An extended AE-rich N-terminal trunk in secreted pineapple cystatin enhances inhibition of fruit bromelain and is posttranslationally removed during ripening

Crystal structure and interconversion of monomers and domain-swapped dimers of the walnut tree phytocystatin

Biotechnological applications of phytocystatins have garnered significant interest due to their potential applications in crop protection and improve crop resistance to abiotic stress factors. Cof1 and Wal1 are phytocystatins derived from Coffea arabica and Juglans regia, respectively. These plants hold significant economic value due to coffee's global demand and the walnut tree's production of valuable timber and widely consumed walnuts with culinary and nutritional benefits. The study involved the heterologous expression in E. coli Lemo 21(DE3), purification by immobilized metal ion affinity and size exclusion chromatography, and biophysical characterization of both phytocystatins, focusing on isolating and interconverting their monomers and dimers. The crystal structure of the domain-swapped dimer of Wal1 was determined revealing two domain-swapped dimers in the asymmetric unit, an arrangement reminiscent of the human cystatin C structure. Alphafold models of monomers and Alphafold-Multimer models of domain-swapped dimers of Cof1 and Wal1 were analyzed in the context of the crystal structure. The methodology and data presented here contribute to a deeper understanding of the oligomerization mechanisms of phytocystatins and their potential biotechnological applications in agriculture.

Identification of bromelain subfamily proteases encoded in the pineapple genome

Papain (aka C1A) family proteases, including bromelain enzymes, are widespread across the plant kingdom and play critical regulatory functions in protein turnover during development. The proteolytic activity exhibited by papain family proteases has led to their increased usage for a wide range of cosmetic, therapeutic, and medicinal purposes. Bromelain enzymes, or bromelains in short, are members of the papain family that are specific to the bromeliad plant family. The only major commercial extraction source of bromelain is pineapple. The importance of C1A family and bromelain subfamily proteases in pineapple development and their increasing economic importance led several researchers to utilize available genomic resources to identify protease-encoding genes in the pineapple genome. To date, studies are lacking in screening bromelain genes for targeted use in applied science studies. In addition, the bromelain genes coding for the enzymes present in commercially available bromelain products have not been identified and their evolutionary origin has remained unclear. Here, using the newly developed MD2 v2 pineapple genome, we aimed to identify bromelain-encoding genes and elucidate their evolutionary origin. Orthologous and phylogenetic analyses of all papain-family proteases encoded in the pineapple genome revealed a single orthogroup (189) and phylogenetic clade (XIII) containing the bromelain subfamily. Duplication mode and synteny analyses provided insight into the origin and expansion of the bromelain subfamily in pineapple. Proteomic analysis identified four bromelain enzymes present in two commercially available bromelain products derived from pineapple stem, corresponding to products of four putative bromelain genes. Gene expression analysis using publicly available transcriptome data showed that 31 papain-family genes identified in this study were up-regulated in specific tissues, including stem, fruit, and floral tissues. Some of these genes had higher expression in earlier developmental stages of different tissues. Similar expression patterns were identified by RT-qPCR analysis with leaf, stem, and fruit. Our results provide a strong foundation for future applicable studies on bromelain, such as transgenic approaches to increase bromelain content in pineapple, development of bromelain-producing bioreactors, and studies that aim to determine the medicinal and/or therapeutic viability of individual bromelain enzymes.

The bracteatus pineapple genome and domestication of clonally propagated crops

Domestication of clonally propagated crops such as pineapple from South America was hypothesized to be a 'one-step operation'. We sequenced the genome of Ananas comosus var. bracteatus CB5 and assembled 513 Mb into 25 chromosomes with 29,412 genes. Comparison of the genomes of CB5, F153 and MD2 elucidated the genomic basis of fiber production, color formation, sugar accumulation and fruit maturation. We also resequenced 89 Ananas genomes. Cultivars 'Smooth Cayenne' and 'Queen' exhibited ancient and recent admixture, while 'Singapore Spanish' supported a one-step operation of domestication. We identified 25 selective sweeps, including a strong sweep containing a pair of tandemly duplicated bromelain inhibitors. Four candidate genes for self-incompatibility were linked in F153, but were not functional in self-compatible CB5. Our findings support the coexistence of sexual recombination and a one-step operation in the domestication of clonally propagated crops. This work guides the exploration of sexual and asexual domestication trajectories in other clonally propagated crops.

