Three-dimensional structure of thaumatin I, an intensely sweet protein

Re-evaluation of thaumatin (E 957) as food additive

The present opinion deals with the re-evaluation of thaumatin (E 957) when used as a food additive. Thaumatin is a natural plant protein, consisting of thaumatin I and thaumatin II proteins together with minor amounts of plant constituents, obtained by acidic aqueous extraction of the arils of the fruit of Thaumatococcus daniellii plant. The Panel followed the conceptual framework for the risk assessment of certain food additives and considered that thaumatin is a digestible protein; adequate exposure estimates were available; there was no concern with respect to the genotoxicity; no conclusion on oral allergenicity could be drawn from the available human data; no adverse effects were observed in sub-chronic toxicity studies in rats and dogs at the highest dose tested of up 5,200 and 1,476 mg/kg bodyweight (bw) per day, respectively, and in a prenatal developmental toxicity study up to 2,000 mg/kg bw per day; moderate confidence in the body of evidence supported the absence of association between exposure to thaumatin and adverse health outcomes. Therefore, the Panel concluded that there is no need for a numerical acceptable daily intake (ADI) for thaumatin (E 957) and, based on a margin of safety (MOS) of 5,417, considered to be an underestimate and derived using the highest 95th percentile (P95) exposure of 0.48 mg/kg bw per day in consumers only, there is no safety concern for thaumatin (E 957) at the regulatory maximum level exposure assessment scenario, which was considered the most appropriate. The Panel recommended that European Commission considers introducing in the EU specifications for thaumatin (E 957) a new specification limit for the minimum combined content of thaumatin I and II proteins in E 957, a specification limit for yeast, mould counts and Salmonella spp and lowering the existing maximum limit for arsenic along with the inclusion of maximum limits for mercury and cadmium.

De novo transcriptome analysis and comparative expression profiling of genes associated with the taste-modifying protein neoculin in Curculigo latifolia and Curculigo capitulata fruits

Background

Curculigo latifolia is a perennial plant endogenous to Southeast Asia whose fruits contain the taste-modifying protein neoculin, which binds to sweet receptors and makes sour fruits taste sweet. Although similar to snowdrop (Galanthus nivalis) agglutinin (GNA), which contains mannose-binding sites in its sequence and 3D structure, neoculin lacks such sites and has no lectin activity. Whether the fruits of C. latifolia and other Curculigo plants contain neoculin and/or GNA family members was unclear.

Results

Through de novo RNA-seq assembly of the fruits of C. latifolia and the related C. capitulata and detailed analysis of the expression patterns of neoculin and neoculin-like genes in both species, we assembled 85,697 transcripts from C. latifolia and 76,775 from C. capitulata using Trinity and annotated them using public databases. We identified 70,371 unigenes in C. latifolia and 63,704 in C. capitulata. In total, 38.6% of unigenes from C. latifolia and 42.6% from C. capitulata shared high similarity between the two species. We identified ten neoculin-related transcripts in C. latifolia and 15 in C. capitulata, encoding both the basic and acidic subunits of neoculin in both plants. We aligned these 25 transcripts and generated a phylogenetic tree. Many orthologs in the two species shared high similarity, despite the low number of common genes, suggesting that these genes likely existed before the two species diverged. The relative expression levels of these genes differed considerably between the two species: the transcripts per million (TPM) values of neoculin genes were 60 times higher in C. latifolia than in C. capitulata, whereas those of GNA family members were 15,000 times lower in C. latifolia than in C. capitulata.

Conclusions

The genetic diversity of neoculin-related genes strongly suggests that neoculin genes underwent duplication during evolution. The marked differences in their expression profiles between C. latifolia and C. capitulata may be due to mutations in regions involved in transcriptional regulation. Comprehensive analysis of the genes expressed in the fruits of these two Curculigo species helped elucidate the origin of neoculin at the molecular level.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07674-3.

