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

OsVPE2, a Member of Vacuolar Processing Enzyme Family, Decreases Chilling Tolerance of Rice

Chilling is a major abiotic stress affecting rice growth, development and geographical distribution. Plant vacuolar processing enzymes (VPEs) contribute to the seed storage protein processing and mediate the programmed cell death by abiotic and biotic stresses. However, little is known about the roles of plant VPEs in cold stress responses and tolerance regulation. Here, we found that OsVPE2 was a chilling-responsive gene. The early-indica rice variety Xiangzaoxian31 overexpressing OsVPE2 was more sensitive to chilling stress, whereas the OsVPE2-knockout mutants generated by the CRISPR-Cas9 technology exhibited significantly enhanced chilling tolerance at the seedling stage without causing yield loss. Deficiency of OsVPE2 reduces relative electrolyte leakage, accumulation of toxic compounds such as reactive oxygen species and malondialdehyde, and promotes antioxidant enzyme activities under chilling stress conditions. It was indicated that OsVPE2 mediated the disintegration of vacuoles under chilling stress, accompanied by the entry of swollen mitochondria into vacuoles. OsVPE2 suppressed the expression of genes that have a positive regulatory role in antioxidant process. Moreover, haplotype analysis suggested that the natural variation in the OsVPE2 non-coding region may endow OsVPE2 with different expression levels, thereby probably conferring differences in cold tolerance between japonica and indica sub-population. Our results thus reveal a new biological function of the VPE family in regulating cold resistance, and suggest that the gene editing or natural variations of OsVPE2 can be used to create cold tolerant rice varieties with stable yield.

Phytocystatin 6 is a context-dependent, tight-binding inhibitor of Arabidopsis thaliana legumain isoform β

Plant legumains are crucial for processing seed storage proteins and are critical regulators of plant programmed cell death. Although research on legumains boosted recently, little is known about their activity regulation. In our study, we used pull-down experiments to identify AtCYT6 as a natural inhibitor of legumain isoform β (AtLEGβ) in Arabidopsis thaliana. Biochemical analysis revealed that AtCYT6 inhibits both AtLEGβ and papain-like cysteine proteases through two separate cystatin domains. The N-terminal domain inhibits papain-like proteases, while the C-terminal domain inhibits AtLEGβ. Furthermore, we showed that AtCYT6 interacts with legumain in a substrate-like manner, facilitated by a conserved asparagine residue in its reactive center loop. Complex formation was additionally stabilized by charged exosite interactions, contributing to pH-dependent inhibition. Processing of AtCYT6 by AtLEGβ suggests a context-specific regulatory mechanism with implications for plant physiology, development, and programmed cell death. These findings enhance our understanding of AtLEGβ regulation and its broader physiological significance.

Involvement of Vacuolar Processing Enzyme CgVPE1 in Vacuole Rupture in the Programmed Cell Death during the Development of the Secretory Cavity in Citrus grandis 'Tomentosa' Fruits

Vacuolar processing enzymes (VPEs) with caspase-1-like activity are closely associated with vacuole rupture. The destruction of vacuoles is one of the characteristics of programmed cell death (PCD) in plants. However, whether VPE is involved in the vacuole destruction of cells during secretory cavity formation in Citrus plants remains unclear. This research identified a CgVPE1 gene that encoded the VPE and utilized cytology and molecular biology techniques to explore its temporal and spatial expression characteristics during the PCD process of secretory cavity cells in the Citrus grandis 'Tomentosa' fruit. The results showed that CgVPE1 is an enzyme with VPE and caspase-1-like activity that can self-cleave into a mature enzyme in an acidic environment. CgVPE1 is specifically expressed in the epithelial cells of secretory cavities. In addition, it mainly accumulates in vacuoles before it is ruptured in the secretory cavity cells. The spatial and temporal immunolocalization of CgVPE1 showed a strong relationship with the change in vacuole structure during PCD in secretory cavity cells. In addition, the change in the two types of VPE proteins from proenzymes to mature enzymes was closely related to the change in CgVPE1 localization. Our results indicate that CgVPE1 plays a vital role in PCD, causing vacuole rupture in cells during the development of the secretory cavity in C. grandis 'Tomentosa' fruits.

Programmed Cell Death in Developing Brachypodium distachyon Grain

The normal developmental sequence in a grass grain entails the death of several maternal and filial tissues in a genetically regulated process termed programmed cell death (PCD). The progression and molecular aspects of PCD in developing grains have been reported for domesticated species such as barley, rice, maize and wheat. Here, we report a detailed investigation of PCD in the developing grain of the wild model species Brachypodium distachyon. We detected PCD in developing Brachypodium grains using molecular and histological approaches. We also identified in Brachypodium the orthologs of protease genes known to contribute to grain PCD and surveyed their expression. We found that, similar to cereals, PCD in the Brachypodium nucellus occurs in a centrifugal pattern following anthesis. However, compared to cereals, the rate of post-mortem clearance in the Brachypodium nucellus is slower. However, compared to wheat and barley, mesocarp PCD in Brachypodium proceeds more rapidly in lateral cells. Remarkably, Brachypodium mesocarp PCD is not coordinated with endosperm development. Phylogenetic analysis suggests that barley and wheat possess more vacuolar processing enzymes that drive nucellar PCD compared to Brachypodium and rice. Our expression analysis highlighted putative grain-specific PCD proteases in Brachypodium. Combined with existing knowledge on grain PCD, our study suggests that the rate of nucellar PCD moderates grain size and that the pattern of mesocarp PCD influences grain shape.