Differential response of silencing HvIcy2 barley plants against Magnaporthe oryzae infection and light deprivation

BACKGROUND

Phytocystatins (PhyCys) act as endogenous regulators of cysteine proteases (CysProt) involved in various physiological processes. Besides, PhyCys are involved in plant reactions to abiotic stresses like drought or darkness and have been used as effective molecules against different pests and pathogens. The barley PhyCys-CysProt system is considered a model of protease-inhibitor regulation of protein turnover. Thirteen barley cystatins (HvCPI-1 to HvCPI-13) have been previously identified and characterized. Among them HvCPI-2 has been shown to have a relevant role in plant responses to pathogens and pests, as well as in the plant response to drought.

RESULTS

The present work explores the multiple role of this barley PhyCys in response to both, biotic and abiotic stresses, focusing on the impact of silencing this gene. HvIcy-2 silencing lines behave differentially against the phytopathogenic fungus Magnaporthe oryzae and a light deprivation treatment. The induced expression of HvIcy-2 by the fungal stress correlated to a higher susceptibility of silencing HvIcy-2 plants. In contrast, a reduction in the expression of HvIcy-2 and in the cathepsin-L and -B like activities in the silencing HvIcy-2 plants was not accompanied by apparent phenotypical differences with control plants in response to light deprivation.

CONCLUSION

These results highlight the specificity of PhyCys in the responses to diverse external prompts as well as the complexity of the regulatory events leading to the response to a particular stress. The mechanism of regulation of these stress responses seems to be focused in maintaining the balance of CysProt and PhyCys levels, which is crucial for the modulation of physiological processes induced by biotic or abiotic stresses.

Identification of a type II cystatin in Fragaria chiloensis: A proteinase inhibitor differentially regulated during achene development and in response to biotic stress-related stimuli

The equilibrium between protein synthesis and degradation is key to maintaining efficiency in different physiological processes. The proteinase inhibitor cystatin regulates protease activities in different developmental and physiological contexts. Here we describe for the first time the identification and the biological function of the cysteine protease inhibitor cystatin of Fragaria chiloensis, FchCYS1. Based on primary sequence and 3D-structural homology modelling, FchCYS1 is a type II phytocystatin with high identity to other cystatins of the Fragaria genus. Both the papain-like and the legumain-like protease inhibitory domains are indeed functional, based on in vitro assays performed with Escherichia coli protein extracts containing recombinant FchCYS1. FchCYS1 is differentially-expressed in achenes of F. chiloensis fruits, with highest expression as the fruit reaches the ripened stage, suggesting a role in preventing degradation of storage proteins that will nourish the embryo during seed germination. Furthermore, FchCYS1 responds transcriptionally to the application of salicylic acid and to mechanical injury, strongly suggesting that FchCYS1 could be involved in the response against pathogen attack. Overall these results point to a role for FchCYS1 in diverse physiological processes in F. chiloensis.

Molecular Cloning, Recombinant Expression and Antifungal Activity of BnCPI, a Cystatin in Ramie (Boehmeria nivea L.)

Phytocystatins play multiple roles in plant growth, development and resistance to pests and other environmental stresses. A ramie (Boehmeria nivea L.) phytocystatin gene, designated as BnCPI, was isolated from a ramie cDNA library and its full-length cDNA was obtained by rapid amplification of cDNA ends (RACE). The full-length cDNA sequence (691 bp) consisted of a 303 bp open reading frame (ORF) encoding a protein of 100 amino acids with deduced molecular mass of 11.06 kDa and a theoretical isoelectric point (pI) of 6.0. The alignment of genome DNA (accession No. MF153097) and cDNA sequences of BnCPI showed that an intron (~104 bp) exists in the coding region. The BnCPI protein contains most of the highly conserved blocks including Gly⁵-Gly⁶ at the N-terminal, the reactive site motif QxVxG (Q⁴⁹V⁵⁰V⁵¹S⁵²G⁵³), the L⁷⁹-W⁸⁰ block and the [LVI]-[AGT]-[RKE]-[FY]-[AS]-[VI]-x-[EDQV]-[HYFQ]-N (L²²G²³R²⁴ F²⁵A²⁶V²⁷ D²⁸D²⁹H³⁰ N³¹) block that is common among plant cystatins. BLAST analysis indicated that BnCPI is similar to cystatins from Glycine max (77%), Glycine soja (76%), Hevea brasiliensis (75%) and Ricinus communis (75%). The BnCPI was subcloned into expression vector pSmart-I and then overexpressed in Escherichia coli BL21 (DE3) as a His-tagged recombinant protein. The purified reBnCPI has a molecular mass of 11.4 kDa determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS–PAGE). Purified reBnCPI can efficiently inhibit the protease activity of papain and ficin toward BANA (Nα-benzoyl-L-arginine-2-naphthyamide), as well as the mycelium growth of some important plant pathogenic fungi. The data further contribute to our understanding of the molecular functions of BnCPI.

