A unique vacuolar processing enzyme responsible for conversion of several proprotein precursors into the mature forms

Two novel asparaginyl endopeptidase-like cysteine proteinases from the protist Trichomonas vaginalis: their evolutionary relationship within the clan CD cysteine proteinases

Cysteine proteinases (CPs) are important virulence factors of the protozoan parasite Trichomonas vaginalis. A total of six genes coding for cathepsin L-like CPs belonging to clan CA have been identified in T. vaginalis. At least 23 distinct spots with proteolytic activity have been detected by two-dimensional (2-D) substrate gel electrophoresis from in vitro grown parasites; however, only few of them have been characterized. In this work, we detected six spots with proteolytic activity and molecular weights between 25 and 35 kDa. The six proteinases correspond to two distinct CP families: the papain-like family, represented by four spots with pIs between 4.5 and 5.5; and the legumain-like family represented by two spots with pI 6.3 and 6.5. Next, we obtained two cDNAs encoding for legumain-like CPs from T. vaginalis, which were named Tvlegu-1 and Tvlegu-2. The size of these cDNA clones were 1225 and 1364 bp, which encoded for 388 and 415 amino acids, respectively. Their putative translation products have molecular masses of 42.8 and 47.2 kDa, corresponding to inactive legumain-like CP precursors. The two sequences share approximately 40% identity at the amino acid level. These protein products can be classified within a branch of the legumain-like family in clan CD cysteine proteinases due to their sensitivity to specific proteinases inhibitors, their DNA sequences, and phylogenetic reconstruction. However, they do not correspond either to the typical asparaginyl endopeptidase or the glycosylphosphatidylinositol (GPI): protein transamidase subfamilies. These results suggest that the TVLEGU-1 and TVLEGU-2 peptidases are likely to be part of a new subfamily within the legumain-like family of clan CD cysteine proteinases. Furthermore, they could be one of the missing links between prokaryotic and eukaryotic CPs in clan CD enzymes.

Storage protein accumulation in the absence of the vacuolar processing enzyme family of cysteine proteases

The role(s) of specific proteases in seed protein processing is only vaguely understood; indeed, the overall role of processing in stable protein deposition has been the subject of more speculation than direct investigation. Seed-type members of the vacuolar processing enzyme (VPE) family were hypothesized to perform a unique function in seed protein processing, but we demonstrated previously that Asn-specific protein processing in developing Arabidopsis seeds occurs independently of this VPE activity. Here, we describe the unexpected expression of vegetative-type VPEs in developing seeds and test the role(s) of all VPEs in seed storage protein accumulation by systematically stacking knockout mutant alleles of all four members (alphaVPE, betaVPE, gammaVPE, and deltaVPE) of the VPE gene family in Arabidopsis. The complete removal of VPE function in the alphavpe betavpe gammavpe deltavpe quadruple mutant resulted in a total shift of storage protein accumulation from wild-type processed polypeptides to a finite number of prominent alternatively processed polypeptides cleaved at sites other than the conserved Asn residues targeted by VPE. Although alternatively proteolyzed legumin-type globulin polypeptides largely accumulated as intrasubunit disulfide-linked polypeptides with apparent molecular masses similar to those of VPE-processed legumin polypeptides, they showed markedly altered solubility and protein assembly characteristics. Instead of forming 11S hexamers, alternatively processed legumin polypeptides were deposited primarily as 9S complexes. However, despite the impact on seed protein processing, plants devoid of all known functional VPE genes appeared unchanged with regard to protein content in mature seeds, relative mobilization rates of protein reserves during germination, and vegetative growth. These findings indicate that VPE-mediated Asn-specific proteolytic processing, and the physiochemical property changes attributed to this specific processing step, are not required for the successful deposition and mobilization of seed storage protein in the protein storage vacuoles of Arabidopsis seeds.

Expression of sea anemone equistatin in potato. Effects of plant proteases on heterologous protein production

Plants are increasingly used as production platforms of various heterologous proteins, but rapid protein turnover can seriously limit the steady-state expression level. Little is known about specific plant proteases involved in this process. In an attempt to obtain potato (Solanum tuberosum cv Desirée) plants resistant to Colorado potato beetle (Leptinotarsa decemlineata Say) larvae, the protease inhibitor equistatin was expressed under the control of strong, light-inducible and constitutive promoters and was targeted to the secretory pathway with and without endoplasmic reticulum retention signal. All constructs yielded similar stepwise protein degradation patterns, which considerably reduced the amount of active inhibitor in planta and resulted in insufficient levels for resistance against Colorado potato beetle larvae. Affinity purification of the degradation products and N-terminal sequencing allowed the identification of the amino acid P(1)-positions (asparagine [Asn]-13, lysine-56, Asn-82, and arginine-151) that were cleaved in planta. The proteases involved in the equistatin degradation were characterized with synthetic substrates and inhibitors. Kininogen domain 3 completely inhibited equistatin degradation in vitro. The results indicate that arginine/lysine-specific and legumain-type Asn-specific cysteine proteases seriously impede the functional accumulation of recombinant equistatin in planta. General strategies to improve the resistance to proteases of heterologous proteins in plants are proposed.

Biosynthetic processing of cathepsins and lysosomal degradation are abolished in asparaginyl endopeptidase-deficient mice

Asparaginyl endopeptidase (AEP)/legumain, an asparagine-specific cysteine proteinase in animals, is an ortholog of plant vacuolar processing enzyme (VPE), which processes the exposed asparagine residues of various vacuolar proteins. In search for its physiological role in mammals, here we generated and characterized AEP-deficient mice. Although their body weights were significantly reduced, they were normally born and fertile. In the wild-type kidney where the expression of AEP was exceedingly high among various organs, the localization of AEP was mainly found in the lamp-2-positive late endosomes in the apical region of the proximal tubule cells. In these cells of AEP-deficient mice, the lamp-2-positive membrane structures were found to be greatly enlarged. These aberrant lysosomes, merged with the late endosomes, accumulated electron-dense and membranous materials. Furthermore, the processing of the lysosomal proteases, cathepsins B, H, and L, from the single-chain forms into the two-chain forms was completely defected in the deficient mice. Thus, the AEP deficiency caused the accumulation of macromolecules in the lysosomes, highlighting a pivotal role of AEP in the endosomal/lysosomal degradation system.

