Identification of stable plant cystatin/nematode proteinase complexes using mildly denaturing gelatin/polyacrylamide gel electrophoresis

Biotic stress-induced expression of mulberry cystatins and identification of cystatin exhibiting stability to silkworm gut proteinases

Biotic stresses induce the expression of mulberry cystatins. MaCPI-4 protein is stable in silkworm digestive fluid and accumulates in gut food debris and frass. Plant cystatins are considered to be involved in defense responses to insect herbivores though little is known about how cystatins from the natural host respond to a specialist herbivory and the following postingestive interaction is also poorly understood. Here, we studied the biotic stress-mediated inductions of cystatins from mulberry tree, and examined the stability of mulberry cystatin proteins in the gut of silkworm, Bombyx mori, a specialist insect feeding on mulberry leaf. First, we cloned and characterized six cystatin genes from a mulberry cultivar, Morus atropurpurea Roxb., named as MaCPI-1 to MaCPI-6. The recombinant MaCPI-1, MaCPI-3 and MaCPI-4 proteins, which showed inhibitory effects against papain in vitro, were produced. Silkworm herbivory as well as methyl jasmonate (MeJA) treatment induced the expression of five mulberry cystatin genes, and the highest inductions were observed from MaCPI-1 and MaCPI-6. Mechanical wounding led to the inductions of four cystatin genes. The differential induction occurred in MaCPI-2. The induced protein changes were detected from three mulberry cystatins comprising MaCPI-1, MaCPI-3 and MaCPI-4. In vivo and in vitro assays showed that MaCPI-1 and MaCPI-3 proteins were susceptible to silkworm digestive fluid and MaCPI-4 had an antidigestive stability, and was detected in silkworm gut and frass. Collectively, our data indicated that biotic stresses resulted in the transcriptional inductions and protein changes of mulberry cystatins (MaCPIs), and identified MaCPI-4 with stability in the gut of its specialist herbivore.

Molecular characterization of putative parasitism genes in the plant-parasitic nematode Meloidogyne hispanica

Meloidogyne hispanica (Mhi) is a difficult-to-control polyphagous root-knot nematode (RKN) species of emerging importance for economically valuable crops. Nematode secretions are likely to be the first signals perceived by the plant and are thought to be involved in various aspects of the plant-nematode interaction. The aims of this work were to identify and characterize M. hispanica parasitism genes: cathepsin L cysteine protease (cpl-1), calreticulin (crt-1), β-1,4-endoglucanase-1 (eng-1) and manganese superoxide dismutase (mnsod). As there are no genomic data available for M. hispanica, primers were designed from the conserved regions of the putative parasitism genes in M. incognita and M. hapla and used to amplify the genes in M. hispanica, which led to the successful amplification of these genes in M. hispanica. Partial gene sequences were also obtained for M. arenaria, M. hapla, M. hispanica, M. incognita and M. javanica cpl-1, crt-1, eng-1 and mnsod genes, and their phylogenetic relationship analysed. In order to determine whether these genes are differentially expressed during M. hispanica development, cDNA was amplified from mRNA isolated from eggs, second-stage juveniles (J2) and females. Amplification products were observed from cDNA of all developmental stages for the Mhi-cpl-1 and Mhi-crt-1 genes. However, the gene Mhi-crt-1 exhibited intense amplification bands in females, while the Mhi-eng-1 gene was equally amplified in eggs and J2 and the Mhi-mnsod gene was only expressed in eggs. In comparison to the other RKN species, the genes Mhi-eng-1 and Mhi-mnsod showed transcription in different nematode developmental stages.

Tomato cystatin SlCYS8 as a stabilizing fusion partner for human serpin expression in plants

Studies have reported the usefulness of fusion proteins to bolster recombinant protein yields in plants. Here, we assess the potential of tomato SlCYS8, a Cys protease inhibitor of the cystatin protein superfamily, as a stabilizing fusion partner for human alpha-1-antichymotrypsin (α1ACT) targeted to the plant cell secretory pathway. Using the model expression platform Nicotiana benthamiana, we show that the cystatin imparts a strong stabilizing effect when expressed as a translational fusion with α1ACT, allowing impressive accumulation yields of over 2 mg/g of fresh weight tissue for the human serpin, a 25-fold improvement on the yield of α1ACT expressed alone. Natural and synthetic peptide linkers inserted between SlCYS8 and α1ACT have differential effects on protease inhibitory potency of the two protein partners in vitro. They also have a differential impact on the yield of α1ACT, dependent on the extent to which the hybrid protein may remain intact in the plant cell environment. The stabilizing effect of SlCYS8 does not involve Cys protease inhibition and can be partly reproduced in the cytosol, where peptide linkers are less susceptible to degradation. The effect of SlCYS8 on α1ACT yields could be explained by: (i) an improved translation of the human protein coding sequence; and/or (ii) an overall stabilization of its tertiary structure preventing proteolytic degradation and/or polymerization. These findings suggest the potential of plant cystatins as stabilizing fusion partners for recombinant proteins in plant systems. They also underline the need for an empirical assessment of peptide linker functions in plant cell environments.

