Action on bovine alpha s1-casein of cardosins A and B, aspartic proteinases from the flowers of the cardoon Cynara cardunculus L

The journey of cardosin A in young Arabidopsis seedlings leads to evidence of a Golgi-independent pathway to the protein storage vacuole

The aspartic proteinase cardosin A is a vacuolar enzyme found to accumulate in protein storage and lytic vacuoles in the flowers and protein bodies in the seeds of the native plant cardoon. Cardosin A was first isolated several decades ago and has since been extensively characterized, both in terms of tissue distribution and enzyme biochemistry. In the native system, several roles have been attributed to cardosin A, such as reproduction, reserve mobilization, and membrane remodeling. To participate in such diverse events, cardosin A must accumulate and travel to different compartments within the cell: protein storage vacuoles, lytic vacuoles, and the cytoplasmic membrane (and eventually outside the cell). Several studies have approached the expression of cardosin A in Arabidopsis thaliana and Nicotiana tabacum with promising results for the use of these systems to study of cardosin A trafficking. A poly-sorting mechanism has been uncovered for this protein, as two different vacuolar sorting determinants, mediating different vacuolar routes, have been described. The first is a conventional C-terminal domain, which delivers the protein to the vacuole via the Golgi, and the second is a more unconventional signal-the plant-specific insert (PSI)-that mediates a Golgi-independent route. The hypothesis that these two signals are activated according to cell needs and in organs with high metabolic activity is investigated here. An Arabidopsis line expressing cardosin A under an inducible promoter was used to understand the dynamics of cardosin A regarding vacuolar accumulation during seed germination events. Using antibodies against different regions of the protein and combining them with immunofluorescence and immunocytochemistry assays in different young seedling tissues, cardosin A was detected along the secretory pathway to the protein storage vacuole, often associated with the endoplasmic reticulum. More interestingly, upon treatment with the drug Brefeldin A, cardosin A was still detected in protein storage vacuoles, indicating that the intact protein can bypass the Golgi in this system, contrary to what was observed in N. tabacum. This study is a good starting point for further research involving the use of fluorescent fusions and exploring in more detail the relationship between cardosin A trafficking and plant development.

Characterization of Proteolytic Activity of Artichoke (Cynara scolymus L.) Flower Extracts on Bovine Casein to Obtain Bioactive Peptides

Simple Summary

Recently, dairy proteins, in addition to their basic nutritional role in the diet, were recognized as a source of bioactive peptides. Such peptides are encoded within the primary structure of the protein and can be released by enzymatic hydrolysis. The growing interest in the development of functional foods for the benefit of consumer health led to a recent increase in research on the production of bioactive peptides from different matrices and production methods. The use of aspartic proteases from stigmas of mature artichoke (Cynara scolymus L.) flowers to obtain hydrolytic enzymes (cinarases) in the production of bioactive peptides would involve the utilization of an agricultural residue of a plant species of great socio-economic importance. In the present study, the characterization of the optimal hydrolysis conditions of artichoke flower extracts was carried out for the production of peptides from bovine casein. Furthermore, the angiotensin-converting enzyme-I inhibitory activity and the antioxidant capacity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid (ABTS) free radicals in vitro were determined for the obtained hydrolysates. The results revealed that the water-soluble extract of artichoke flower could be suitable for the production of bioactive peptides from whole bovine casein.

Abstract

The aim of this work is to establish the most suitable proteolysis conditions to obtain bovine casein hydrolysates containing peptides with antioxidant and antihypertensive capacity. To this end, the proteolytic activity of Cynara scolymus L. flower extracts was characterized on whole bovine casein, evaluating the effect of several factors (pH, temperature, substrate concentration, enzyme concentration, and hydrolysis time). The optimal conditions to carry out the hydrolysis with the C. scolymus L. extract were as follows: pH 6.2, 50 °C, and 0.023 mg·mL⁻¹ of extract-protein concentration. A Michaelis constant (K_m) value of 5.66 mg·mL⁻¹ and a maximum rate of reaction (V_max) of 8.47 mUAbs∙min⁻¹ were observed. The optimal hydrolysis time was 17 h. The casein hydrolysates obtained with these conditions contained peptides with antioxidant activity (1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity: 30.89%; Trolox equivalent antioxidant capacity (TEAC) against 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) free radical (ABTS^●+): 4.43 mM Trolox equivalent·mg⁻¹ peptide) and antihypertensive activity, showing 55.05% angiotensin-converting enzyme-I inhibition in vitro.