Vacuolar processing enzymes are essential for proper processing of seed storage proteins in Arabidopsis thaliana

The proprotein precursors of storage proteins are post-translationally processed to produce their respective mature forms within the protein storage vacuoles of maturing seeds. To investigate the processing mechanism in vivo, we isolated Arabidopsis mutants that accumulate detectable amounts of the precursors of the storage proteins, 12 S globulins and 2 S albumins, in their seeds. All six mutants isolated have a defect in the beta VPE gene. VPE (vacuolar processing enzyme) is a cysteine proteinase with substrate specificity toward an asparagine residue. We further generated various mutants lacking different VPE isoforms: alpha VPE, beta VPE, and/or gamma VPE. More than 90% of VPE activity is abolished in the beta vpe-3 seeds, and no VPE activity is detected in the alpha vpe-1/beta vpe-3/gamma vpe-1 seeds. The triple mutant seeds accumulate no properly processed mature storage proteins. Instead, large amounts of storage protein precursors are found in the seeds of this mutant. In contrast to beta vpe-3 seeds, which accumulate both precursors and mature storage proteins, the other single (alpha vpe-1 and gamma vpe-1) and double (alpha vpe-1/gamma vpe-1) mutants accumulate no precursors in their seeds at all. Therefore, the vegetative VPEs, alpha VPE and gamma VPE, are not necessary for precursor processing in the presence of beta VPE, but partly compensates for the deficiency in beta VPE in beta vpe-3 seeds. In the absence of functional VPEs, a proportion of pro2S albumin molecules are alternatively cleaved by aspartic proteinase. This cleavage by aspartic proteinase is promoted by the initial processing of pro2S albumins by VPE. Our overall results suggest that seed-type beta VPE is most essential for the processing of storage proteins, and that the vegetative-type VPEs and aspartic proteinase complement beta VPE activity in this processing.

Legumains and their functions in plants

Legumains are a family of plant and animal Asn-specific cysteine proteinases with extra-cytoplasmic localization in vacuoles or cell walls. Plant legumains are involved in Asn-specific propolypeptide processing during, for example, storage-protein deposition in maturing seeds, when these proteins are resistant against degradation by legumains. With the transition to germination and subsequent seedling growth, storage proteins are opened to unlimited cleavage by legumains, which now contribute to protein mobilization. Here, we suggest a hypothesis that unifies both functions of legumains. Their action as propolypeptide-processing or protein-degrading enzymes is naturally controlled by the conformational state of their substrates, which undergo development- or environment-dependent changes. The suggested substrate conformation-dependent differential roles of legumains might not be restricted to seeds but could also apply to cells of different tissues in vegetative organs.

Calcium-mediated association of a putative vacuolar sorting receptor PV72 with a propeptide of 2S albumin

PV72, a type I membrane protein with three epidermal-growth factor (EGF)-like motifs, was found to be localized on the membranes of the precursor-accumulating (PAC) vesicles that accumulated precursors of various seed storage proteins. To clarify the function of PV72 as a sorting receptor, we expressed four modified PV72s and analyzed their ability to bind the internal propeptide (the 2S-I peptide) of pro2S albumin by affinity chromatography and surface plasmon resonance. The recombinant PV72 specifically bound to the 2S-I peptide with a K(D) value of 0.2 microm, which was low enough for it to function as a receptor. The EGF-like motifs modulated the Ca(2+)-dependent conformational change of PV72 to form a functional pocket for the ligand binding. The binding of Ca(2+) stabilizes the receptor-ligand complex even at pH 4.0. The association and dissociation of PV72 with the ligand is modulated by the Ca(2+) concentration (EC(50) value = 40 microm) rather than the environmental pH. Overall results suggest that Ca(2+) regulates the vacuolar sorting mechanism in higher plants.

Activation of Arabidopsis vacuolar processing enzyme by self-catalytic removal of an auto-inhibitory domain of the C-terminal propeptide

Vacuolar processing enzyme (VPE) is a cysteine proteinase responsible for the maturation of various vacuolar proteins in higher plants. To clarify the mechanism of maturation and activation of VPE, we expressed the precursors of Arabidopsis gamma VPE in insect cells. The cells accumulated a glycosylated proprotein precursor (pVPE) and an unglycosylated preproprotein precursor (ppVPE) which might be unfolded. The N-terminal sequence of pVPE revealed that ppVPE had a 22-amino-acid signal peptide to be removed co-translationally. Under acidic conditions, the 56-kDa pVPE was self-catalytically converted to a 43-kDa intermediate form (iVPE) and then to the 40-kDa mature form (mVPE). N-terminal sequencing of iVPE and mVPE showed that sequential removal of the C-terminal propeptide and N-terminal propeptide produced mVPE. Both iVPE and mVPE exhibited the activity, while pVPE exhibited no activity. These results imply that the removal of the C-terminal propeptide is essential for activating the enzyme. Further removal of the N-terminal propeptide from iVPE is not required to activate the enzyme. To demonstrate that the C-terminal propeptide functions as an inhibitor of VPE, we expressed the C-terminal propeptide and produced specific antibodies against it. We found that the C-terminal propeptide reduced the activity of VPE and that this inhibitory activity was suppressed by specific antibodies against it. Our findings suggest that the C-terminal propeptide functions as an auto-inhibitory domain that masks the catalytic site. Thus, the removal of the C-terminal propeptide of pVPE might expose the catalytic site of the enzyme.

Ricin A chain without its partner B chain is degraded after retrotranslocation from the endoplasmic reticulum to the cytosol in plant cells

When expressed in tobacco cells, the catalytic subunit of the dimeric ribosome inactivating protein, ricin, is first inserted into the endoplasmic reticulum (ER) and then degraded in a manner that can be partially inhibited by the proteasome inhibitor clasto-lactacystin beta-lactone. Consistent with the implication of cytosolic proteasomes, degradation of ricin A chain is brefeldin A-insensitive and the polypeptides that accumulate in the presence of the proteasome inhibitor are not processed in a vacuole-specific fashion. Rather, these stabilized polypeptides are in part deglycosylated by a peptide:N-glycanase-like activity. Taken together, these results indicate that ricin A chain, albeit a structurally native protein, can behave as a substrate for ER to cytosol export, deglycosylation in the cytosol, and proteasomal degradation. Furthermore, retrotranslocation of this protein is not tightly coupled to proteasomal activity. These data are consistent with the hypothesis that ricin A chain can exploit the ER-associated protein degradation pathway to reach the cytosol. Although well characterized in mammalian and yeast cells, the operation of a similar pathway to the cytosol of plant cells has not previously been demonstrated.

Cloning and sequence analysis of cDNA encoding thiamin-binding proteins from sesame seeds

The amino acid sequences of the large polypeptides of thiamin-binding proteins (TBPs) from sesame (Sesamum indicum L.) seeds (STBP-I, -II and -III) were analyzed. The large polypeptides of STBP-I, -II and -III had the same amino acid sequences as did their small polypeptides. The peptide sequence information obtained from STBPs was used to synthesize DNA primers for amplification of the gene(s) encoding STBPs. A 200-bp fragment was amplified from cDNA synthesized from RNA from sesame seeds 4 weeks after flowering. The 200-bp fragment was used to clone full-length cDNA(s) encoding STBP(s) with RACE techniques. A 644-bp fragment was amplified, cloned and sequenced. The cDNA was a full-length clone encoding STBP(s). It contained an open reading frame, which defined a 143-residue polypeptide. The identified small and large polypeptide sequences of STBPs exactly matched the sequence encoded within the cDNA clone. These results indicated that the small and large polypeptides of STBPs were encoded on the mRNA as a single large proprotein precursor and that the final mature forms were generated by post-translational processing in the same manner as the other 2S albumins of plant seeds.