Vacuolar processing enzymes in the plant life cycle

Vacuolar processing enzyme (VPE) is a cysteine-type endopeptidase that has a substrate-specificity for asparagine or aspartic acid residues and cleaves peptide bonds at their carboxyl-terminal side. Various vacuolar proteins are synthesized as larger proprotein precursors, and VPE is an important initiator of maturation and activation of these proteins. It mediates programmed cell death (PCD) by provoking vacuolar rupture and initiating the proteolytic cascade leading to PCD. Vacuolar processing enzyme also possesses a peptide ligation activity, which is responsible for producing cyclic peptides in several plant species. These unique functions of VPE support developmental and environmental responses in plants. The number of VPE homologues is higher in angiosperm species, indicating that there has been differentiation and specialization of VPE function over the course of evolution. Angiosperm VPEs are separated into two major types: the γ-type VPEs, which are expressed mainly in vegetative organs, and the β-type VPEs, whose expression occurs mainly in storage organs; in eudicots, the δ-type VPEs are further separated within γ-type VPEs. This review also considers the importance of processing and peptide ligation by VPE in vacuolar protein maturation.

Molecular cloning and functional characterization of a seed-specific VvβVPE gene promoter from Vitis vinifera

The grapevine VvβVPE promoter is specifically expressed in the seed. The - 1306~- 1045 bp core region restricts expression in other tissues and organs. Vacuolar processing enzyme (VPE) is a cysteine proteinase regulating vacuolar protein maturation and executing programmed cell death (PCD) in plants. Vitis vinifera (Vv)βVPE is a β-type VPE showing seed-specific expression that processes seed proteins during ovule development. However, the regulation of the seed-specific gene expression is far from understood. In this study, we characterize VvβVPE promoter (pVvβVPE) from 12 seeded and seedless grape genotypes. 94.56% of the pVvβVPE coding sequence is consistent. Two βVPE promoters were constructed and transformed into Arabidopsis thaliana via β-glucuronidase (GUS) fused expression vectors, using cv. Pinot Noir and cv. Thompson as seed and seedless candidates. GUS staining in different tissues and organs revealed that VvβVPE expresses specifically in the embryo, including the cotyledon, hypocotyl and suspensor, but not in the leaf, stem, root or flowers of the seedling. Using promoter deletion analysis, we created four incomplete VvβVPE promoters and found each pVvβVPE deletion could drive GUS gene to express in seeds. Interestingly, seed specificity disappeared when the promoter missed the core - 1306~- 1045 bp region. All deletion promoters presenting various quantified GUS activities indicate that the region - 1704~- 1306 bp inhibits, and the region - 705~- 861 bp promotes gene expression of VvβVPE. Our results demonstrate that pVvβVPE is a seed-specific promoter in both seeded and seedless grapes. Moreover, the core region of pVvβVPE (- 1306~- 1045 bp) is the key one responsible for seed-specific expression.

Expression of IbVPE1 from sweet potato in Arabidopsis affects leaf development, flowering time and chlorophyll catabolism

BACKGROUND

Since their discovery, vacuolar processing enzymes (VPEs) have consistently been investigated as programmed cell death (PCD) initiators and participants in plant development and responses to biotic or abiotic stresses, in part due to similarities with the apoptosis regulator caspase-1. However, recent studies show additional functions of VPE in tomatoes, specifically in sucrose accumulation and fruit ripening.

RESULTS

Herein, we evaluated the functions of VPE from sweetpotato, initially in expression pattern analyses of IbVPE1 during development and senescence. Subsequently, we identified physiological functions by overexpressing IbVPE1 in Arabidopsis thaliana, and showed reduced leaf sizes and numbers and early flowering, and elucidated the underlying molecular mechanisms.

CONCLUSIONS

The present data demonstrate functions of the VPE gene family in development and senescence and in regulation of flowering times, leaf sizes and numbers, and senescence phenotypes in Arabidopsis thaliana.