Cardoon-based rennets for cheese production

The use of crude aqueous extracts of Cynara cardunculus flowers as coagulants in the production of high-quality sheep and goat cheeses-as are the cases of several Portuguese and Spanish cheese varieties with Protected Designation of Origin status-has been maintained since ancient times. The unique rheological attributes and sensory properties characteristic of these cheeses have always suggested that this plant coagulant (and, therefore, its isolated milk-clotting proteases) could be used as alternative rennet in the dairy industry, particularly suited for the production of sheep and goat cheeses. However, the lack of standardization of C. cardunculus crude flower extracts, whose quality and performance depends on numerous factors, has always hampered the application of this plant rennet in industrial production scales. To overcome these limitations, and to aim at developing more effective solutions with potential for scalability of production and commercial application, several strategies have been undertaken in more recent years to establish new cardoon-based rennets. This review provides an overview on these developments and on the currently available solutions, which range from producing standardized formulations of native cardoon enzymes, to the optimization of the heterologous production of cardosins and cyprosins to generate synthetic versions of these milk-clotting enzymes. Challenges and emerging opportunities are also discussed.

Use of artichoke (Cynara scolymus) flower extract as a substitute for bovine rennet in the manufacture of Gouda-type cheese: characterization of aspartic proteases

Artichoke (Cynara scolymus L.) flower extract was assayed with the aim of replacing animal rennet in the manufacture of Gouda-type cheeses from bovine milk. Floral extract coagulated milk within a suitable time for use on an industrial scale, while the yield of cheese obtained was equal to that achieved with bovine abomasum. Five proteolytic fractions with milk-clotting activity were isolated in a two-step purification protocol, three belonging to the cardosin group. Cheeses made with C. scolymus proteases must be brined for a longer period (40 h) to prevent overproteolysis and avoid the development of a background flavor. The type of coagulant (bovine or vegetable) had no significant effect on the cheeses' chemical parameters analyzed throughout ripening, and no significant organoleptic differences were detected between those manufactured with C. scolymus or animal rennet. The results indicate that C. scolymus flower extract is suitable for replacing animal rennet in the production of Gouda-type cheeses.

Properties and applications of phytepsins from thistle flowers

Aqueous extracts of thistle flowers from the genus Cynara-Cardueae tribe Cass. (Cynareae Less.), Asteraceae Dumortier-are traditionally used in the Mediterranean region for production of artisanal cheeses. This is because of the presence of aspartic proteases (APs) with the ability to coagulate milk. Plant APs, collectively known as phytepsins (EC 3.4.23.40), are bilobed endopeptidases present in an ample variety of plant species with activity mainly at acidic pHs, and have two aspartic residues located on each side of a catalytic cleft that are responsible for catalysis. The cleavage of the scissile peptide-bond occurs primarily between residues with large hydrophobic side-chains. Even when aspartylendopeptidase activity in plants is normally present at relatively low levels overall, the flowers of several species of the Cardueae tribe possess APs with extremely high specific activities in certain tissues. For this reason, in the last two decades, APs present in thistle flowers have been the subject of intensive study. Present here is a compilation of work that summarizes the known chemical and biological properties of these proteases, as well as their biomedical and biotechnological applications.