Legumain forms from plants and animals differ in their specificity

We purified forms of legumain from a plant source (seeds of kidney bean, Phaseolus vulgaris) and a mammal (kidney of pig, Sus scropha) for comparison of their properties. Both forms were found to be stable only under moderately acidic pH conditions, and were maximally active at about pH 6; the plant enzyme was somewhat less stable and had a slightly higher pH optimum. With benzyloxycarbonyl-Xaa-Ala-Asn-aminomethylcoumarylamide substrates, the two forms of legumain showed distinctly different specificities for the P3 residue, the plant legumain preferring amino acids with bulky hydrophobic side chains because of lower Km values. Both forms of legumain were highly specific for hydrolysis of asparaginyl bonds in the arylamide substrates and in neurotensin. Aspartyl bonds were hydrolysed about 100-fold more slowly with lower pH optima. Potential substrates containing other amino acids structurally similar to asparagine were not hydrolysed. There were clear differences in specificity of hydrolysis of protein substrates. The plant legumain differed from pig legumain in its action on tetanus toxoid C-fragment, cleaving at Asn97 but not at Asn337, and produced more extensive digestion of phaseolin. The plant form of legumain was much more weakly inhibited by egg-white cystatin than was the mammalian form.

Biogenesis of the protein storage vacuole crystalloid

We identify new organelles associated with the vacuolar system in plant cells. These organelles are defined biochemically by their internal content of three integral membrane proteins: a chimeric reporter protein that moves there directly from the ER; a specific tonoplast intrinsic protein; and a novel receptor-like RING-H2 protein that traffics through the Golgi apparatus. Highly conserved homologues of the latter are expressed in animal cells. In a developmentally regulated manner, the organelles are taken up into vacuoles where, in seed protein storage vacuoles, they form a membrane-containing crystalloid. The uptake and preservation of the contents of these organelles in vacuoles represents a unique mechanism for compartmentalization of protein and lipid for storage.

Identification of a cDNA encoding an active asparaginyl endopeptidase of Schistosoma mansoni and its expression in Pichia pastoris

Asparaginyl endopeptidases, or legumains, are a recently identified family of cysteine-class endopeptidases. A single gene encoding a Schistosoma mansoni asparaginyl endopeptidase (a.k.a. Sm32 or schistosome legumain) has been reported, but by sequence homology it would be expected to yield an inactive product as the active site C197 had been replaced by N. We now describe a new S. mansoni gene in which C197 is present. Both gene products were expressed in Pichia pastoris. Autocatalytic processing to fully active C197 Sm32 occurred at acid pH. In contrast, N197 Sm32 was not processed and this is consistent with the hypothesis that C197 is essential for catalysis. This was confirmed by mutation of N197 to C and re-expression in Pichia. The availability of recombinant active Sm32 allows detailed analysis of its catalytic mechanism and its function(s) in the biology of this important human parasite.

Vacuolar processing enzyme is up-regulated in the lytic vacuoles of vegetative tissues during senescence and under various stressed conditions

Vacuolar processing enzyme (VPE) has been shown to be responsible for maturation of various seed proteins in protein-storage vacuoles. Arabidopsis has three VPE homologues; betaVPE is specific to seeds and alphaVPE and gammaVPE are specific to vegetative organs. To investigate the activity of the vegetative VPE, we expressed the gammaVPE in a pep4 strain of the yeast Saccharomyces cerevisiae and found that gammaVPE has the ability to cleave the peptide bond at the carbonyl side of asparagine residues. An immunocytochemical analysis revealed the specific localization of the gammaVPE in the lytic vacuoles of Arabidopsis leaves that had been treated with wounding. These findings indicate that gammaVPE functions in the lytic vacuoles as the betaVPE does in the protein-storage vacuoles. The betaVPE promoter was found to direct the expression of the beta-glucuronidase reporter gene in seeds and the root tip of transgenic Arabidopsis plants. On the other hand, both the alphaVPE and gammaVPE promoters directed the expression in senescent tissues, but not in young intact tissues. The mRNA levels of both alphaVPE and gammaVPE were increased in the primary leaves during senescence in parallel with the increase of the mRNA level of a senescence-associated gene (SAG2). Treatment with wounding, ethylene and salicylic acid up-regulated the expression of alphaVPE and gammaVPE, while jasmonate slightly up-regulated the expression of gammaVPE. These gene expression patterns of the VPEs were associated with the accumulation of vacuolar proteins that are known to respond to these treatments. Taken together, the results suggest that vegetative VPE might regulate the activation of some functional proteins in the lytic vacuoles.

Posttranslational removal of the carboxyl-terminal KDEL of the cysteine protease SH-EP occurs prior to maturation of the enzyme

SH-EP is a cysteine protease from germinating mung bean (Vigna mungo) that possesses a carboxyl-terminal endoplasmic reticulum (ER) retention sequence, KDEL. In order to examine the function of the ER retention sequence, we expressed a full-length cDNA of SH-EP and a minus-KDEL control in insect Sf-9 cells using the baculovirus system. Our observations on the synthesis, processing, and trafficking of SH-EP in Sf-9 cells suggest that the KDEL ER-retention sequence is posttranslationally removed either while the protein is still in the ER or immediately after its exit from the ER, resulting in the accumulation of proSH-EP minus its KDEL signal. It is this intermediate form that appears to progress through the endomembrane system and is subsequently processed to form mature active SH-EP. The removal of an ER retention may regulate protein delivery to a functional site and present an alternative role for ER retention sequences in addition to their well established role in maintaining the protein composition of the ER lumen.

Vacuolar processing enzyme is self-catalytically activated by sequential removal of the C-terminal and N-terminal propeptides

A vacuolar processing enzyme (VPE) responsible for maturation of various vacuolar proteins is synthesized as an inactive precursor. To clarify how to convert the VPE precursor into the active enzyme, we expressed point mutated VPE precursors of castor bean in the pep4 strain of Saccharomyces cerevisiae. A VPE with a substitution of the active site Cys with Gly showed no ability to convert itself into the mature form, although a wild VPE had the ability. The mutated VPE was converted by the action of the VPE that had been purified from castor bean. Substitution of the conserved Asp-Asp at the putative cleavage site of the C-terminal propeptide with Gly-Gly abolished both the conversion into the mature form and the activation of the mutated VPE. In vitro assay with synthetic peptides demonstrated that a VPE exhibited activity towards Asp residues and that a VPE cleaved an Asp-Gln bond to remove the N-terminal propeptide. Taken together, the results indicate that the VPE is self-catalytically maturated to be converted into the active enzyme by removal of the C-terminal propeptide and subsequent removal of the N-terminal one.