Differential expression of antioxidant enzymes and PR-proteins in compatible and incompatible interactions of cowpea (Vigna unguiculata) and the root-knot nematode Meloidogyne incognita

This study aimed to evaluated the resistance and susceptibility of 10 cowpea cultivars to Meloidogyne incognita in field studies and to analyze the kinetics of the enzymes superoxide dismutase, catalase, peroxidase, chitinase, β-1,3-glucanases and cystein proteinase inhibitors in the root system of two contrasting cowpea cultivars after inoculation with M. incognita. The cultivars CE-31 and Frade Preto were highly resistant; CE-28, CE-01, CE-315, CE-237, were very resistant; CE-70 and CE-216 were moderately resistant, whereas Vita-3 and CE-109 were slightly resistant. In the roots of the highly resistant cultivar CE-31 the activity of the antioxidant enzyme superoxide dismutase increased and catalase decreased and those of the pathogenesis-related proteins chitinase, β-1,3-glucanase, peroxidase and cystein proteinase inhibitor increased in comparison with the root system of the slightly resistant CE-109, during the course of M. incognita infestation. Thus the changes in the activities of these enzymes might be related to the smaller final population of M. incognita in CE-31 and may contribute to the high resistance of this cowpea cultivar against infection and colonization by this nematode species.

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.

Effects of a novel pathogenesis-related class 10 (PR-10) protein from Crotalaria pallida Roots with papain inhibitory activity against root-knot nematode Meloidogyne incognita

A novel pathogenesis-related class 10 (PR-10) protein with papain inhibitory activity, named CpPRI, was purified from Crotalaria pallida roots by ammonium sulfate precipitation followed by three reverse-phase high-performance liquid chromatographies (HPLCs). CpPRI is made up of a single polypeptide chain with a M(r) of 15 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). This protein exhibited a K(i) value of 1.8 x 10(-9) M and operates via a noncompetitive inhibition mechanism. The alignment of the N-terminal amino acid sequence of CpPRI with other proteins revealed its identity with PR-10 proteins. CpPRI acts against digestive proteinase from root-knot nematode Meloidogyne incognita and demonstrated nematostatic and nematicide effects on this parasite in bioassays. In a localization study, fluorescein-5-isothiocyanate (FITC)-CpPRI was observed to internalize and diffuse over the entire J2 body after 6 h of incubation. This fact could explain the natural tolerance of this plant species to nematodes.

An AC-5 cathepsin B-like protease purified from Haemonchus contortus excretory secretory products shows protective antigen potential for lambs

The immunogenic properties of cysteine proteases obtained from excretory/secretory products (ES) of Haemonchus contortus were investigated with a fraction purified with a recombinant H. contortus cystatin affinity column. The enrichment of H. contortus ES for cysteine protease was confirmed with substrate SDS-PAGE gels since the cystatin-binding fraction activity was three times higher than total ES, despite representing only 3% of total ES. This activity was inhibited by a specific cysteine protease inhibitor (E64) and by recombinant cystatin. The one-dimensional profile of the cystatin-binding fraction displayed a single band with a molecular mass of 43 kDa. Mass spectrometry showed this to be AC-5, a cathepsin B-like cysteine protease which had not been identified in ES products of H. contortus before. The cystatin binding fraction was tested as an immunogen in lambs which were vaccinated three times (week 0, 2.5 and 5), challenged with 10 000 L3 H. contortus (week 6) before necropsy and compared to unvaccinated challenge controls and another group given total ES (n = 10 per group). The group vaccinated with cystatin-binding proteins showed 36% and 32% mean worm burden and eggs per gram of faeces (EPG) reductions, respectively, compared to the controls but total ES was almost without effect. After challenge the cystatin-binding proteins induced significantly higher local and systemic ES specific IgA and IgG responses.