Exploring the milk-clotting properties of a plant coagulant from the berries of S. elaeagnifolium var. Cavanilles

Solanum elaeagnifolium (trompillo or silverleaf nightshade) is an endemic plant from the northeast of Mexico and southwest of United States. This plant is considered as a weed with negative impact on agriculture and livestock production. Nevertheless, in some places of Chihuahua, Mexico, the berries of this plant have been used for decades in the manufacture of artisanal filata-type asadero cheese. The milk-clotting enzyme of S. elaeagnifolium has been scarcely studied; for this reason, the aim of this work was to explore some properties of this plant coagulant. Protein extracts (PEs) from ripe berries of S. elaeagnifolium were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and assessed for milk-clotting activity. In addition, milk gels and miniature fresh-type cheeses manufactured with the plant coagulant were analyzed for their texture properties. The PE from the berries of S. elaeagnifolium contained 8 proteins with molecular weights from 22 to 62 kDa. Some bands observed in the PE had similar molecular weights as reported for aspartic proteinases such as chymosin. The extracts from the berries of S. elaeagnifolium had lower milk-cloting activities than observed with rennin or chymosin, but this plant coagulant produced firm gels under acidic conditions. The mini-cheeses manufactured with this coagulant were softer than cheeses manufactured with rennin or chymosin. For this reason, the coagulant from the ripe berries of S. elaeagnifolium could be suitable not only for the manufacture of filata-type cheeses but also for the manufacture of soft cheeses such as cream cheese.

PRACTICAL APPLICATION

Silverleaf nightshade (trompillo) is a plant that grows in northern Mexico and the southwestern United States. This plant is considered a weed with negative impact on agriculture and livestock production. However, the ripe berries of this plant have been used by ancient Pima Indians as a substitute of rennin in making cheese. In this work, it was observed that this plant coagulant had lower activity and produced softer cheeses than did rennin or chymosin. For this reason, the coagulant from berries of S. elaeagnifolium could be used for the manufacture of soft cheeses such as cream cheese.

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.

Allocation of proteolytic activity in the seedling of Cynara cardunculus L. in the initial growth stages

Cynara cardunculus L. seeds were germinated in vitro under environmentally controlled conditions. Seeds showed a 60% germination rate, and three growth stages were established based on the seedling mean relative growth rate (RGR). Root, stem and cotyledons were compared in these stages with respect to the emergence of total proteases and cardosin activity and its allocation in the seedling. In growth stage I (1st-5th post-germinative days), seedlings grew very slowly. Total proteases and cardosins were already active at the onset of seedlings in the stem. Total soluble protein remained constant in cardoon seedlings during stage I, and the content of all free amino acids (aa) but proline (Pro) was equally allocated on the 1st post-germinative day. In growth stage II (5th-10th post-germinative days), seedlings grew intensively and exhibited fully developed cotyledons. A pronounced increase in the content of all free aa up to the middle of growth stage II in both stems and roots was observed. In addition, the allocation of the total proteolytic activity and cardosins followed a gradient from the root to the seedling shoot. However, the whole seedling soluble protein remained constant up to the 7th day in and tended to peak on the 10th post-germinative day, being allocated mainly to the seedling stem. In growth stage III (10th-15th post-germinative days), cardoon seedlings exhibited the lowest mean RGR and the highest R/S growth ratio. An intensive degradation of total soluble protein present in the whole seedling except for cotyledons (ca. 5-fold) was observed. Nevertheless, in growth stage III, both the gradients exhibited by total proteases and cardosins activities between the root and the seedling shoot were enhanced, as were contents of all aa except Pro, exhibiting the highest levels in cotyledons on the 15th post-germinative day.

Processing and trafficking of a single isoform of the aspartic proteinase cardosin A on the vacuolar pathway