Plant Vacuolar Processing Enzymes

Plant proteomes contain hundreds of proteases divided into different families based on evolutionary and functional relationship. In particular, plant cysteine proteases of the C1 (papain-like) and C13 (legumain-like) families play key roles in many physiological processes. The legumain-like proteases, also called vacuolar processing enzymes (VPEs), perform a multifunctional role in different plant organs and during different stages of plant development and death. VPEs are similar to animal caspases, and although caspase activity was identified in plants almost 40 years ago, there still remains much research to be done to gain a complete understanding of their various roles and functions in plants. Here we not only summarize the current existing knowledge of plant VPEs, including recent developments in the field, but also highlight the future prospective areas to be investigated to obtain a more detailed understanding of the role of VPEs in plants.

Vacuolar processing enzyme (VvβVPE) from Vitis vinifera, processes seed proteins during ovule development, and accelerates seed germination in VvβVPE heterologously over-expressed Arabidopsis

Vacuolar processing enzymes (VPEs), belonging to cysteine protease, are responsible for processing seed protein during maturation. Stenospermocarpic grapes occur self-abortion in fertilized embryos during the ovule development, which affects the formation of matured seed proteins. However, little is known about VPE functions in ovule self-defeating. Here, we investigated the role of one seed-type VPE gene, VvβVPE. Sequence analysis showed that all ORFs (Open reading frames) of VvβVPE from 19 seed/seedless genotypes are highly conserved. At the transcriptional level, VvβVPE was specifically expressed during ovule development, with distinct expression patterns: it increased gradually in seeded grapes; while weakly expressed in seedless grapes. Whereas, at the translational level, 3 forms of VvβVPE were expressed during ovule development in seeded grape: precursor βVPE (pβVPE), intermediate βVPE (iβVPE) and finally, active mature βVPE (mβVPE). By contrast, in seedless grape, VvβVPE only exists as pβVPE at whole developmental stage of ovule. for confirming these expression patterns, 12 seeded/seedless genotypes were sampled and analyzed. Furthermore, VPE enzyme activity was increased in Arabidopsis overexpressing VvβVPE, leading to faster germination. Our study indicated that VvβVPE is essential for grapevine ovule maturation through various forms and is involved in seed germination.

Vacuolar processing enzymes, AmVPE1 and AmVPE2, as potential executors of ethylene regulated programmed cell death in the lace plant (Aponogeton madagascariensis)

Perforation formation in Aponogeton madagascariensis (Mirb.) H.Bruggen (lace plant) is an excellent model for studying developmentally regulated programmed cell death (PCD). In this study, we isolated and identified two lace plant vacuolar processing enzymes (VPEs) and investigated their involvement in PCD and throughout leaf development. Lace plant VPE transcript levels were determined during seven different stages of leaf development. PCD and non-PCD cells from “window” stage leaves (in which perforations are forming) were separated through laser-capture microscopy and their transcript levels were also determined. VPE activity was also studied between the cell types, through a VPE activity-based probe JOPD1. Additionally, VPE transcript levels were studied in plants treated with an ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG). The two isolated VPEs, AmVPE1 and AmVPE2, are vegetative type VPEs. AmVPE1 had higher transcript levels during a pre-perforation developmental stage, immediately prior to visible signs of PCD. AmVPE2 transcript levels were higher later during window and late window stages. Both VPEs had higher transcript and activity levels in PCD compared with the non-PCD cells. AVG treatment inhibited PCD and associated increases in VPE transcript levels. Our results suggested that VPEs are involved in the execution of the ethylene-related PCD in the lace plant.

Vacuolar processing enzyme activates programmed cell death in the apical meristem inducing loss of apical dominance

The potato (Solanum tuberosum L.) tuber is a swollen underground stem that can sprout in an apical dominance (AD) pattern. Bromoethane (BE) induces loss of AD and the accumulation of vegetative vacuolar processing enzyme (S. tuberosum vacuolar processing enzyme [StVPE]) in the tuber apical meristem (TAM). Vacuolar processing enzyme activity, induced by BE, is followed by programmed cell death in the TAM. In this study, we found that the mature StVPE1 (mVPE) protein exhibits specific activity for caspase 1, but not caspase 3 substrates. Optimal activity of mVPE was achieved at acidic pH, consistent with localization of StVPE1 to the vacuole, at the edge of the TAM. Downregulation of StVPE1 by RNA interference resulted in reduced stem branching and retained AD in tubers treated with BE. Overexpression of StVPE1 fused to green fluorescent protein showed enhanced stem branching after BE treatment. Our data suggest that, following stress, induction of StVPE1 in the TAM induces AD loss and stem branching.