Meloidogyne incognita: molecular cloning and characterization of a cDNA encoding a cathepsin D-like aspartic proteinase

Herein we describe the cloning and characterization of a cDNA encoding an aspartic proteinase from the root-knot nematode Meloidogyne incognita. Using PCR techniques, a 1471-bp cDNA fragment encoding a cathepsin D-like (Mi-asp1) transcript was isolated from second-stage larvae mRNA. Its predicted amino acid sequence comprises a pro-region of 71 amino acid residues and a mature protease of 378 amino acid residues with a predicted molecular mass of 41.502kDa. Protein sequence comparisons of Mi-asp1 with GenBank (DQ360827) sequences showed 59-71% identity with nematode-specific cathepsin D-like aspartic proteinases. Southern blot analysis, RT-PCR amplification and EST mining suggest the existence of a developmentally expressed gene family encoding aspartic proteinases in M. incognita. Mi-asp1 may represent a potential target for molecular intervention for the purposes of plant-parasitic nematode control.

MsCYS1, a developmentally-regulated cystatin from alfalfa

Several roles have been attributed to cystatins in plants, ranging from the regulation of host [endogenous] cysteine proteases to the inhibition of herbivorous pest [exogenous] proteases. We report here the cloning, expression and functional characterization of a novel cystatin from alfalfa, Medicago sativa L. The new sequence, isolated from a cDNA expression library prepared from young leaves, encodes a protein, MsCYS1, with the typical inhibitory motifs of cystatins, namely the central signature motif QxVxG, a GG doublet in the N-terminal trunk, and a W residue in the C-terminal region, about 30 amino acids distant from the central inhibitory motif. As shown by a protein-based phylogenetic reconstruction, MsCYS1 is a close relative of other cystatins from Fabaceae presumably involved in the regulation of endogenous proteases. This cystatin is developmentally regulated in stems and leaves, and not induced by stress signals including methyl jasmonate, known to activate cystatins involved in plant defense. A recombinant form of MsCYS1 expressed in Escherichia coli was shown to strongly inhibit alfalfa leaf cysteine proteases while showing weak affinity for the digestive cysteine proteases of different herbivorous pests. Overall, these observations suggest an endogenous protease regulatory role for MsCYS1, possibly associated with the early development of stems and leaves.

A multicomponent, elicitor-inducible cystatin complex in tomato, Solanum lycopersicum

We assessed the ability of the fungal elicitor arachidonic acid to induce cystatin genes in tomato (Solanum lycopersicum), using a cDNA expression library from arachidonate-treated leaves. The cDNAs of two novel cystatins were isolated, coding for an approx. 11-kDa protein, SlCYS10; and for a 23.6-kDa protein, SlCYS9, bearing an N-terminal signal peptide and a long, 11.5-kDa extension at the C terminus. Both genes were induced by arachidonate but not by methyl jasmonate, an inducer of the 88-kDa eight-unit cystatin, multicystatin, accumulated in the cytosol of leaf cells upon herbivory. A truncated form of SlCYS9, tSlCYS9, was produced by deletion of the C-terminal extension to assess the influence of this structural element on the cystatin moiety. As shown by kinetic and stability assays with recombinant variants expressed in Escherichia coli, deleting the extension influenced both the overall stability and inhibitory potency of SlCYS9 against cysteine proteases of herbivorous organisms. These findings provide evidence for a multicomponent elicitor-inducible cystatin complex in tomato, including at least 10 cystatin units produced via two metabolic routes.

Modulating the proteinase inhibitory profile of a plant cystatin by single mutations at positively selected amino acid sites

Cysteine proteinase inhibitors of the cystatin superfamily have several important functions in plants, including the inhibition of exogenous cysteine proteinases during herbivory or infection. Here we used a maximum-likelihood approach to assess whether plant cystatins, like other proteins implicated in host-pest interactions, have been subject to positive selection during the course of their evolution. Several amino acid sites were identified as being positively selected in cystatins from either Poaceae (monocots) and Solanaceae (dicots). These hypervariable sites were located at strategic positions on the protein: on each side of the conserved glycine residues in the N-terminal trunk, within the first and second inhibitory loops entering the active site of target enzymes, and surrounding the larfav motif, a sequence of unknown function conserved among plant cystatins. Supporting the assumption that positively selected, hypervariable sites are indicative of amino acid sites implicated in functional diversity, mutants of the 8th cystatin unit of tomato multicystatin including alternative residues at positively selected sites in the N-terminal trunk exhibited highly variable affinities for the cysteine proteases papain, cathepsin B and cathepsin H. Overall, these observations support the hypothesis that plant cystatins have been under selective pressure to evolve in response to predatory challenges by herbivorous enemies. They also indicate the potential of site-directed mutagenesis at positively selected sites for the generation of cystatins with improved binding properties.