Structural and functional characterization of the triticale (x Triticosecale Wittm.) phytocystatin TrcC-8 and its dimerization-dependent inhibitory activity

Phytocystatins are a group of proteins with significant potential to regulate activities of cysteine proteinases of native and pest/pathogen origins. The two-domain triticale (x Triticosecale Wittm.) phytocystatin TrcC-8 was characterized in this study. This protein belongs to the second group of phytocystatins and contains all the conserved sequences and motifs as well as both N-terminal (CY) and C-terminal (CY-L) domains that are characteristic of phytocystatins with the C-terminal extension. We demonstrated that TrcC-8 forms stable dimers with a significantly reduced inhibitory activity against papain compared to the activity of monomers, indicating the regulatory nature of the oligomerization. Moreover, according to our research, only the N-terminal domain possesses the ability to form dimers, indicating that this part of TrcC-8 is involved in the dimerization of the full-length protein. Homology modelling of TrcC-8 strongly suggests distinct specificities for the CY and CY-L domains, confirmed in experiments with inhibition of the papain. Our results suggest that the CY domain of TrcC-8 may, although markedly weakly and suboptimally, interact with papain in an analogous mode to tarocystatin, while the CY-L domain of TrcC-8 has distinct specificity than tarocystatin.

Molecular Characterization and Expression Profiling of Brachypodium distachyon L. Cystatin Genes Reveal High Evolutionary Conservation and Functional Divergence in Response to Abiotic Stress

Cystatin is a class of proteins mainly involved in cysteine protease inhibition and plant growth and development, as well as tolerance under various abiotic stresses. In this study, we performed the first comprehensive analysis of the molecular characterization and expression profiling in response to various abiotic stresses of the cystatin gene family in Brachypodium distachyon, a novel model plant for Triticum species with huge genomes. Comprehensive searches of the Brachypodium genome database identified 25 B. distachyon cystatin (BdC) genes that are distributed unevenly on chromosomes; of these, nine and two were involved in tandem and segmental duplication events, respectively. All BdC genes had similar exon/intron structural organization, with three conserved motifs similar to those from other plant species, indicating their high evolutionary conservation. Expression profiling of 10 typical BdC genes revealed ubiquitous expression in different organs at varying expression levels. BdC gene expression in seedling leaves was particularly highly induced by various abiotic stresses, including the plant hormone abscisic acid and various environmental cues (cold, H₂O₂, CdCl₂, salt, and drought). Interestingly, most BdC genes were significantly upregulated under multiple abiotic stresses, including BdC15 under all stresses, BdC7-2 and BdC10 under five stresses, and BdC7-1, BdC2-1, BdC14, and BdC12 under four stresses. The putative metabolic pathways of cytastin genes in response to various abiotic stresses mainly involve the aberrant protein degradation pathway and reactive oxygen species (ROS)-triggered programmed cell death signaling pathways. These observations provide a better understanding of the structural and functional characteristics of the plant cystatin gene family.

Defense response in non-genomic model species: methyl jasmonate exposure reveals the passion fruit leaves' ability to assemble a cocktail of functionally diversified Kunitz-type trypsin inhibitors and recruit two of them against papain