Gene Cloning, Expression and Enzyme Activity of Vitis vinifera Vacuolar Processing Enzymes (VvVPEs)

Vacuolar processing enzymes (VPEs) have received considerable attention due to their caspase-1-like activity and ability to regulate programmed cell death (PCD), which plays an essential role in the development of stenospermocarpic seedless grapes ovules. To characterize VPEs and the relationship between stenospermocarpic grapes and the VPE gene family, we identified 3 Vitis vinifera VPE genes (VvβVPE, VvγVPE, and VvδVPE) from the PN40024 grape genome and cloned the full-length complementary DNAs (cDNAs) from the ‘Vitis vinifera cv. Pinot Noir’ and ‘Vitis vinifera cv. Thompson Seedless’ varietals. Each of the VPEs contained a typical catalytic dyad [His (177), Cys (219)] and substrate binding pocket [Arg (112), Arg (389), Ser (395)], except that Ser (395) in the VvγVPE protein sequence was replaced with alanine. Phylogenetic analysis of 4 Arabidopsis thaliana and 6 Vitis vinifera VPEs revealed that the 10 VPEs form 3 major branches. Furthermore, the 6 grapevine VPEs share a similar gene structure, with 9 exons and 8 introns. The 6 grapevine VPEs are located on 3 different chromosomes. We also tested the enzymatic activity of recombinant VPEs expressed in the Pichia Pastoris expression system and found that the VvVPEs exhibit cysteine peptidase activity. Tissue-specific expression analysis showed that VvδVPE is only expressed in flowers, buds and ovules, that VvγVPE is expressed in various tissues, and that VvβVPE was expressed in roots, flowers, buds and ovules. The results of quantitative real-time PCR (qRT-PCR) suggested that VvβVPE in seeded grapes increased significantly at 30 days after full-bloom (DAF), close to the timing of endosperm abortion at 32 DAF. These results suggested that VvβVPE is related to ovule abortion in seedless grapes. Our experiments provide a new perspective for understanding the mechanism of stenospermocarpic seedlessness and represent a useful reference for the further study of VPEs.

Bcl-2△21 and Ac-DEVD-CHO Inhibit Death of Wheat Microspores

Microspore cell death and low green plant production efficiency are an integral obstacle in the development of doubled haploid production in wheat. The aim of the current study was to determine the effect of anti-apoptotic recombinant human B-cell lymphoma-2 (Bcl-2△21) and caspase-3-inhibitor (Ac-DEVD-CHO) in microspore cell death in bread wheat cultivars AC Fielder and AC Andrew. Induction medium containing Bcl-2△21 and Ac-DEVD-CHO yielded a significantly higher number of viable microspores, embryo-like structures and total green plants in wheat cultivars AC Fielder and AC Andrew. Total peroxidase activity was lower in Bcl-2△21 treated microspore cultures at 96 h of treatment compared to control and Ac-DEVD-CHO. Electron paramagnetic resonance study of total microspore protein showed a different scavenging activity for Bcl-2△21 and Ac-DEVD-CHO. Bcl-2△21 scavenged approximately 50% hydroxyl radical (HO^•) formed, whereas Ac-DEVD-CHO scavenged approximately 20% of HO^•. Conversely, reduced caspase-3-like activities were detected in the presence of Bcl-2△21 and Ac-DEVD-CHO, supporting the involvement of Bcl-2△21 and Ac-DEVD-CHO in increasing microspore viability by reducing oxidative stress and caspase-3-like activity. Our results indicate that Bcl-2△21 and Ac-DEVD-CHO protects cells from cell death following different pathways. Bcl-2△21 prevents cell damage by detoxifying HO^• and suppressing caspase-3-like activity, while Ac-DEVD-CHO inhibits the cell death pathways by modulating caspase-like activity.

Characterization of the legumains encoded by the genome of Theobroma cacao L

Legumains are cysteine proteases related to plant development, protein degradation, programmed cell death, and defense against pathogens. In this study, we have identified and characterized three legumains encoded by Theobroma cacao genome through in silico analyses, three-dimensional modeling, genetic expression pattern in different tissues and as a response to the inoculation of Moniliophthora perniciosa fungus. The three proteins were named TcLEG3, TcLEG6, and TcLEG9. Histidine and cysteine residue which are part of the catalytic site were conserved among the proteins, and they remained parallel in the loop region in the 3D modeling. Three-dimensional modeling showed that the propeptide, which is located in the terminal C region of legumains blocks the catalytic cleft. Comparing dendrogram data with the relative expression analysis, indicated that TcLEG3 is related to the seed legumain group, TcLEG6 is related with the group of embryogenesis activities, and protein TcLEG9, with processes regarding the vegetative group. Furthermore, the expression analyses proposes a significant role for the three legumains during the development of Theobroma cacao and in its interaction with M. perniciosa.

Structure and function of legumain in health and disease

The last years have seen a steady increase in our understanding of legumain biology that is driven from two largely uncoupled research arenas, the mammalian and the plant legumain field. Research on legumain, which is also referred to as asparaginyl endopeptidase (AEP) or vacuolar processing enzyme (VPE), is slivered, however. Here we summarise recent important findings and put them into a common perspective. Legumain is usually associated with its cysteine endopeptidase activity in lysosomes where it contributes to antigen processing for class II MHC presentation. However, newly recognized functions disperse previously assumed boundaries with respect to their cellular compartmentalisation and enzymatic activities. Legumain is also found extracellularly and even translocates to the cytosol and the nucleus, with seemingly incompatible pH and redox potential. These different milieus translate into changes of legumain's molecular properties, including its (auto-)activation, conformational stability and enzymatic functions. Contrasting its endopeptidase activity, legumain can develop a carboxypeptidase activity which remains stable at neutral pH. Moreover, legumain features a peptide ligase activity, with intriguing mechanistic peculiarities in plant and human isoforms. In pathological settings, such as cancer or Alzheimer's disease, the proper association of legumain activities with the corresponding cellular compartments is breached. Legumain's increasingly recognized physiological and pathological roles also indicate future research opportunities in this vibrant field.