Multiple functional proteins are produced by cleaving Asn-Gln bonds of a single precursor by vacuolar processing enzyme

Precursor-accumulating vesicles mediate transport of the precursors of seed proteins to protein storage vacuoles in maturing pumpkin seeds. We isolated the precursor-accumulating vesicles and characterized a 100-kDa component (PV100) of the vesicles. Isolated cDNA for PV100 encoded a 97,310-Da protein that was composed of a hydrophobic signal peptide and the following three domains: an 11-kDa Cys-rich domain with four CXXXC motifs, a 34-kDa Arg/Glu-rich domain composed of six homologous repeats, and a 50-kDa vicilin-like domain. Both immunocytochemistry and immunoblots with anti-PV100 antibodies showed that <10-kDa proteins and the 50-kDa vicilin-like protein were accumulated in the vacuoles. To identify the mature proteins derived from PV100, soluble proteins of the vacuoles were separated, and their molecular structures were determined. Mass spectrometry and peptide sequencing showed that two Cys-rich peptides, three Arg/Glu-rich peptides, and the vicilin-like protein were produced by cleaving Asn-Gln bonds of PV100 and that all of these proteins had a pyroglutamate at their NH2 termini. To clarify the cleavage mechanism, in vitro processing of PV100 was performed with purified vacuolar processing enzyme (VPE). Taken together, these results suggested that VPE was responsible for cleaving Asn-Gln bonds of a single precursor, PV100, to produce multiple seed proteins. It is likely that the Asn-Gln stretches not only provide cleavage sites for VPE but also produce aminopeptidase-resistant proteins. We also found that the Cys-rich peptide functions as a trypsin inhibitor. Our findings suggested that PV100 is converted into different functional proteins, such as a proteinase inhibitor and a storage protein, in the vacuoles of seed cells.

cDNA cloning of FRIL, a lectin from Dolichos lablab, that preserves hematopoietic progenitors in suspension culture

Ex vivo culture of hematopoietic stem cells is limited by the inability of cytokines to maintain primitive cells without inducing proliferation, differentiation, and subsequent loss of repopulating capacity. We identified recently in extracts of kidney bean and hyacinth bean a mannose-binding lectin, called FRIL, and provide here evidence that this protein appears to satisfy properties of a stem cell preservation factor. FRIL was first identified based on its ability to stimulate NIH 3T3 cells transfected with Flt3, a tyrosine kinase receptor central to regulation of stem cells. Molecular characterization from polypeptide sequencing and identification of the cDNA of hyacinth bean FRIL shows 78% amino acid identity with a mannose-binding lectin of hyacinth beans. Treatment of primitive hematopoietic progenitors in suspension culture with purified hyacinth FRIL alone is able to preserve cells for 1 month without medium changes. In vitro progenitor assays for human hematopoietic cells cultured 3 weeks in FRIL displayed small blast-like colonies that were capable of serial replating and persisted even in the presence of cytokines known to induce differentiation. These results suggest that FRIL is capable of preserving primitive progenitors in suspension culture for prolonged periods. FRIL's clinical utility involving procedures for stem cell transplantation, tumor cell purging before autologous transplantation, and ex vivo cultures used for expansion and stem cell gene therapy currently are being explored.

Nucellain, a barley homolog of the dicot vacuolar-processing protease, is localized in nucellar cell walls

The nucellus is a complex maternal grain tissue that embeds and feeds the developing cereal endosperm and embryo. Differential screening of a barley (Hordeum vulgare) cDNA library from 5-d-old ovaries resulted in the isolation of two cDNA clones encoding nucellus-specific homologs of the vacuolar-processing enzyme of castor bean (Ricinus communis). Based on the sequence of these barley clones, which are called nucellains, a homolog from developing corn (Zea mays) grains was also identified. In dicots the vacuolar-processing enzyme is believed to be involved in the processing of vacuolar storage proteins. RNA-blot and in situ-hybridization analyses detected nucellain transcripts in autolysing nucellus parenchyma cells, in the nucellar projection, and in the nucellar epidermis. No nucellain transcripts were detected in the highly vacuolate endosperm or in the other maternal tissues of developing grains such as the testa or the pericarp. Using an antibody raised against castor bean vacuolar-processing protease, a single polypeptide was recognized in protein extracts from barley grains. Immunogold-labeling experiments with this antibody localized the nucellain epitope not in the vacuoles, but in the cell walls of all nucellar cell types. We propose that nucellain plays a role in processing and/or turnover of cell wall proteins in developing cereal grains.

Deposition of storage proteins

Plants store amino acids for longer periods in the form of specific storage proteins. These are deposited in seeds, in root and shoot tubers, in the wood and bark parenchyma of trees and in other vegetative organs. Storage proteins are protected against uncontrolled premature degradation by several mechanisms. The major one is to deposit the storage proteins into specialized membrane-bounded storage organelles, called protein bodies (PB). In the endosperm cells of maize and rice prolamins are sequestered into PBs which are derived from the endoplasmic reticulum (ER). Globulins, the typical storage proteins of dicotyledonous plants, and prolamins of some cereals are transported from the ER through the Golgi apparatus and then into protein storage vacuoles (PSV) which later become transformed into PBs. Sorting and targeting of storage proteins begins during their biosynthesis on membrane-bound polysomes where an N-terminal signal peptide mediates their segregation into the lumen of the ER. After cleavage of the signal peptide, the polypeptides are glycosylated and folded with the aid of chaperones. While still in the ER, disulfide bridges are formed which stabilize the structure and several polypeptides are joined to form an oligomer which has the proper conformation to be either deposited in ER-derived PB or to be further transferred to the PSV. At the trans-Golgi cisternae transport vesicles are sequestered which carry the storage proteins to the PSV. Several storage proteins are also processed after arriving in the PSVs in order to generate a conformation that is capable of final deposition. Some storage protein precursors have short N- or C-terminal targeting sequences which are detached after arrival in the PSV. Others have been shown to have internal sequence regions which could act as targeting information. In some cases positive targeting information is known to mediate sorting into the PSV whereas in other cases aggregation and membrane association seem to be major sorting mechanisms.

Two ways of legumin-precursor processing in conifers. Characterization and evolutionary relationships of Metasequoia cDNAs representing two divergent legumin gene subfamilies

Subunit monomers and oligomers of crystalloid-type legumins are major components of SDS-soluble fractions from Metasequoia glyptostroboides (Dawn redwood, Taxodiaceae) seed proteins. The subunits are made up of disulfide linked alpha-polypeptides and beta-polypeptides with molecular masses of 33 kDa and 23-25 kDa, respectively. Unusually for legumins, those from Metasequoia are glycosylated and the carbohydrate moieties are residing in the C-terminal region of the respective beta-polypeptides. A Metasequoia endosperm cDNA library has been constructed and legumin-encoding transcripts representing two divergent gene subfamilies have been characterized. Intersubfamily comparisons reveal 75% identity at the amino acid level and the values range from 53-35% when the legumin precursors deduced were compared with those from angiosperms. The predicted sequences together with data from amino acid sequencing prove that post-translational processing of Metasequoia prolegumins is directed to two different processing sites, each of them specific for one of the legumin subfamilies. The sites involved differ in their relative position and in the junction to be cleaved: Metasequoia legumin precursors MgLeg18 and MgLeg26 contain the conventional post-translational Asn-Gly processing site, which is generally regarded as highly conserved. In contrast, the MgLeg4 precursor is lacking this site and post-translational cleavage is directed to an unusual Asn-Thr processing site located in its hypervariable region, causing N-terminal extension of the beta-polypeptide relative to those hitherto known. Evidence is given that the unusual variant of processing also occurs in other conifers. Phylogenetic analysis reveals the precursors concerned as representatives of a distinct legumin subfamily, originating from duplication of an ancestral gene prior to or at the beginning of Taxodiaceae diversification.