Multiplicity of protease inhibitors induced by predators may increase the understanding of a plant's intelligent behavior toward environmental challenges. Information about defense mechanisms of non-genomic model plant passion fruit (Passiflora edulis Sims) in response to predator attack is still limited. Here, via biochemical approaches, we showed its flexibility to build-up a broad repertoire of potent Kunitz-type trypsin inhibitors (KTIs) in response to methyl jasmonate. Seven inhibitors (20-25 kDa) were purified from exposed leaves by chromatographic techniques. Interestingly, the KTIs possessed truncated Kunitz motif in their N-terminus and some of them also presented non-consensus residues. Gelatin-Native-PAGE established multiple isoforms for each inhibitor. Significant differences regarding inhibitors' activity toward trypsin and chymotrypsin were observed, indicating functional polymorphism. Despite its rarity, two of them also inhibited papain, and such bifunctionality suggests a recruiting process onto another mechanistic class of target protease (cysteine-type). All inhibitors acted strongly on midgut proteases from sugarcane borer, Diatraea saccharalis (a lepidopteran insect) while in vivo assays supported their insecticide properties. Moreover, the bifunctional inhibitors displayed activity toward midgut proteases from cowpea weevil, Callosobruchus maculatus (a coleopteran insect). Unexpectedly, all inhibitors were highly effective against midgut proteases from Aedes aegypti a dipteran insect (vector of neglected tropical diseases) opening new avenues for plant-derived PIs for vector control-oriented research. Our results reflect the KTIs' complexities in passion fruit which could be wisely exploited by influencing plant defense conditions. Therefore, the potential of passion fruit as source of bioactive compounds with diversified biotechnological application was strengthened.

C1A cysteine protease-cystatin interactions in leaf senescence

Senescence-associated proteolysis in plants is a crucial process to relocalize nutrients from leaves to growing or storage tissues. The massive net degradation of proteins involves broad metabolic networks, different subcellular compartments, and several types of proteases and regulators. C1A cysteine proteases, grouped as cathepsin L-, B-, H-, and F-like according to their gene structures and phylogenetic relationships, are the most abundant enzymes responsible for the proteolytic activity during leaf senescence. Besides, cystatins as specific modulators of C1A peptidase activities exert a complex regulatory role in this physiological process. This overview article covers the most recent information on C1A proteases in leaf senescence in different plant species. Particularly, it is focussed on barley, as the unique species where the whole gene family members of C1A cysteine proteases and cystatins have been analysed.

Functional properties of a cysteine proteinase from pineapple fruit with improved resistance to fungal pathogens in Arabidopsis thaliana

In plant cells, many cysteine proteinases (CPs) are synthesized as precursors in the endoplasmic reticulum, and then are subject to post-translational modifications to form the active mature proteinases. They participate in various cellular and physiological functions. Here, AcCP2, a CP from pineapple fruit (Ananas comosus L.) belonging to the C1A subfamily is analyzed based on the molecular modeling and homology alignment. Transcripts of AcCP2 can be detected in the different parts of fruits (particularly outer sarcocarps), and gradually increased during fruit development until maturity. To analyze the substrate specificity of AcCP2, the recombinant protein was overexpressed and purified from Pichia pastoris. The precursor of purified AcCP2 can be processed to a 25 kDa active form after acid treatment (pH 4.3). Its optimum proteolytic activity to Bz-Phe-Val-Arg-NH-Mec is at neutral pH. In addition, the overexpression of AcCP2 gene in Arabidopsis thaliana can improve the resistance to fungal pathogen of Botrytis cinerea. These data indicate that AcCP2 is a multifunctional proteinase, and its expression could cause fruit developmental characteristics of pineapple and resistance responses in transgenic Arabidopsis plants.

The expression patterns of bromelain and AcCYS1 correlate with blackheart resistance in pineapple fruits submitted to postharvest chilling stress

Blackheart is a physiological disorder induced by postharvest chilling storage during pineapple fruit export shipping. The aim of this study was to check the involvement of bromelain, the cysteine protease protein family abundantly present in pineapple fruits, and AcCYS1, an endogenous inhibitor of bromelain, in the development of blackheart. For this we checked the response to postharvest chilling treatment of two pineapple varieties (MD2 and Smooth Cayenne) differing in their resistance to blackheart. Quantitative RT-PCR analyses showed that postharvest chilling treatment induced a down-regulation of bromelain transcript accumulation in both varieties with the most dramatic drop in the resistant variety. Regarding AcCYS1 transcript accumulation, the varieties showed opposite trends with an up-regulation in the case of the resistant variety and a down-regulation in the susceptible one. Taken together our results suggest that the control of bromelain and AcCYS1 expression levels directly correlates to the resistance to blackheart development in pineapple fruits.