Peptide macrocyclization by a bifunctional endoprotease

Proteases usually cleave peptides, but under some conditions, they can ligate them. Seeds of the common sunflower contain the 14-residue, backbone-macrocyclic peptide sunflower trypsin inhibitor 1 (SFTI-1) whose maturation from its precursor has a genetic requirement for asparaginyl endopeptidase (AEP). To provide more direct evidence, we developed an in situ assay and used (18)O-water to demonstrate that SFTI-1 is excised and simultaneously macrocyclized from its linear precursor. The reaction is inefficient in situ, but a newfound breakdown pathway can mask this inefficiency by reducing the internal disulfide bridge of any acyclic-SFTI to thiols before degrading it. To confirm AEP can directly perform the excision/ligation, we produced several recombinant plant AEPs in E. coli, and one from jack bean could catalyze both a typical cleavage reaction and cleavage-dependent, intramolecular transpeptidation to create SFTI-1. We propose that the evolution of ligating endoproteases enables plants like sunflower and jack bean to stabilize bioactive peptides.

Vacuolar processing enzyme in plant programmed cell death

Vacuolar processing enzyme (VPE) is a cysteine proteinase originally identified as the proteinase responsible for the maturation and activation of vacuolar proteins in plants, and it is known to be an ortholog of animal asparaginyl endopeptidase (AEP/VPE/legumain). VPE has been shown to exhibit enzymatic properties similar to that of caspase 1, which is a cysteine protease that mediates the programmed cell death (PCD) pathway in animals. Although there is limited sequence identity between VPE and caspase 1, their predicted three-dimensional structures revealed that the essential amino-acid residues for these enzymes form similar pockets for the substrate peptide YVAD. In contrast to the cytosolic localization of caspases, VPE is localized in vacuoles. VPE provokes vacuolar rupture, initiating the proteolytic cascade leading to PCD in the plant immune response. It has become apparent that the VPE-dependent PCD pathway is involved not only in the immune response, but also in the responses to a variety of stress inducers and in the development of various tissues. This review summarizes the current knowledge on the contribution of VPE to plant PCD and its role in vacuole-mediated cell death, and it also compares VPE with the animal cell death executor caspase 1.

Uncovering legumain genes in rice

Legumains are Asn specific cysteine proteases physiologically related to the biosynthesis of vacuolar components, degradation of storage proteins and programmed cell death. The present work identifies and characterizes the genic family of legumains in rice (Oryza sativa), which comprises five different loci. Rice legumains (OsaLegs) were ubiquitously detected in all plant tissues analyzed. However, phylogenetic analyses and gene expression studies demonstrated greater association of OsaLeg2 and OsaLeg3 to seed-related legumains, whereas OsaLeg1, 4 and 5 would act as vegetative-related proteases. Additionally, OsaLeg1 mRNA is strongly induced in senescent leaves. All rice legumain genes respond in different ways to environmental conditions such as wounding, salt and abscisic acid treatments. Mainly, wounding is capable of inducing all the four expressed genes OsaLeg1, 2, 3 and 4. Alternative splicing isoforms, with potential to generate pre-activated OsaLeg1 and OsaLeg2 nonvacuolar enzymes under different environmental situations were also observed.

Differential expression of cysteine peptidase genes in the inner integument and endosperm of developing seeds of Jatropha curcas L. (Euphorbiaceae)

In several plant tissues, programmed cell death (PCD) is mediated by the combined action of cysteine peptidases, namely KDEL-tailed cysteine peptidases (KDEL-CysEP) and vacuolar processing enzymes (VPE). Here, we performed a search of the draft genome of Jatropha curcas L. (Euphorbiaceae) and identified 2 genes for KDEL-CysEP (Jc-CysEP1 and Jc-CysEP2) and 3 genes for VPE (Jc-βVPE, Jc-γVPE and Jc-δVPE) and determined the expression patterns of these genes by RT-qPCR in integument and cellular endosperm of seeds collected at seven different developmental stages. We were able to demonstrate that the expression of Jc-CysEP1, Jc-CysEP2, Jc-βVPE and Jc-γVPE proceeded rapidly from Stage IV, with Jc-CysEP2 displaying the highest relative expression; expression of Jc-δVPE could not be detected in any of the tissues/developmental stages analyzed. Additionally, we showed that the expression pattern of these peptidases correlates with anatomical changes in integument and cellular endosperm, thus suggesting a role for both classes of peptidases in PCD and in protein processing, both of which occur simultaneously in each of these tissues.