Isolation and characterization of a cDNA encoding a serine proteinase from the root-knot nematode Meloidogyne incognita

This report describes the first serine proteinase gene isolated from the sedentary nematode Meloidogyne incognita. Using degenerate primers, a 1372bp cDNA encoding a chymotrypsin-like serine proteinase (Mi-ser1) was amplified from total RNA of adult females by RT-PCR and 5' and 3' rapid amplification of cDNA ends. The deduced amino acid sequence of Mi-ser1 encoded a putative signal peptide and a prodomain of 22 and 33 amino acids, respectively, and a mature proteinase of 341 amino acids with a predicted molecular mass of 37,680Da. Sequence identity with the top serine proteinases matches from the databases ranged from 23 to 27%, including sequences from insects, mammals, and other nematodes. Southern blot analysis suggested that Mi-ser1 is encoded by a single or few gene copies. The pattern of developmental expression analyzed by Northern blot and RT-PCR indicated that Mi-ser1 was transcribed mainly in females. The domain architecture composed of a single chymotrypsin-like catalytic domain and the detection of a putative signal peptide suggested a digestive role for Mi-ser1.

Engineering plants for nematode resistance

Biotechnology offers sustainable solutions to the problem of plant parasitic nematode control. There are several possible approaches for developing transgenic plants with improved nematode resistance; these include anti-invasion and migration strategies, feeding-cell attenuation, and antinematode feeding and development strategies. The essential elements of an effective control strategy are (a) genes that encode an antinematode effector protein, peptide or interfering RNA and (b) promoters that direct a specific pattern of expression for that effector. This review summarizes information on effectors that act directly against the nematode as well as those aimed at disrupting the nematode feeding site. We discuss patterns of promoter activity that could deliver expression of these effectors in a restricted and directed manner. Societal opposition to the technology of GM-nematode control is also discussed.

An electroblotting, two-step procedure for the detection of proteinases and the study of proteinase/inhibitor complexes in gelatin-containing polyacrylamide gels

A two-step gelatin/polyacrylamide gel electrophoresis (gelatin/PAGE) procedure was devised for the detection of proteinases and the study of proteinase/inhibitor interactions in complex biological extracts. The proteins are first resolved by sodium dodecyl sulfate (SDS)-PAGE under reducing or nonreducing conditions, and electrotransferred into a 0.75 mm-thick accompanying polyacrylamide slab gel containing 0.1% w/v porcine gelatin. The active proteinase bands are developed by a gelatin proteolysis step in the accompanying gel in the presence or absence of diagnostic proteinase inhibitors, allowing the assessment of proteinase classes and the visual discrimination of inhibitor-'sensitive' and -'insensitive' proteinases in complex extracts. Alternatively, protein extracts are preincubated with specific reversible inhibitors before electrophoresis, allowing a rapid discrimination of strong and weak interactions implicating proteinases and reversible inhibitors. In comparison with the standard gelatin/PAGE procedure, that involves copolymerization of gelatin with acrylamide in the resolving gel, this new procedure simplifies proteinase patterns, avoids overestimation of proteinase numbers in complex extracts, and allows in certain conditions the estimation of proteinase molecular weights. Stem bromelain (EC 3.4.22.32), bovine trypsin (EC 3.4.21.4), papain (EC 3.4.22.2), and the extracellular (digestive) cysteine proteinases of five herbivorous pests are used as model enzymes to illustrate the usefulness of this approach in detecting proteinases and in studying their interactions with specific proteinaceous inhibitors potentially useful in biotechnology.

Adaptation of Spodoptera exigua (Lepidoptera: Noctuidae) to barley trypsin inhibitor BTI-CMe expressed in transgenic tobacco

Nicotiana tabacum plants were transformed with the cDNA of barley trypsin inhibitor BTI-CMe under the control of the 35S CaMV promoter. Although the transgene was expressed and the protein was active in the homozygous lines selected, growth of Spodoptera exigua (Lepidoptera: Noctuidae) larvae reared on transgenic plants was not affected. The protease activity in larval midgut extracts after 2 days feeding on transformed tobacco leaves from the highest expressing plant showed a reduction of 25% in the trypsin-like activity compared to that from insects fed on non-transformed controls. The susceptibility of digestive serine-proteases to inhibition by BTI-CMe was confirmed by activity staining gels. This decrease was compensated with a significant induction of leucine aminopeptidase-like and carboxipeptidase A-like activities, while chymotrypsin-, elastase-, and carboxipeptidase B-like proteases were not affected.