Plant Thaumatin-like Proteins: Function, Evolution and Biotechnological Applications

Thaumatin-like proteins (TLPs) are a highly complex protein family associated with host defense and developmental processes in plants, animals, and fungi. They are highly diverse in angiosperms, for which they are classified as the PR-5 (Pathogenesis-Related-5) protein family. In plants, TLPs have a variety of properties associated with their structural diversity. They are mostly associated with responses to biotic stresses, in addition to some predicted activities under drought and osmotic stresses. The present review covers aspects related to the structure, evolution, gene expression, and biotechnological potential of TLPs. The efficiency of the discovery of new TLPs is below its potential, considering the availability of omics data. Furthermore, we present an exemplary bioinformatics annotation procedure that was applied to cowpea (Vigna unguiculata) transcriptome, including libraries of two tissues (root and leaf), and two stress types (biotic/abiotic) generated using different sequencing approaches. Even without using genomic sequences, the pipeline uncovered 56 TLP candidates in both tissues and stresses. Interestingly, abiotic stress (root dehydration) was associated with a high number of modulated TLP isoforms. The nomenclature used so far for TLPs was also evaluated, considering TLP structure and possible functions identified to date. It is clear that plant TLPs are promising candidates for breeding purposes and for plant transformation aiming a better performance under biotic and abiotic stresses. The development of new therapeutic drugs against human fungal pathogens also deserves attention. Despite that, applications derived from TLP molecules are still below their potential, as it is evident in our review.

The Sweet Taste of Acarbose and Maltotriose: Relative Detection and Underlying Mechanism

Although sweet-tasting saccharides possess similar molecular structures, their relative sweetness often varies to a considerable degree. Current understanding of saccharide structure/sweetness interrelationships is limited. Understanding how certain structural features of saccharides and/or saccharide analogs correlate to their relative sweetness can provide insight on the mechanisms underlying sweetness potency. Maltotriose is a short-chain glucose-based oligosaccharide, which we recently reported to elicit sweet taste. Acarbose, an α-glucosidase inhibitor, is a pseudo-saccharide that has an overall resemblance to a glucose-based oligosaccharide and thus may be viewed as a structural analog. During other studies, we recognized that acarbose can also elicit sweet taste. Here, we formally investigated the underlying taste detection mechanism of acarbose, while confirming our previous findings for maltotriose. We found that subjects could detect the sweet taste of acarbose and maltotriose in aqueous solutions but were not able to detect them in the presence of a sweet taste inhibitor lactisole. These findings support that both are ligands of the human sweet taste receptor, hT1R2/hT1R3. In a separate experiment, we measured the relative sweetness detection of acarbose, maltotriose, and other sweet-tasting mono- and disaccharides (glucose, fructose, maltose, and sucrose). Whereas maltotriose was found to have a similar discriminability profile to glucose and maltose, the discriminability of acarbose matched that of fructose at the concentrations tested (18, 32, and 56 mM). These findings are discussed in terms of how specific molecular features (e.g., degree of polymerization and monomer composition) may contribute to the relative sweetness of saccharides.

Identification and functional characterization of HbOsmotin from Hevea brasiliensis

Latex in the laticiferous cell network of Hevea brasiliensis tree is composed of cytoplasm that synthesizes natural rubber. Ethylene stimulation of the tree bark enhances latex production partly by prolonging the duration of latex flow during the tapping process. Here, we identified an osmotin-like cDNA sequence (HbOsmotin) from H. brasiliensis that belongs to the pathogenesis-related 5 (PR-5) gene family. The HbOsmotin protein is present in the lutoids of latex in H. brasiliensis, whereas in onion epidermal cells, this protein is predominantly distributed around the cell wall, suggesting that it may be secreted from the cytoplasm. We investigated the effects of exogenous ethylene on HbOsmotin transcription and protein accumulation in rubber latex, and further determined the protein function after osmotic stress in Arabidopsis. In regularly tapped trees, HbOsmotin expression was drastically inhibited in rubber latex after tapping, although the expression was subsequently recovered by ethylene stimulation. However, in virgin plants that had never been tapped, exogenous ethylene application slightly decreased HbOsmotin expression. HbOsmotin overexpression in Arabidopsis showed that HbOsmotin reduced the osmotic stress tolerance of the plant, which likely occurred by raising the water potential. These data indicated that HbOsmotin may contribute to osmotic regulation in laticiferous cells.