Morphological and biochemical characterization of Erwinia amylovora-induced hypersensitive cell death in apple leaves

In attached apple leaves, spot-inoculated with Erwinia amylovora, the phenotypic appearance of the hypersensitive response (HR) and the participation of ethylene, reactive oxygen species (ROS) and of vacuolar processing enzyme (VPE) (a plant caspase-1-like protease) were analysed. The HR in both the resistant and susceptible genotypes expressed a similar pattern of distinguishable micro HR lesions that progressed into confined macro HR lesions. The HR symptoms in apple were compared to those in non-host tobacco. The morphology of dead cells (protoplast shrinkage and retraction from cell wall) in apple leaves resembled necrotic programmed cell death (PCD). Lesion formation in both cv. Free Redstar (resistant) and cv. Idared (highly susceptible) was preceded by ROS accumulation and elevation of ethylene levels. Treatment of infected leaves with an inhibitor of ethylene synthesis led to a decrease of ethylene emission and suppression of lesion development in both cultivars. In the resistant but not in the susceptible apple cultivar an early and late increase in VPE gene expression was detected. This suggests that VPE might be an underlying component of the response to E. amylovora in resistant apple cultivars. The findings show that in the studied pathosystem the cell death during the HR proceeds through a signal transduction cascade in which ROS, ethylene and VPE pathways play a role.

Production of plant proteases in vivo and in vitro--a review

In the latest two decades, the interest received by plant proteases has increased significantly. Plant enzymes such as proteases are widely used in medicine and the food industry. Some proteases, like papain, bromelain and ficin are used in various processes such as brewing, meat softening, milk-clotting, cancer treatment, digestion and viral disorders. These enzymes can be obtained from their natural source or through in vitro cultures, in order to ensure a continuous source of plant enzymes. The focus of this review will be the production of plant proteases both in vivo and in vitro, with particular emphasis on the different types of commercially important plant proteases that have been isolated and characterized from naturally grown plants. In vitro approaches for the production of these proteases is also explored, focusing on the techniques that do not involve genetic transformation of the plants and the attempts that have been made in order to enhance the yield of the desired proteases.

The role of vacuolar processing enzymes in plant immunity

Proteases play important roles in plant innate immunity. In this mini-review, we describe the current view on the role of a plant protease, vacuolar processing enzyme (VPE), and the first identified plant caspase-1-like protein, in plant immunity. In the past several years, VPEs were determined to play important roles in various types of cell death in plants. Early studies demonstrated the identification of VPE as a vacuolar hydrolytic protein responsible for maturation of vacuolar proteins. Later, Nicotiana benthamiana VPE was reported to mediate virus-induced hypersensitive response by regulating membrane collapse. The ortholog of VPE in Arabidopsis is also suggested to be involved in both mycotoxin-induced cell death and developmental cell death. However, the role of VPE in elicitor-signaling is still unclear. Our recent studies demonstrated the involvement of VPE in elicitor signal transduction to induce stomatal closure and defense responses, including defense gene expression and hypersensitive cell death.

The role of vacuolar processing enzyme (VPE) from Nicotiana benthamiana in the elicitor-triggered hypersensitive response and stomatal closure

Elicitors/pathogen-associated molecular patterns (PAMPs) trigger the plant immune system, leading to rapid programmed cell death (hypersensitive response, HR) and stomatal closure. Previous reports have shown that the vacuolar processing enzyme (VPE), a cysteine proteinase responsible for the maturation of vacuolar proteins, has caspase-1-like activity and mediates TMV- and mycotoxin-induced cell death. The role of VPE from Nicotiana benthamiana in the response to three elicitors: bacterial harpin, fungal Nep1, and oomycete boehmerin, is described here. Single-silenced (NbVPE1a or NbVPE1b) and dual-silenced (NbVPE1a/1b) N. benthamiana plants were produced by virus-induced gene silencing. Although NbVPE silencing does not affect H(2)O(2) accumulation triggered by boehmerin, harpin, or Nep1, the HR is absent in NbVPE1a- and NbVPE1a/1b-silenced plants treated with harpin alone. However, NbVPE-silenced plants develop a normal HR after boehmerin and Nep1 treatment. These results suggest that harpin-triggered HR is VPE-dependent. Surprisingly, all gene-silenced plants show significantly impaired elicitor-induced stomatal closure and elicitor-promoted nitric oxide (NO) production in guard cells. Dual-silenced plants show increased elicitor-triggered AOS production in guard cells. The accumulation of transcripts associated with defence and cell redox is modified by VPE silencing in elicitor signalling. Overall, these results indicate that VPE from N. benthamiana functions not only in elicitor-induced HR, but also in elicitor-induced stomatal closure, suggesting that VPE may be involved in elicitor-triggered immunity.