In vivo inhibition of cysteine proteases provides evidence for the involvement of 'senescence-associated vacuoles' in chloroplast protein degradation during dark-induced senescence of tobacco leaves

Breakdown of leaf proteins, particularly chloroplast proteins, is a massive process in senescing leaves. In spite of its importance in internal N recycling, the mechanism(s) and the enzymes involved are largely unknown. Senescence-associated vacuoles (SAVs) are small, acidic vacuoles with high cysteine peptidase activity. Chloroplast-targeted proteins re-localize to SAVs during senescence, suggesting that SAVs might be involved in chloroplast protein degradation. SAVs were undetectable in mature, non-senescent tobacco leaves. Their abundance, visualized either with the acidotropic marker Lysotracker Red or by green fluorescent protein (GFP) fluorescence in a line expressing the senescence-associated cysteine protease SAG12 fused to GFP, increased during senescence induction in darkness, and peaked after 2-4 d, when chloroplast dismantling was most intense. Increased abundance of SAVs correlated with higher levels of SAG12 mRNA. Activity labelling with a biotinylated derivative of the cysteine protease inhibitor E-64 was used to detect active cysteine proteases. The two apparently most abundant cysteine proteases of senescing leaves, of 40kDa and 33kDa were detected in isolated SAVs. Rubisco degradation in isolated SAVs was completely blocked by E-64. Treatment of leaf disks with E-64 in vivo substantially reduced degradation of Rubisco and leaf proteins. Overall, these results indicate that SAVs contain most of the cysteine protease activity of senescing cells, and that SAV cysteine proteases are at least partly responsible for the degradation of stromal proteins of the chloroplast.

DREB2C acts as a transcriptional activator of the thermo tolerance-related phytocystatin 4 (AtCYS4) gene

Phytocystatins are proteinaceous inhibitors of cysteine proteases. They have been implicated in the regulation of plant protein turnover and in defense against pathogens and insects. Here, we have characterized an Arabidopsis phytocystatin family gene, Arabidopsis thaliana phytocystatin 4 (AtCYS4). AtCYS4 was induced by heat stress. The heat shock tolerance of AtCYS4-overexpressing transgenic plants was greater than that of wild-type and cys4 knock-down plants, as measured by fresh weight and root length. Although no heat shock elements were identified in the 5'-flanking region of the AtCYS4 gene, canonical ABA-responsive elements (ABREs) and dehydration-responsive elements (DREs) were found. Transient promoter activity measurements showed that AtCYS4 expression was up-regulated in unstressed protoplasts by co-expression of DRE-binding factor 2s (DREB2s), especially by DREB2C, but not by bZIP transcription factors that bind to ABREs (ABFs, ABI5 and AREBs). DREB2C bound to and activated transcription from the two DREs on the AtCYS4 promoter although some preference was observed for the GCCGAC DRE element over the ACCGAC element. AtCYS4 transcript and protein levels were elevated in transgenic DREB2C overexpression lines with corresponding decline of endogenous cysteine peptidase activity. We propose that AtCYS4 functions in thermotolerance under the control of the DREB2C cascade.

The group IV-A cyclic nucleotide-gated channels, CNGC19 and CNGC20, localize to the vacuole membrane in Arabidopsis thaliana

The cyclic nucleotide-gated channels, CNGC19 and CNGC20, are the sole members of the highly isolated evolutionary group IV-A in Arabidopsis plants. Prior studies have shown that the expression of both CNGC19 and CNGC20 genes are induced by salinity and biotic stress. In this report, CNGC19 and CNGC20 were determined to localize to the vacuolar membrane. Co-expression of CNGC19 and CNGC20 increased the efficiency of vacuolar localization. CNGC19 and CNGC20 are, therefore, vacuolar membrane channels that are hypothesized to mediate plant response to salinity and biotic stress.