An aspartic endopeptidase is involved in the breakdown of propeptides of storage proteins in protein-storage vacuoles of plants

To understand the mechanism of the maturation of various proteins in protein-storage vacuoles, we purified a 48-kDa aspartic endopeptidase composed of 32-kDa and 16-kDa subunits from castor bean. Immunocytochemical and cell fractionation analyses of the endosperm of maturing castor bean seed showed that the aspartic endopeptidase was localized in the matrix of the protein-storage vacuoles, where a variety of seed storage proteins were also present. The amount of the aspartic endopeptidase increased at the mid-maturation stage of the seeds before accumulation of the storage proteins. To determine how the aspartic endopeptidase is responsible for maturation of seed proteins in concert with the vacuolar processing enzyme, we prepared 35S-labeled proproteins of seed proteins from the endoplasmic reticulum fraction of pulse-labeled maturing endosperm and used the authentic proproteins as substrates for in vitro processing experiments. The purified aspartic endopeptidase was unable to convert any of three endosperm proproteins, pro2S albumin, proglobulin, and proricin, into their mature sizes, while the purified vacuolar processing enzyme could convert all three proproteins. We further examined the activity of aspartic endopeptidase on the cleavage of an internal propeptide of Arabidopsis pro2S albumin, which is known to be removed post-translationally. The aspartic endopeptidase cleaved the propeptide at three sites under acidic conditions. These results suggest that aspartic endopeptidase cannot directly convert pro2S albumin into the mature form, but it may play a role in trimming the C-terminal propeptides from the subunits that are produced by the action of the vacuolar processing enzyme.

Cloning, isolation, and characterization of mammalian legumain, an asparaginyl endopeptidase

Legumain is a cysteine endopeptidase that shows strict specificity for hydrolysis of asparaginyl bonds. The enzyme belongs to peptidase family C13, and is thus unrelated to the better known cysteine peptidases of the papain family, C1 (Rawlings, N. D., and Barrett, A. J. (1994) Methods Enzymol. 244, 461-486). To date, legumain has been described only from plants and a blood fluke, Schistosoma mansoni. We now show that legumain is present in mammals. We have cloned and sequenced human legumain and part of pig legumain. We have also purified legumain to homogeneity (2200-fold, 8% yield) from pig kidney. The mammalian sequences are clearly homologous with legumains from non-mammalian species. Pig legumain is a glycoprotein of about 34 kDa, decreasing to 31 kDa on deglycosylation. It is an asparaginyl endopeptidase, hydrolyzing Z-Ala-Ala-Asn-7-(4-methyl)coumarylamide and benzoyl-Asn-p-nitroanilide. Maximal activity is seen at pH 5.8 under normal assay conditions, and the enzyme is irreversibly denatured at pH 7 and above. Mammalian legumain is a cysteine endopeptidase, inhibited by iodoacetamide and maleimides, but unaffected by compound E64 (trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane). It is inhibited by ovocystatin (cystatin from chicken egg white) and human cystatin C with Ki values < 5 nM. We discuss the significance of the discovery of a cysteine endopeptidase of a new family and distinctive specificity in man and other mammals.

Processing of thionin precursors in barley leaves by a vacuolar proteinase

Thionins are synthesized as precursors with a signal peptide and a long C-terminal acidic peptide that is post-translationally processed. A fusion protein including the maltose-binding protein from Escherichia coli (MalE), thionin DG3 from barley leaves, and its acidic C-terminal peptide has been used to obtain antibodies that recognize both domains of the precursor. In barley leaf sections, mature thionins accumulated in the vacuolar content, while the acidic peptide was not detected in any cell fraction. Brefeldin A and monensin inhibited processing of the precursor but its export from the microsomal fraction was not inhibited. Both purified vacuoles and an acid (pH 5.5) extract from leaves processed the fusion protein into a MalE-thionin and an acidic peptide fragment. A 70-kDa proteinase that effected this cleavage was purified from the acid extract. Processing of the fusion protein by both lysed vacuoles and the purified proteinase was inhibited by Zn2+ and by Cu2+, but not by inhibitors of the previously described vacuolar processing thiol or aspartic proteinases. In vivo processing of the thionin precursor in leaf sections was also inhibited by Zn2+ and Cu2+. Variants of the fusion protein with altered processing sites that represented those of thionin precursors from different taxa were readily processed by the proteinase, whereas changing the polarity of either the C-terminal or N-terminal residues of the processing site prevented cleavage by the proteinase.

1H NMR assignment and global fold of napin BnIb, a representative 2S albumin seed protein

Napin BnIb is a representative member of the 2S albumin seed proteins, which consists of two polypeptide chains of 3.8 and 8.4 kDa linked by two disulfide bridges. In this work, a complete assignment of the 1H spectra of napin BnIb has been carried out by two-dimensional NMR sequence-specific methods and its secondary structure determined on the basis of spectral data. A calculation of the tertiary structure has been performed using approximately 500 distance constraints derived from unambiguously assigned NOE cross-correlations and distance geometry methods. The resulting global fold consists of five helices and a C-terminal loop arranged in a right-handed spiral. The folded protein is stabilized by two interchain disulfide bridges and two additional ones between cysteine residues in the large chain. The structure of napin BnIb represents a third example of a new and distinctive folding pattern first described for the hydrophobic protein from soybean and nonspecific lipid transfer proteins from wheat and maize. The presence of an internal cavity is not at all evident, which rules out in principle the napin BnIb as a carrier of lipids. The determined structure is compatible with activities attributed to these proteins such as phospholipid vesicle interaction, allergenicity, and calmodulin antagonism. Given the sequence homology of BnIb with other napins and napin-type 2S albumin seed proteins from different species, it is likely that all these proteins share a common architecture. The determined structure will be crucial to establish structure-function relationships and to explore the mechanisms of folding, processing, and deposition of these proteins. It will also provide a firm basis for a rational use of genetic engineering in order to develop improved transgenic plants.

Purification and sequencing of napin-like protein small and large chains from Momordica charantia and Ricinus communis seeds and determination of sites phosphorylated by plant Ca(2+)-dependent protein kinase

The basic protein fraction from seeds of castor bean (Ricinus communis L.) contains 4732 Da and 4603 Da proteins phosphorylated in vitro by plant Ca(2+)-dependent protein kinase (CDPK). These proteins, RS1A and RS1B respectively, were purified by cation-exchange HPLC (SP5PW column) and reverse-phase HPLC (C18 column) and identified as napin-like protein small chains by Edman sequencing and electrospray ionization mass spectrometry (ESMS). The other R. communis 4 kDa small chains (RS2A, RS2B, RS2C and RS2D) are not phosphorylated by CDPK and neither is the corresponding 7332 Da large chain (RL) that forms 1:1 disulfide-linked complexes with RS2(A-D). RS1A/B is one of the best substrates found for plant CDPK (K(m) = 1.8 +/- 0.8 microM). RS2(A-D) (but not RL or RS1A/B) strongly inhibit calmodulin (CaM)-dependent myosin light chain protein kinase (MLCK) (IC50 = 0.25 microM) and inhibit the Ca(2+)-dependent enhancement of dansyl-CaM fluorescence. The basic protein fraction from seeds of bitter melon (Momordica charantia) also contains napin-like proteins that are 1:1 disulfide-linked complexes of a small chain (MS1, MS2, MS3 or MS4) and a large chain (ML). The M. charantia small chains were purified and completely sequenced by Edman degradation and ESMS. M. charantia small chains MS1, MS2, and MS4 (but not MS3) are phosphorylated by CDPK to unit stoichiometry on S21 within the sequence R17SCES21FLR. The R. communis small chain RS1A is phosphorylated on S34 within the sequence R31QSS34SRR. Both of these phosphorylation site motifs are consistent with those found for other plant CDPK substrates.