Cardosin A is the major vacuolar aspartic proteinase (APs) (E.C.3.4.23) in pistils of Cynara cardunculus L. (cardoon). Plant APs carry a unique domain, the plant-specific-insert (PSI), and a pro-segment which are separated from the catalytic domains during maturation but the sequence and location of processing steps for cardosins have not been established. Here transient expression in tobacco and inducible expression in Arabidopsis indicate that processing of cardosin A is conserved in heterologous species. Pulse chase analysis in tobacco protoplasts indicated that cleavage at the carboxy-terminus of the PSI could generate a short-lived 50 kDa intermediate which was converted to a more stable 35 kDa intermediate by removal of the PSI. Processing intermediates detected immunologically in tobacco leaves and Arabidopsis seedlings confirmed that cleavage at the amino-terminus of the PSI either preceded or followed quickly after cleavage at its carboxy-terminus. Thus removal of PSI preceded the loss of the prosegment in contrast to the well-characterised barley AP, phytepsin. PreprocardosinA acquired a complex glycan and its processing was inhibited by brefeldin A and dominant-inhibitory AtSAR1 or AtRAB-D2(a )mutants indicating that it was transported via the Golgi and that processing followed ER export. The 35 kDa intermediate was present in the cell wall and protoplast culture medium as well as the vacuole but the 31 kDa mature subunit, lacking the amino-terminal prosegment, was detected only in the vacuole. Thus maturation appears to occur only after sorting from the trans-Golgi to the vacuole. Processing or transport of cardosin A was apparently slower in tobacco protoplasts than in whole cells.

Cardosins in postembryonic development of cardoon: towards an elucidation of the biological function of plant aspartic proteinases

Following on from previous work, the temporal and spatial accumulation of the aspartic proteinases (EC 3.4.23) cardosin A and cardosin B during postembryonic seed development of cardoon (Cynara cardunculus) was studied, mRNA and protein analyses of both cardosins suggested that the proteins accumulate during seed maturation, and that cardosin A is later synthesised de novo at the time of radicle emergence. Immunocytochemistry revealed that the precursor form of cardosin A accumulates in protein bodies and cell walls. This localisation in seeds is different from that previously described for cardoon flowers, suggesting a tissue-dependent targeting of the protein. It is known that procardosins are active and may have a role in proteolysis and processing of storage proteins. However, the presence of procardosin A in seeds could be related to the proposed role of the plant-specific insert in membrane lipid conversion during water uptake and solute leakage in actively growing tissues. This is in accordance with the recently proposed bifunctional role of aspartic proteinase precursor molecules that possess a membrane-destabilising domain in addition to a protease domain. Mature cardosin B, but not its mRNA, was detected in the first hours after seed imbibition and disappeared at the time of radicle emergence. This extracellular aspartic protease has already been implicated in cell wall loosening and remodelling, and its role in seed germination could be related to loosening tissue constraints for radicle protusion. The described pattern of cardosin A and B expression suggests a finely tuned developmental regulation and prompts an analysis of their possible roles in the physiology of postembryonic development.

Structural characterization of the stigma-style complex of Cynara cardunculus (Asteraceae) and immunolocalization of cardosins A and B during floral development

Studies were carried out on the structure of the stigma and style of Cynara cardunculus L. (cardoon) during flower development. The stigma is of the dry type with a papillate cuticularized epidermis. During development, the unicellular papillae become match-stick shaped, cuticularize, and show an increase in vacuolar volume. In mature papillae, two morphologically different vacuoles were observed, one electron-dense and the other electron-transparent, putatively corresponding to distinct vacuolar populations. These vacuoles label differently for cardosin A, specifically detected in the electron-dense compartments. The style is solid with a cuticularized epidermis and a central core of transmitting tissue (TT) several cell layers thick. The TT cells show abundant rough endoplasmic reticulum and Golgi bodies, associated with active secretion. During maturation, TT cells become increasingly separated by a polysaccharide-rich extracellular matrix. Communication between TT cells is maintained via plasmodesmata in longitudinal walls. Distribution of cardosins A and B in developing C. cardunculus flowers was also characterized. The presence of aspartic proteinases (APs) in flowers is unusual, generally occurring at low levels. Cardosins A and B are always present in cardoon florets and localize at distinct pistil levels: stigma (papillae) and style (TT), respectively. This differential localization suggests distinct biological functions for cardosins, most likely essential for reproduction in this species.