Plant cyclic nucleotide-gated channels (CNGCs) are implicated in the uptake of both essential and toxic cations, Ca²⁺ signalling, and responses to biotic and abiotic stress. The 20 CNGC paralogues of Arabidopsis are divided into five evolutionary groups. Group IV-A is highly isolated and consists only of two closely spaced genes, CNGC19 and CNGC20. Prior studies have shown that both genes are induced by salinity and biotic stress. A unique feature of CNGC19 and CNGC20 is their long hydrophilic N-termini. To determine the subcellular locations of CNGC19 and CNGC20, partial and full-length fusions to GFP(S65T) were generated. Translational fusions of the N-termini of CNGC19 (residues 1–171) and CNGC20 (residues 1–200) to GFP(S65T) were targeted to punctate structures when transiently expressed in leaf protoplasts. In the case of CNGC20, but not CNGC19, the punctate structures were co-labelled with a marker for the Golgi. The full-length CNGC19-GFP fusion co-localized with markers for the vacuole membrane (αTIP- and γTIP-mCherry). Vacuole membrane labelling by the full-length CNGC20-GFP fusion was also observed, but the signal was weak and accompanied by numerous punctate signals that did not co-localize with αTIP- or γTIP-mCherry. These punctate structures diminished, and localization of full-length CNGC20-GFP to the vacuole increased, when it was co-expressed with the full-length CNGC19-mCherry. Vacuole membrane labelling was also detected in planta via immunoelectron microscopy using a CNGC20-antiserum on cryopreserved ultrathin sections of roots. We hypothesize that the role of group IV-A CNGCs is to mediate the movement of cations between the central vacuole and the cytosol in response to certain types of abiotic and biotic stress.

Solution structure of a phytocystatin from Ananas comosus and its molecular interaction with papain

The structure of a recombinant pineapple cystatin (AcCYS) was determined by NMR with the RMSD of backbone and heavy atoms of twenty lowest energy structures of 0.56 and 1.11 Å, respectively. It reveals an unstructured N-terminal extension and a compact inhibitory domain comprising a four-stranded antiparallel β-sheet wrapped around a central α-helix. The three structural motifs (G(45), Q(89)XVXG, and W(120)) putatively responsible for the interaction with papain-like proteases are located in one side of AcCYS. Significant chemical shift perturbations in two loop regions, residues 45 to 48 (GIYD) and residues 89 to 91 (QVV), of AcCYS strongly suggest their involvement in the binding to papain, consistent with studies on other members of the cystatin family. However, the highly conserved W120 appears not to be involved in the binding with papain as no chemical shift perturbation was observed. Chemical shift index analysis further indicates that the length of the α-helix is shortened upon association with papain. Collectively, our data suggest that AcCYS undergoes local secondary structural rearrangements when papain is brought into close contact. A molecular model of AcCYS/papain complex is proposed to illustrate the interaction between AcCYS and papain, indicating a complete blockade of the catalytic triad by AcCYS.

Passion fruit flowers: Kunitz trypsin inhibitors and cystatin differentially accumulate in developing buds and floral tissues

In order to better understand the physiological functions of protease inhibitors (PIs) the PI activity in buds and flower organs of passion fruit (Passiflora edulis Sims) was investigated. Trypsin and papain inhibitory activities were analyzed in soluble protein extracts from buds at different developmental stages and floral tissues in anthesis. These analyses identified high levels of inhibitory activity against both types of enzymes at all bud stages. Intriguingly, the inhibitory activity against both proteases differed remarkably in some floral tissues. While all organs tested were very effective against trypsin, only sepal and petal tissues exhibited strong inhibitory activity against papain. The sexual reproductive tissues (ovary, stigma-style and stamen) showed either significantly lower activity against papain or practically none. Gelatin-SDS-PAGE assay established that various trypsin inhibitors (TIs) homogenously accumulated in developing buds, although some were differentially present in floral organs. The N-terminal sequence analysis of purified inhibitors from stamen demonstrated they had homology to the Kunitz family of serine PIs. Western-blot analysis established presence of a ∼60 kDa cystatin, whose levels progressively increased during bud development. A positive correlation between this protein and strong papain inhibitory activity was observed in buds and floral tissues, except for the stigma-style. Differences in temporal and spatial accumulation of both types of PIs in passion fruit flowers are thus discussed in light of their potential roles in defense and development.

Protein disulfide isomerase-2 of Arabidopsis mediates protein folding and localizes to both the secretory pathway and nucleus, where it interacts with maternal effect embryo arrest factor