COMPARTMENTATION OF PROTEINS IN THE ENDOMEMBRANE SYSTEM OF PLANT CELLS

This review focuses on four interrelated processes in the plant endomembrane system: compartmentation of proteins in subdomains of the endoplasmic reticulum, mechanisms that determine whether storage proteins are retained within the ER lumen or transported out, the origin and function of biochemically distinct vacuoles or prevacuolar organelles, and the cellular processes by which proteins are sorted to vacuolar compartments. We postulate that ER-localized protein bodies are formed by a series of orderly events of protein synthesis, protein concentration, and protein assembly in subdomains of the ER. Protein concentration, which facilitates protein-to-protein interactions and subsequent protein assembly, may be achieved by the interactions with chaperones and by the localization of storage protein mRNAs. We also describe recent developments on the coexistence of two biochemically distinguishable vacuolar compartments, the possible direct role of the ER in vacuole biogenesis, and proposed mechanisms for transport of proteins from the ER or Golgi apparatus to the vacuole.

An acidic amino acid-specific protease from germinating soybeans

The degradation of the beta-conglycinin protein reserves in soybean seeds during germination and early growth begins with the proteolysis of its alpha and alpha' subunits by an enzyme called Protease C1. In the pathway, a number of proteolytic intermediates are produced and subsequently degraded. Determination of the N-terminal sequences of these intermediates provides insight regarding the requirements of the cleavage sites. The N-terminal sequence of three such proteolytic intermediates has been determined. The sequence has been located in the published sequences of the beta-conglycinin subunits. Comparing these cleavage sites, plus those of two others previously delineated, shows that the P1' and P4' positions always bear either a Glu or an Asp residue while the P1 position always bears either a Glu or a Gln residue. In addition, other sites from P3 to P7' are also rich in either Glu or Asp, and the whole region is predicted to be in a alpha-helix. Consistent with the observation, synthetic poly-L-Glu inhibits the Protease C1-catalysed degradation of the alpha and alpha' subunits of beta-conglycinin. Poly-L-Glu (av. M(r) = 1000) at 12.5 mM was more effective at inhibiting the reaction than poly-L-Glu (av. M(r) = 600) or poly-L-Glu (av. M(r) = 14,300) at the same concentration. Comparing large synthetic polypeptides at 12.5mM, inhibition by poly-L-Asp (av. M(r) = 15,000) is as effective as poly-L-Glu (av. M(r) = 14,300), while poly-L-Ser (av. M(r) = 15,000) had no effect at all. Poly-D-Glu (av. M(r) = 15,000) is a better inhibitor than poly-L-Glu of the same size. A serine protease of similar molecular weight as Protease C1 and also capable of catalysing the proteolysis of the alpha and alpha' subunits of beta-conglycinin to generate proteolytic intermediates of the same size has been found in mung bean.

Mass spectrometric amino acid sequencing of a mixture of seed storage proteins (napin) from Brassica napus, products of a multigene family

The amino acid sequences of a number of closely related proteins ("napin") isolated from Brassica napus were determined by mass spectrometry without prior separation into individual components. Some of these proteins correspond to those previously deduced (napA, BngNAP1, and gNa), chiefly from DNA sequences. Others were found to differ to a varying extent (BngNAP1', BngNAP1A, BngNAP1B, BngNAP1C, gNa', and gNaA). The short chains of gNa and gNa' and of BngNAP1 and BngNAP1' differ by the replacement of N-terminal proline by pyroglutamic acid; the long chains of gNaA and BngNAP1B contain a six amino acid stretch, MQGQQM, which is present in gNa (according to its DNA sequence) but absent from BngNAP1 and BngNAP1C. These alternations of sequences between napin isoforms are most likely due to homologous recombination of the genetic material, but some of the changes may also be due to RNA editing. The amino acids that follow the untruncated C termini of those napin chains for which the DNA sequences are known (napA, BngNAP1, and gNa) are aromatic amino acids. This suggests that the processing of the proprotein leading to the C termini of the two chains is due to the action of a protease that specifically cleaves a G/S-F/Y/W bond.

Rapid appearance and asymmetric distribution of glucose transporter SGTP4 at the apical surface of intramammalian-stage Schistosoma mansoni

Adult Schistosoma mansoni blood flukes reside in the mesenteric veins of their vertebrate hosts, where they absorb immense quantities of glucose through their tegument by facilitated diffusion. Previously, we obtained S. mansoni cDNAs encoding facilitated-diffusion schistosome glucose transporter proteins 1 and 4 (SGTP1 and SGTP4) and localized SGTP1 to the basal membranes of the tegument and the underlying muscle. In this study, we characterize the expression and localization of SGTP4 during the schistosome life cycle. Antibodies specific to SGTP4 appear to stain only the double-bilayer, apical membranes of the adult parasite tegument, revealing an asymmetric distribution relative to the basal transporter SGTP1. On living worms, SGTP4 is available to surface biotinylation, suggesting that it is exposed at the hose-parasite interface. SGTP4 is detected shortly after the transformation of free-living, infectious cercariae into schistosomula and coincides with the appearance of the double membrane. Within 15 min after transformation, anti-SGTP4 staining produces a bright, patchy distribution at the surface of schistosomula, which becomes contiguous over the entire surface of the schistosomula by 24 hr after transformation. SGTP4 is not detected in earlier developmental stages (eggs, sporocysts, and cercariae) that do not possess the specialized double membrane. Thus, SGTP4 appears to be expressed only in the mammalian stages of the parasite's life cycle and specifically localized within the host-interactive, apical membranes of the tegument.

Legumin encoding sequences from the redwood family (Taxodiaceae) reveal precursors lacking the conserved Asn-Gly processing site

We have cloned and sequenced two different cDNAs encoding legumins from Japanese red cedar (Cryptomeria japonica, Taxodiaceae). The derived amino acid sequences show between 34% and 55% identity when compared with legumins from angiosperms and from Pinaceae, respectively. The predicted precursors are unusual in that they contain potential glycosylation signals, and we have found the corresponding beta-polypeptides actually to be glycosylated. As most outstanding feature one of the precursors is lacking the Asn-Gly processing site which has been assumed to be highly conserved in legumin gene evolution. Legumin encoding sequences amplified from genomic DNA suggest that these unusual precursors are widespread if not ubiquitous in the Taxodiaceae family. From previous reports on legumin precursors with divergent processing sites, on the proteases involved in legumin precursor processing and from the results presented here it is concluded that the Asn-Gly processing site has been acquired rather than conserved during legumin gene evolution.