Molecular Characterization of Peptides Released from β-Lactoglobulin and α-Lactalbumin via Cardosins A and B

Crude mixtures of aspartic proteases from flowers of the plant Cynara cardunculus have been studied frequently, as have activities of such enzymes (in pure form) on caseins from bovine, ovine, and caprine sources. This research study addressed pure bovine whey protein as substrates; that is, alpha-lactalbumin (alphaLA) and beta-lactoglobulin (alpha-LG), submitted to hydrolysis by 1 of 2 aspartic proteases (cardosins A and B), previously extracted and purified from C. cardunculus. Samples collected, following incubation at 55 degrees C and pH 5.2, were assayed by fast protein liquid chromatography, reversed phase-high performance liquid chromatography, and tricine-sodium dodecyl sulfate polyacrylamide gel electrophoresis; the major peptides released were then collected and sequenced by Edman degradation. Cardosin B and, to a lesser degree, cardosin A showed proteolytic activity toward alpha-LA, but the hydrolyzates produced were characterized by distinct peptide profiles. Cardosin B possesses a broad specificity, and produces several hydrophobic peptides (at least 5, with molecular mass in the range 2 to 8 kDa) in the early stages, which eventually become more hydrophilic (with molecular mass below 2 kDa) at later stages of hydrolysis. Cardosin A was found to cleave alpha-LA at the peptide bonds Phe28-Arg29, Gly54-Tyr55, Ala59-Ile60, Leu71-Phe72, and Leu105-Thr106, whereas cardosin B cleaved Ala19-Glu20, Phe28-Arg29, Glu30-Leu31, Tyr37-Gly38, Trp45-Val46, Phe50-His51, Ala59-Ile60, Ser66-Thr67, Leu71-Phe72, Phe72-Gln73, Gln73-Ile74, Ile78-Trp79, Leu115-Asp116, and Leu124-Ala125. Conversely, cardosins A and B are apparently not active on beta-LG.

Molecular analysis of the interaction between cardosin A and phospholipase D(alpha). Identification of RGD/KGE sequences as binding motifs for C2 domains

Here we report the identification of phospholipase Dalpha as a cardosin A-binding protein. The interaction was confirmed by coimmunoprecipitation studies and pull-down assays. To investigate the structural and molecular determinants involved in the interaction, pull-down assays with cardosin A and various glutathione S-transferase-fused phospholipase Dalpha constructs were performed. Results revealed that the C2 domain of phospholipase Dalpha contains the cardosin A-binding activity. Further assays with mutated recombinant forms of cardosin A showed that the RGD motif as well as the unprecedented KGE motif, which is structurally and charge-wise very similar to RGD, are indispensable for the interaction. Taken together our results indicate that the C2 domain of plant phospholipase Dalpha can act as a cardosin A-binding domain and suggest that plant C2 domains may have an additional role as RGD/KGE-recognition domains.

The three typical aspartic proteinase genes of Arabidopsis thaliana are differentially expressed

Genomic sequencing has identified three different typical plant aspartic proteinases in the genome of Arabidopsis thaliana, named Pasp-A1, A2 and A3. A1 is identical to a cDNA we had previously isolated and the two others produce proteins 81 and 63% identical to that predicted protein. Sequencing of the aspartic proteinase protein purified from Arabidopsis seeds showed that the peptides are derived from two of these genes, A1 and A2. Using gene specific probes, we have analyzed RNA from different tissues and found these three genes are differentially expressed. A1 mRNA is detected in all tissues analyzed and more abundant in leaves during the light phase of growth. The other two genes are expressed either primarily in flowers (A3) or in seeds (A2). Insitu hybridization demonstrated that all three genes are expressed in many cells of the seeds and developing seed pods. The A1 and A3 genes are expressed in the sepals and petals of flowers as well as the outer layer of the style, but are not expressed in the transmitting tract or on the stigmatal surface. The A2 gene is weakly expressed only in the transmitting tissue of the style. All three genes are also expressed in the guard cells of sepals. These data suggest multiple roles for aspartic proteinases besides those proposed in seeds.