Storage reserve accumulation in Arabidopsis: metabolic and developmental control of seed filling

In the life cycle of higher plants, seed development is a key process connecting two distinct sporophytic generations. Seed development can be divided into embryo morphogenesis and seed maturation. An essential metabolic function of maturing seeds is the deposition of storage compounds that are mobilised to fuel post-germinative seedling growth. Given the importance of seeds for food and animal feed and considering the tremendous interest in using seed storage products as sustainable industrial feedstocks to replace diminishing fossil reserves, understanding the metabolic and developmental control of seed filling constitutes a major focus of plant research. Arabidopsis thaliana is an oilseed species closely related to the agronomically important Brassica oilseed crops. The main storage compounds accumulated in seeds of A. thaliana consist of oil stored as triacylglycerols (TAGs) and seed storage proteins (SSPs). Extensive tools developed for the molecular dissection of A. thaliana development and metabolism together with analytical and cytological procedures adapted for very small seeds have led to a good description of the biochemical pathways producing storage compounds. In recent years, studies using these tools have shed new light on the intricate regulatory network controlling the seed maturation process. This network involves sugar and hormone signalling together with a set of developmentally regulated transcription factors. Although much remains to be elucidated, the framework of the regulatory system controlling seed filling is coming into focus.

What happened to plant caspases?

The extent of conservation in the programmed cell death pathways that are activated in species belonging to different kingdoms is not clear. Caspases are key components of animal apoptosis; caspase activities are detected in both animal and plant cells. Yet, while animals have caspase genes, plants do not have orthologous sequences in their genomes. It is 10 years since the first caspase activity was reported in plants, and there are now at least eight caspase activities that have been measured in plant extracts using caspase substrates. Various caspase inhibitors can block many forms of plant programmed cell death, suggesting that caspase-like activities are required for completion of the process. Since plant metacaspases do not have caspase activities, a major challenge is to identify the plant proteases that are responsible for the caspase-like activities and to understand how they relate, if at all, to animal caspases. The protease vacuolar processing enzyme, a legumain, is responsible for the cleavage of caspase-1 synthetic substrate in plant extracts. Saspase, a serine protease, cleaves caspase-8 and some caspase-6 synthetic substrates. Possible scenarios that could explain why plants have caspase activities without caspases are discussed.

Phosphorylation of the 12 S globulin cruciferin in wild-type and abi1-1 mutant Arabidopsis thaliana (thale cress) seeds

Cruciferin (a 12 S globulin) is the most abundant storage protein in the seeds of Arabidopsis thaliana (thale cress) and other crucifers, sharing structural similarity with the cupin superfamily of proteins. Cruciferin is synthesized as a precursor in the rough endoplasmic reticulum. Subunit assembly is accompanied by structural rearrangements involving proteolysis and disulfide-bond formation prior to deposition in protein storage vacuoles. The A. thaliana cv. Columbia genome contains four cruciferin loci, two of which, on the basis of cDNA analysis, give rise to three alternatively spliced variants. Using MS, we confirmed the presence of four variants encoded by genes At4g28520.1, At5g44120.3, At1g03880.1 and At1g3890.1 in A. thaliana seeds. Two-dimensional gel electrophoresis, along with immunological detection using anti-cruciferin antiserum and antibodies against phosphorylated amino acid residues, revealed that cruciferin was the major phosphorylated protein in Arabidopsis seeds and that polymorphism far exceeded that predicted on the basis of known isoforms. The latter may be attributed, at least in part, to phosphorylation site heterogeneity. A total of 20 phosphorylation sites, comprising nine serine, eight threonine and three tyrosine residues, were identified by MS. Most of these are located on the IE (interchain disulfide-containing) face of the globulin trimer, which is involved in hexamer formation. The implications of these findings for cruciferin processing, assembly and mobilization are discussed. In addition, the protein phosphatase 2C-impaired mutant, abi1-1, was found to exhibit increased levels of cruciferin phosphorylation, suggesting either that cruciferin may be an in vivo target for this enzyme or that abi1-1 regulates the protein kinase/phosphatase system required for cruciferin phosphorylation.

A cellular suicide strategy of plants: vacuole-mediated cell death

Programmed cell death (PCD) occurs in animals and plants under various stresses and during development. Recently, vacuolar processing enzyme (VPE) was identified as an executioner of plant PCD. VPE is a cysteine protease that cleaves a peptide bond at the C-terminal side of asparagine and aspartic acid. VPE exhibited enzymatic properties similar to that of a caspase, which is a cysteine protease that mediates the PCD pathway in animals, although there is limited sequence identity between the two enzymes. VPE and caspase-1 share several structural properties: the catalytic dyads and three amino acids forming the substrate pockets (Asp pocket) are conserved between VPE and caspase-1. In contrast to such similarities, subcellular localizations of these proteases are completely different from each other. VPE is localized in the vacuoles, while caspases are localized in the cytosol. VPE functions as a key molecule of plant PCD through disrupting the vacuole in pathogenesis and development. Cell death triggered by vacuolar collapse is unique to plants and has not been seen in animals. Plants might have evolved a VPE-mediated vacuolar system as a cellular suicide strategy.