Changes in physical properties of vacuolar membrane during transformation of protein bodies into vacuoles in germinating pumpkin seeds

Changes in membrane molecular dynamics associated with the transformation of protein body membranes into vacuolar membranes during pumpkin seed germination, were monitored by EPR-spin probe technique. Using highly purified membrane preparations as well as 5-SASL and 16-SASL spin labels, parameters like general membrane lipid fluidity, order parameter, semicone angle, rotational correlation times tau 2B and tau 2C, ratio of immobilized to mobile lipids were determined and the activation energy for rotational diffusion of 16-SASL was calculated. Analysis of these parameters at different temperatures indicated a more rigid nature of protein body membrane comparing to vacuolar membrane, as a result of a more restricted motional freedom of lipids. These differences are discussed in terms of protein composition and various functional specialization of both types of membranes.

Homologues of a vacuolar processing enzyme that are expressed in different organs in Arabidopsis thaliana

Vacuolar processing enzymes (VPEs) are responsible for the maturation of seed proteins. These processing enzymes belong to a novel group of cysteine proteinases with molecular masses of 37 to 39 kDa. We isolated two genes of VPEs from a genomic library of Arabidopsis. The gene products were designated alpha-VPE and beta-VPE, and they were 56% identical in terms of amino acid sequence. The amino acid sequences of alpha-VPE and beta-VPE were also 55% and 67% identical to that of castor bean VPE, respectively. The gene for alpha-VPE had 7 introns, while that of beta-VPE had 8 introns. Northern blot analysis revealed that alpha-VPE is expressed in rosette leaves, cauline leaves and stems of Arabidopsis, while beta-VPE is predominantly expressed in the flowers and buds. Neither alpha-VPE nor beta-VPE is expressed in the siliques. This result strongly suggests that the isolated genes encode isozymes of VPE that are specific to vegetative organs.

Characterization of two integral membrane proteins located in the protein bodies of pumpkin seeds

Two integral membrane proteins, MP28 and MP23, were found in protein bodies isolated from pumpkin (Cucurbita sp.) seeds. Molecular characterization revealed that both MP28 and MP23 belong to the seed TIP (tonoplast intrinsic protein) subfamily. The predicted 29 kDa precursor to includes six putative membrane-spanning domains, and the loop between the first and second transmembrane domains is larger than that of MP28. The N-terminal sequence of the mature MP23 starts from residue 66 in the first loop, indicating that an N-terminal 7 kDa fragment that contains one transmembrane domain is post-translationally removed. During maturation of pumpkin seeds, mRNAs for MP28 and MP23 became detectable in cotyledons at the early stage, and their levels increased slightly until a rapid decrease occurred at the late stage. This is consistent with the accumulation of the 29 kDa precursor and MP28 in the cotyledons at the early stage. By contrast, MP23 appeared at the late stage simultaneously with the disappearance of the 29 kDa precursor. Thus, it seems possible that the conversion of the 29 kDa precursor to the mature MP23 might occur in the vacuoles after the middle stage of seed maturation. Both proteins were localized immunocytochemically on the membranes of the vacuoles at the middle stage and the protein bodies at the late stage. These results suggest that both MP28 and the precursor to MP23 accumulate on vacuolar membranes before the deposition of storage proteins, and then the precursor is converted to the mature MP23 at the late stage. These two TIPs might have a specific function during the maturation of pumpkin seeds.

Purification of a processing enzyme (VmPE-1) that is involved in post-translational processing of a plant cysteine endopeptidase (SH-EP)

A cysteine endopeptidase, designated SH-EP, occurs in the cotyledons of germinated seeds of Vigna mungo and acts to degrade the seed storage protein in protein storage vacuoles. SH-EP is synthesized on membrane-bound ribosomes as an inactive 45-kDa precursor, which is cotranslationally processed to a 43-kDa intermediate through cleavage of the signal sequence; the 43-kDa intermediate of SH-EP is further processed to the 33-kDa mature enzyme via 39-kDa and 36-kDa intermediates [Mitsuhashi, W. & Minamikawa, T. (1989) Plant Physiol. 89, 274-279]. The present in vitro processing experiments indicated that at least two processing enzymes, designated VmPE-1 and VmPE-2 (V. mungo processing enzymes 1 and 2), were involved in post-translational processing of SH-EP in cotyledons of V. mungo seedlings. VmPE-1 was purified from the cotyledons as a protease that was involved in the processing of the 43-kDa intermediate to the 36-kDa intermediate. The enzyme has a molecular mass of 33 kDa as estimated by SDS/polyacrylamide gel electrophoresis, and showed high similarity to the jackbean asparaginyl endopeptidase in terms of the primary structure and substrate specificity. We discuss the function of VmPE-1 in the processing of SH-EP and related proteases in the cotyledons of germinated seeds.

Characterization of the protease processing sites in a multidomain proteinase inhibitor precursor from Nicotiana alata

A gene encoding a 40.3-kDa serine proteinase inhibitor (PI) precursor is expressed at high levels in the stigma of the ornamental tobacco, Nicotiana alata. The precursor is processed proteolytically in vivo to release five homologous proteinase inhibitors of approximately 6 kDa, as well as two flanking peptides. The five PIs have been purified from stigmas and identified by N-terminal sequencing, electrospray mass spectrometry and inhibition activity against chymotrypsin or trypsin. One of the PIs inhibits chymotrypsin and the other four are most active on trypsin. Cleavage occurs in a linker region (EEKKND) that is repeated six times in the precursor molecule. In the plant, the initial cleavage probably occurs between asparagine and the aspartate residues and ragged ends are formed by subsequent trimming. In vitro, the protease-sensitive linker region is selectively cleaved by the endoproteinases Asp-N, Glu-C and Lys-C to release fully active approximately 6-kDa PIs that are resistant to further proteolytic digestion. The precursor, produced by a recombinant baculovirus, inhibits chymotrypsin more effectively than trypsin. The stoichiometry of 2.6 trypsin molecules/1 precursor molecule indicates that processing is required to activate or expose all of the four trypsin inhibitory sites.

Multiple isoforms of Pisum trypsin inhibitors result from modification of two primary gene products

Characterization of Pisum (pea) seed trypsin inhibitors (TI) and their corresponding cDNAs indicates that the pea TI gene family contains two genes. The existence of multiple TI isoforms can be attributed to post-translational modifications of primary gene products. Post-translational processing at the C-terminus during the desiccation stage of seed development results in the appearance of TI isoforms with increased affinity for the target enzyme, trypsin.

