from cauliflower mosaic virus and introduced into the genome of suspension-cultured tobacco cells by Agrobacterium-mediated transformation. In contrast to the vacuolar localization of sporamin in cells that expressed the wild-type precursor, sporamin was secreted into the culture medium from cells in which the delta pro precursor was expressed. The secreted form of sporamin was shorter by two amino acids at its N terminus than authentic sporamin; it migrated anomalously during electrophoresis on SDS/polyacrylamide gel as a result of the presence of intramolecular disulfide bridges, as does authentic sporamin. The kinetics of secretion of sporamin from the cell were similar to those of proteins normally secreted by the host tobacco cells. These results indicate that the propeptide of the precursor to sporamin is required for correct targeting of sporamin to the vacuole and that proteins can be secreted from plant cells by a bulk-flow default pathway in the absence of a functional sorting signal.