Chemical and microbiological characteristics of ewes' milk cheese manufactured with extracts from flowers of Cynara cardunculus and Cynara humilis as coagulants

The chemical and microbial characteristics as well as the flavor and aroma of Los Pedroches cheese made using aqueous extracts of Cynara cardunculus L. flowers were compared with those of cheeses manufactured with extracts of Cynara humilis L. throughout ripening. The two thistle species assayed were found to have no appreciable effect on the moisture, fat, protein, and NaCl contents of the cheese or on its water activity, flavor, and aroma; however, the use of C. humilis resulted in reduced lactic acid content (p < 0.001) and higher pH values (p < 0.05) relative to those of cheese specimens produced with C. cardunculus. The protein breakdown of the cheeses was assessed in terms of soluble nitrogen (SN), nonprotein nitrogen (NPN), and amino acid nitrogen (AAN). Proteolysis was more marked and rapid in cheese containing C. cardunculus as coagulant, the SN and NPN contents of which were significantly higher (p < 0. 01) than those of the cheese obtained with the species C. humilis; AAN contents were similar in both species of Cynara throughout ripening. Although total viable, coliform, and lactobacilli counts were similar in cheeses produced with both types of plant coagulant throughout ripening, enterobacteria and yeasts counts (p < 0.01) and molds counts (p < 0.05) were higher in cheese produced with C. humilis than in cheese obtained with C. cardunculus.

Cloning and characterization of cDNA encoding cardosin A, an RGD-containing plant aspartic proteinase

Cardosin A is an abundant aspartic proteinase from pistils of Cynara cardunculus L. whose milk-clotting activity has been exploited for the manufacture of cheese. Here we report the cloning and characterization of cardosin A cDNA. The deduced amino acid sequence contains the conserved features of plant aspartic proteinases, including the plant-specific insertion (PSI), and revealed the presence of an Arg-Gly-Asp (RGD) motif, which is known to function in cell surface receptor binding by extracellular proteins. Cardosin A mRNA was detected predominantly in young flower buds but not in mature or senescent pistils, suggesting that its expression is likely to be developmentally regulated. Procardosin A, the single chain precursor, was found associated with microsomal membranes of flower buds, whereas the active two-chain enzyme generated upon removal of PSI is soluble. This result implies a role for PSI in promoting the association of plant aspartic proteinase precursors to cell membranes. To get further insights about cardosin A, the functional relevance of the RGD motif was also investigated. A 100-kDa protein that interacts specifically with the RGD sequence was isolated from octyl glucoside pollen extracts by affinity chromatography on cardosin A-Sepharose. This result suggests that the 100-kDa protein is a cardosin A receptor and indicates that the interaction between these two proteins is apparently mediated through RGD recognition. It is possible therefore that cardosin A may have a role in adhesion-mediated proteolytic mechanisms involved in pollen recognition and growth.

Crystal structure of cardosin A, a glycosylated and Arg-Gly-Asp-containing aspartic proteinase from the flowers of Cynara cardunculus L

Aspartic proteinases (AP) have been widely studied within the living world, but so far no plant AP have been structurally characterized. The refined cardosin A crystallographic structure includes two molecules, built up by two glycosylated peptide chains (31 and 15 kDa each). The fold of cardosin A is typical within the AP family. The glycosyl content is described by 19 sugar rings attached to Asn-67 and Asn-257. They are localized on the molecular surface away from the conserved active site and show a new glycan of the plant complex type. A hydrogen bond between Gln-126 and Manbeta4 renders the monosaccharide oxygen O-2 sterically inaccessible to accept a xylosyl residue, therefore explaining the new type of the identified plant glycan. The Arg-Gly-Asp sequence, which has been shown to be involved in recognition of a putative cardosin A receptor, was found in a loop between two beta-strands on the molecular surface opposite the active site cleft. Based on the crystal structure, a possible mechanism whereby cardosin A might be orientated at the cell surface of the style to interact with its putative receptor from pollen is proposed. The biological implications of these findings are also discussed.