Parasitic nematodes, proteinases and transgenic plants

Parasite proteinases have important functions in host-parasite interactions. Consequently, they have been investigated as targets for the control of both plant and animal parasites. Plant parasitic nematodes cause estimated annual losses to world agriculture of US$100 billion and, currently, their control often relies on highly toxic nematicides, with associated environmental risks. The potential of disrupting digestive proteinases for plant parasitic nematode control, via expression of proteinase inhibitors in transgenic plants, is summarized here by Catherine Lilley, Pauline Devlin, Peter Urwin and Howard Atkinson. They then consider whether the approach of expressing antinematode proteins in plants can be adapted for control of certain animal parasitic nematodes.

Protein hydrolysis by colorado potato beetle, leptinotarsa decemlineata, digestive proteases: the catalytic role of cathepsin D

Although several studies were carried out over the last 15 years to assess the nature and characteristics of digestive proteases in herbivorous insects, little is known about the relative importance of these enzymes in the hydrolysis of specific dietary proteins. In this study, we assessed the involvement of Colorado potato beetle (CPB; Leptinotarsa decemlineata Say, Chrysomelidae) aspartate, cysteine, and serine digestive proteinases in the degradation of two model substrates: ribulose biphosphate carboxylase/oxygenase, the major protein in potato leaves, and the pro-region of papaya proteinase IV, a cysteine protease inhibitor (PI) susceptible to proteolysis by the insect "nontarget" proteases. As shown by the use of various combinations of diagnostic PIs specific to the different classes of CPB proteinases, the insect aspartate (cathepsin D-like) proteinase activity is important in initiating the hydrolysis of both proteins when the insect is feeding on potato, while cysteine (cathepsin B/cathepsin H-like) and serine (chymotrypsin-like) proteinase activities would be involved in subsequent steps of the hydrolytic process. Similar observations were made with diet-induced variants of the insect protease system, suggesting the importance of digestive cathepsin D and the sequential hydrolysis of dietary proteins in CPB, regardless of the diet ingested. Based on these observations, a preliminary model is proposed to explain dietary protein hydrolysis in CPB, also taking into account the information currently available about the distribution of digestive endo- and exopeptidases in the midgut of CPB. The potential of a wound-induced cathepsin D inhibitor from tomato in developing CPB-resistant transgenic potato lines is also discussed, after demonstrating the "pepstatin-like" effect of a recombinant form of this proteinaceous inhibitor against the insect cathepsin D. Arch. Copyright 1999 Wiley-Liss, Inc.

The proregion of papaya proteinase IV inhibits Colorado potato beetle digestive cysteine proteinases

Three distinct digestive protease systems were induced in larvae of the herbivorous pest, Colorado potato beetle (CPB; Leptinotarsa decemlineata Say), and used as a model to assess the ability of the proregion of papaya proteinase IV (PPIV; glycyl endopeptidase, EC 3.4.22.25) to act as an inhibitor of insect digestive cysteine proteinases. As shown by gelatin/PAGE and complementary inhibition assays, a recombinant form of the proregion produced in Escherichia coli inhibited a fraction of the insect proteases also inhibited by the well-characterized inhibitor of cysteine proteinases, oryzacystatin I (OCI). In contrast with OCI, the inhibitory potency of the proregion was affected by an increase of the temperature, suggesting a certain alteration of its structural integrity by the insect non-target proteases. This apparent susceptibility to proteolysis was confirmed by SDS-PAGE, after challenging the proregion with the different insect extracts. As seen on gel, selective inhibition of the insect aspartate proteinase, cathepsin D, with the inhibitor pepstatin A preserved the activity of the proregion against cysteine proteinases by preventing its hydrolysis. Taken together, these observations suggest the potential of plant protease proregions as regulators of cysteine proteinases in biotechnological systems, and show the ability of protease inhibitors to preserve the integrity of 'companion' defense-related proteins from the action of insensitive proteases in target pests.

Two strains of cabbage seed weevil (Coleoptera: Curculionidae) exhibit differential susceptibility to a transgenic oilseed rape expressing oryzacystatin I

The aim of this study was to assess the potential effect of a transgenic line of oilseed rape expressing oryzacystatin I (OCI) on two strains of cabbage seed weevil. The level of OCI expression in seeds was approximately 0.05% of total soluble proteins. The insects were field-collected in two different locations, and their progeny was analyzed after a 3 week-development in pods. Both strains showed a similar pattern of proteolytic activity, and similar levels of OCI-sensitive proteinase activity in vitro. However, the larvae showed differential susceptibility to the transgenic plants. Despite inhibition of digestive proteinases in vitro by OCI in both strains, one strain showed an increased growth rate when fed the transgenic seeds, while the other strain remained unaffected. While suggesting the importance of studying individuals from different populations when assessing the effect of proteinase inhibitor-expressing plants on insect growth, our results also point out the necessity of studying the biochemical interactions taking place in vivo between the recombinant inhibitors and their target proteinases.