Vacuolar processing enzyme: an executor of plant cell death

Apoptotic cell death in animals is regulated by cysteine proteinases called caspases. Recently, vacuolar processing enzyme (VPE) was identified as a plant caspase. VPE deficiency prevents cell death during hypersensitive response and cell death of limited cell layers at the early stage of embryogenesis. Because plants do not have macrophages, dying cells must degrade their materials by themselves. VPE plays an essential role in the regulation of the lytic system of plants during the processes of defense and development. VPE is localized in the vacuoles, unlike animal caspases, which are localized in the cytosol. Thus, plants might have evolved a regulated cellular suicide strategy that, unlike animal apoptosis, is mediated by VPE and the vacuoles.

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.

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.

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.

Identification, classification and expression pattern analysis of sugarcane cysteine proteinases

Cysteine proteases are peptidyl hydrolyses dependent on a cysteine residue at the active center. The physical and chemical properties of cysteine proteases have been extensively characterized, but their precise biological functions have not yet been completely understood, although it is known that they are involved in a number of events such as protein turnover, cancer, germination, programmed cell death and senescence. Protein sequences from different cysteine proteinases, classified as members of the E.C.3.4.22 sub-sub-class, were used to perform a T-BLAST-n search on the Brazilian Sugarcane Expressed Sequence Tags project (SUCEST) data bank. Sequence homology was found with 76 cluster sequences that corresponded to possible cysteine proteinases. The alignments of these SUCEST clusters with the sequence of cysteine proteinases of known origins provided important information about the classification and possible function of these sugarcane enzymes. Inferences about the expression pattern of each gene were made by direct correlation with the SUCEST cDNA libraries from which each cluster was derived. Since no previous reports of sugarcane cysteine proteinases genes exists, this study represents a first step in the study of new biochemical, physiological and biotechnological aspects of sugarcane cysteine proteases.

A slow maturation of a cysteine protease with a granulin domain in the vacuoles of senescing Arabidopsis leaves

Arabidopsis RD21 is a cysteine protease of the papain family. Unlike other members of the papain family, RD21 has a C-terminal extension sequence composed of two domains, a 2-kD proline-rich domain and a 10-kD domain homologous to animal epithelin/granulin family proteins. The RD21 protein was accumulated as 38- and 33-kD proteins in Arabidopsis leaves. An immunoblot showed that the 38-kD protein had the granulin domain, whereas the 33-kD protein did not. A pulse-chase experiment with Bright-Yellow 2 transformant cells expressing RD21 showed that RD21 was synthesized as a 57-kD precursor and was then slowly processed to make the 33-kD mature protein via the 38-kD intermediate. After a 12-h chase, the 38-kD intermediate was still detected in the cells. These results indicate that the N-terminal propeptide was first removed from the 57-kD precursor, and the C-terminal granulin domain was then slowly removed to yield the 33-kD mature protein. Subcellular fractionation of the Bright-Yellow 2 transformant showed that the intermediate and mature forms of RD21 were localized in the vacuoles. Under the acidic conditions of the vacuolar interior, the intermediate was found to be easily aggregated. The intermediate and the mature protein were accumulated in association with leaf senescence. Taken together, these results indicate that the intermediate of RD21 was accumulated in the vacuoles as an aggregate, and then slowly matured to make a soluble protease by removing the granulin domain during leaf senescence.

A new set of Arabidopsis expressed sequence tags from developing seeds. The metabolic pathway from carbohydrates to seed oil

Large-scale single-pass sequencing of cDNAs from different plants has provided an extensive reservoir for the cloning of genes, the evaluation of tissue-specific gene expression, markers for map-based cloning, and the annotation of genomic sequences. Although as of January 2000 GenBank contained over 220,000 entries of expressed sequence tags (ESTs) from plants, most publicly available plant ESTs are derived from vegetative tissues and relatively few ESTs are specifically derived from developing seeds. However, important morphogenetic processes are exclusively associated with seed and embryo development and the metabolism of seeds is tailored toward the accumulation of economically valuable storage compounds such as oil. Here we describe a new set of ESTs from Arabidopsis, which has been derived from 5- to 13-d-old immature seeds. Close to 28,000 cDNAs have been screened by DNA/DNA hybridization and approximately 10,500 new Arabidopsis ESTs have been generated and analyzed using different bioinformatics tools. Approximately 40% of the ESTs currently have no match in dbEST, suggesting many represent mRNAs derived from genes that are specifically expressed in seeds. Although these data can be mined with many different biological questions in mind, this study emphasizes the import of photosynthate into developing embryos, its conversion into seed oil, and the regulation of this pathway.

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.

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.

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.

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.