Protein disulfide isomerase (PDI) is a thiodisulfide oxidoreductase that catalyzes the formation, reduction and rearrangement of disulfide bonds in proteins of eukaryotes. The classical PDI has a signal peptide, two CXXC-containing thioredoxin catalytic sites (a,a'), two noncatalytic thioredoxin fold domains (b,b'), an acidic domain (c) and a C-terminal endoplasmic reticulum (ER) retention signal. Although PDI resides in the ER where it mediates the folding of nascent polypeptides of the secretory pathway, we recently showed that PDI5 of Arabidopsis thaliana chaperones and inhibits cysteine proteases during trafficking to vacuoles prior to programmed cell death of the endothelium in developing seeds. Here we describe Arabidopsis PDI2, which shares a primary structure similar to that of classical PDI. Recombinant PDI2 is imported into ER-derived microsomes and complements the E. coli protein-folding mutant, dsbA. PDI2 interacted with proteins in both the ER and nucleus, including ER-resident protein folding chaperone, BiP1, and nuclear embryo transcription factor, MEE8. The PDI2-MEE8 interaction was confirmed to occur in vitro and in vivo. Transient expression of PDI2-GFP fusions in mesophyll protoplasts resulted in labeling of the ER, nucleus and vacuole. PDI2 is expressed in multiple tissues, with relatively high expression in seeds and root tips. Immunoelectron microscopy with GFP- and PDI2-specific antisera on transgenic seeds (PDI2-GFP) and wild type roots demonstrated that PDI2 was found in the secretory pathway (ER, Golgi, vacuole, cell wall) and the nuclei. Our results indicate that PDI2 mediates protein folding in the ER and has new functional roles in the nucleus.

In silico analysis of sequential, structural and functional diversity of wheat cystatins and its implication in plant defense

Phytocystatins constitute a multigene family that regulates the activity of endogenous and/or exogenous cysteine proteinases. Cereal crops like wheat are continuously threatened by a multitude of pathogens, therefore cystatins offer to play a pivotal role in deciding the plant response. In order to study the need of having diverse specificities and activities of various cystatins, we conducted comparative analysis of six wheat cystatins (WCs) with twelve rice, seven barley, one sorghum and ten corn cystatin sequences employing different bioinformatics tools. The obtained results identified highly conserved signature sequences in all the cystatins considered. Several other motifs were also identified, based on which the sequences could be categorized into groups in congruence with the phylogenetic clustering. Homology modeling of WCs revealed 3D structural topology so well shared by other cystatins. Protein-protein interaction of WCs with papain supported the notion that functional diversity is a consequence of existing differences in amino acid residues in highly conserved as well as relatively less conserved motifs. Thus there is a significant conservation at the sequential and structural levels; however, concomitant variations maintain the functional diversity in this protein family, which constantly modulates itself to reciprocate the diversity while counteracting the cysteine proteinases.

Distinct expression patterns of two Arabidopsis phytocystatin genes, AtCYS1 and AtCYS2, during development and abiotic stresses

The phytocystatins of plants are members of the cystatin superfamily of proteins, which are potent inhibitors of cysteine proteases. The Arabidopsis genome encodes seven phytocystatin isoforms (AtCYSs) in two distantly related AtCYS gene clusters. We selected AtCYS1 and AtCYS2 as representatives for each cluster and then generated transgenic plants expressing the GUS reporter gene under the control of each gene promoter. These plants were used to examine AtCYS expression at various stages of plant development and in response to abiotic stresses. Histochemical analysis of AtCYS1 promoter- and AtCYS2 promoter-GUS transgenic plants revealed that these genes have similar but distinct spatial and temporal expression patterns during normal development. In particular, AtCYS1 was preferentially expressed in the vascular tissue of all organs, whereas AtCYS2 was expressed in trichomes and guard cells in young leaves, caps of roots, and in connecting regions of the immature anthers and filaments and the style and stigma in flowers. In addition, each AtCYS gene has a unique expression profile during abiotic stresses. High temperature and wounding stress enhanced the expression of both AtCYS1 and AtCYS2, but the temporal and spatial patterns of induction differed. From these data, we propose that these two AtCYS genes play important, but distinct, roles in plant development and stress responses.

Plant cystatins

Plant cystatins have been the object of intense research since the publication of a first paper reporting their existence more than 20 years ago. These ubiquitous inhibitors of Cys proteases play several important roles in plants, from the control of various physiological and cellular processes in planta to the inhibition of exogenous Cys proteases secreted by herbivorous arthropods and pathogens to digest or colonize plant tissues. After an overview of current knowledge about the evolution, structure and inhibitory mechanism of plant cystatins, we review the different roles attributed to these proteins in plants. The potential of recombinant plant cystatins as effective pesticidal proteins in crop protection is also considered, as well as protein engineering approaches adopted over the years to improve their inhibitory potency and specificity towards Cys proteases of biotechnological interest.