Processing of the 2S storage protein pronapin in Brassica napus and in transformed tobacco

The 2S protein napin in Brassica napus is synthesized as a proprotein from which an N-terminal an an internal propeptide are removed. In order to investigate the mechanism of 2S storage-protein processing, N-terminal sequences were determined for the light and heavy chains of all major napin isoforms isolated. Mutants of a napin gene, with deletions of different portions of the propeptides, were transformed into tobacco and napin protein was isolated. Napin light and heavy chains were separated and their N-terminal amino acid sequences determined. Further, the C-terminal residues of one napin isoform isolated from B. napus and one mutant napin isolated from tobacco were deduced from molecular-mass determinations of the constituents chains. Analyses suggested that the two propeptides are exposed at the surface of the proprotein. The light chain is processed to the correct length independent of the amino acid sequence in the N-terminal propeptide and the processing site. The internal propeptide is attacked by endoproteases. Aminopeptidases and carboxypeptidases then digest portions of the propeptide to the extent allowed by the primary and the three-dimensional structures, often resulting in 2S protein chains with partly frayed ends.

PCR cloning and expression analysis of cDNAs encoding cysteine proteinases from germinating seeds of Vicia sativa L

cDNA clones encoding cysteine proteinases from cotyledons of germinated seeds of Vicia sativa L. have been obtained by means of PCR. Degenerate oligonucleotide primers were designed according to conserved amino acid regions of known cysteine proteinases. The deduced amino acid sequences of the cDNA clones encoding VSCYSPR1 and VSCYSPR2 display strong homology to cysteine proteinases of the so called papain superfamily. Northern analyses revealed developmentally regulated expression of both the mRNAs in germinating seeds. The transcripts were shown to be products of two distinct single genes, each exhibiting structural polymorphisms as exposed in few nucleotide substitutions.

Ginkgo 11S seed storage protein family mRNA: unusual Asn-Asn linkage as post-translational cleavage site

By reducing the amount of ginkgo water-soluble polysaccharides, which occupy about 35% of the wet seed mass and interfere with the extraction of RNA, cDNA-quality mRNA was obtained from developing seeds of Ginkgo biloba. Based on the NH2-terminal 17-amino acid sequence and an internal 12-amino acid sequence derived from the basic subunit of ginnacin, 11S-seed storage protein family of ginkgo, two degenerate oligonucleotide primers were synthesized and used for polymerase chain reaction (PCR). The resulting PCR product was used for screening the above endosperm cDNA library, and a plaque carrying the 1614 bp cDNA insert, which contained the entire coding region for a precursor of ginnacin was isolated. This is the first reported cloning of cDNA from ginkgo seeds. The deduced primary sequence is composed of a signal peptide segment (25 amino acid residues) and an acidic subunit (248 residues) followed by a basic subunit (187 residues). It was also found that the post-translational cleavage site in the ginnacin precursor is the Asn-Asn rather than the Asn-Gly bond found in a variety of the major subunit precursors in 11S seed protein family known to date. We showed that a purified soybean extract and an extract of ginkgo seeds can specifically hydrolyze -Asn248-Asn249- but not -Asn249-Val250-, in the heptapeptide Gly-Asn248-Asn-Val-Glu-Glu-Leu that corresponds to the ginnacin cleavage region.

A chimeric gene encoding the methionine-rich 2S albumin of the Brazil nut (Bertholletia excelsa H.B.K.) is stably expressed and inherited in transgenic grain legumes

The coding region of the 2S albumin gene of Brazil nut (Bertholletia excelsa H.B.K.) was completely synthesized, placed under control of the cauliflower mosaic virus (CaMV) 35S promoter and inserted into the binary vector plasmid pGSGLUC1, thus giving rise to pGSGLUC1-2S. This was used for transformation of tobacco (Nicotiana tabacum L. cv. Petit Havanna) and of the grain legume Vicia narbonensis L., mediated by the supervirulent Agrobacterium tumefaciens strain EHA 101. Putative transformants were selected by screening for neomycin phosphotransferase (NPT II) and beta-glucuronidase (GUS) activities. Transgenic plants were grown until flowering and fruiting occurred. The presence of the foreign gene was confirmed by Southern analysis. GUS activity was found in all organs of the regenerated transgenic tobacco and legume plants, including the seeds. In the legume, the highest expression levels of the CaMV 35S promoter-controlled 2S albumin gene were observed in leaves and roots. 2S albumin was localized in the vacuoles of leaf mesophyll cells of transgenic tobacco. The Brazil nut protein was present in the 2S fraction after gel filtration chromatography of the legume seed proteins and could be clearly identified by immunoblotting. Analysis of seeds from the R2 progenies of the legume and of transgenic tobacco plants revealed Mendelian inheritance of the foreign gene. Agrobacterium rhizogenes strain RifR 15834 harbouring the binary vector pGSGLUC1-2S was also used to transform Pisum sativum L. and Vicia faba L. Hairy roots expressed the 2S albumin-specific gene. Several shoots were raised but they never completely rooted and no fertile plants were obtained from these transformants.

Pea (Pisum sativum L.) seed isolectins 1 and 2 and pea root lectin result from carboxypeptidase-like processing of a single gene product

The complete amino acid sequences of the alpha-subunits of pea (Pisum sativum L.) seed and root lectin, the C-terminal amino acids of the beta-subunits of pea seed lectin, and most of the sequence of the beta-subunit of pea root lectin were determined. In contrast to earlier reports it was shown that the beta-subunits of both seed isolectins end at Asn-181. The alpha 1 subunits end at Gln-241 (major fraction) or Lys-240 (minor fraction), whereas the alpha 2 subunits end at Ser-239, Ser-238, Ser-237 or Thr-236. psl cDNA clones from seed are identical to psl cDNA clones from root, and root PSL is identical to seed PSL2, ending at Ser-239, Ser-238 or Ser-237. It seems that the presence of Lys-240 is the sole determinant of the charge difference between pea isolectins. PSL1 can be converted into PSL2 by carboxypeptidase P from Penicillium janthinellum. These results confirm that PSL from roots is encoded by the same gene as PSL from seeds. Thus, it seems that, next to an Asn-X specific protease responsible for the processing at positions 181/182 and 187/188, a carboxypeptidase is responsible for the conversion of PSL1 and PSL2, which is probably the final processing product.

A high-order structure of plant storage proprotein allows its second conversion by an asparagine-specific cysteine protease, a novel proteolytic enzyme

During seed embryogenesis, glycinin, the 11-S seed storage protein found in soybeans, undergoes post-translational proteolytic processing, in which a proprotein molecule is cleaved into an acidic and a basic subunit by a one-point cleavage that occurs at the carboxyl side of the asparaginyl residue located at the junction of the subunits. To elucidate the mechanism of this very limited proteolysis, we purified the cysteine endoprotease and used purified proglycinin produced by Escherichia coli as a substrate. This enzyme was separated by isoelectric focusing into three isomeric forms: two had a molecular mass of 33 kDa and the third, 33.8 kDa. The cysteine protease was found both in the proteinaceous vacuoles of cotyledonary tissue of immature seeds and in mature seeds, and is the first proteolytic enzyme to be classified as an asparagine-specific endoprotease. The results also indicate that the above proteolysis is largely attributable to the conformational accessibility of the enzyme to the asparaginyl residue in the cleavage site of proglycinin. The conformation of this single enzyme-accessible region on the proglycinin molecule is relatively flexible and becomes unstable under low salt conditions, or when heat is applied, causing the enzyme to lose its specificity.