Osmotin: a plant sentinel and a possible agonist of mammalian adiponectin

Osmotin is a stress responsive antifungal protein belonging to the pathogenesis-related (PR)-5 family that confers tolerance to both biotic and abiotic stresses in plants. Protective efforts of osmotin in plants range from high temperature to cold and salt to drought. It lyses the plasma membrane of the pathogens. It is widely distributed in fruits and vegetables. It is a differentially expressed and developmentally regulated protein that protects the cells from osmotic stress and invading pathogens as well, by structural or metabolic alterations. During stress conditions, osmotin helps in the accumulation of the osmolyte proline, which quenches reactive oxygen species and free radicals. Osmotin expression results in the accumulation of storage reserves and increases the shelf-life of fruits. It binds to a seven-transmembrane-domain receptor-like protein and induces programmed cell death in Saccharomyces cerevisiae through RAS2/cAMP signaling pathway. Adiponectin, produced in adipose tissues of mammals, is an insulin-sensitizing hormone. Strangely, osmotin acts like the mammalian hormone adiponectin in various in vitro and in vivo models. Adiponectin and osmotin, the two receptor binding proteins do not share sequence similarity at the amino acid level, but interestingly they have a similar structural and functional properties. In experimental mice, adiponectin inhibits endothelial cell proliferation and migration, primary tumor growth, and reduces atherosclerosis. This retrospective work examines the vital role of osmotin in plant defense and as a potential targeted therapeutic drug for humans.

Molecular cloning and functional characterization of an antifungal PR-5 protein from Ocimum basilicum

Pathogenesis-related (PR) proteins are involved in biotic and abiotic stress responses of plants and are grouped into 17 families (PR-1 to PR-17). PR-5 family includes proteins related to thaumatin and osmotin, with several members possessing antimicrobial properties. In this study, a PR-5 gene showing a high degree of homology with osmotin-like protein was isolated from sweet basil (Ocimum basilicum L.). A complete open reading frame consisting of 675 nucleotides, coding for a precursor protein, was obtained by PCR amplification. Based on sequence comparisons with tobacco osmotin and other osmotin-like proteins (OLPs), this protein was named ObOLP. The predicted mature protein is 225 amino acids in length and contains 16 cysteine residues that may potentially form eight disulfide bonds, a signature common to most PR-5 proteins. Among the various abiotic stress treatments tested, including high salt, mechanical wounding and exogenous phytohormone/elicitor treatments; methyl jasmonate (MeJA) and mechanical wounding significantly induced the expression of ObOLP gene. The coding sequence of ObOLP was cloned and expressed in a bacterial host resulting in a 25kDa recombinant-HIS tagged protein, displaying antifungal activity. The ObOLP protein sequence appears to contain an N-terminal signal peptide with signatures of secretory pathway. Further, our experimental data shows that ObOLP expression is regulated transcriptionally and in silico analysis suggests that it may be post-transcriptionally and post-translationally regulated through microRNAs and post-translational protein modifications, respectively. This study appears to be the first report of isolation and characterization of osmotin-like protein gene from O. basilicum.

Identification and characterization of a basic thaumatin-like protein (TLP 2) as an allergen in sapodilla plum (Manilkara zapota)

SCOPE

Cases of oral allergy syndrome following the ingestion of sapodilla plum (Manilkara zapota) have been reported rarely. As the causative allergens are not known, the main objective of this study was to identify and characterize the important allergens in sapodilla.

METHODS AND RESULTS

Allergy to sapodilla was diagnosed by case history, skin prick test, and serum allergen-specific IgE. The allergen was detected by IgE immunoblotting, purified on SP-Sepharose and characterized by native/SDS-PAGE, IEF, MS, and amino acid composition. Several cases of allergy to sapodilla fruit were identified; majority of the sapodilla-allergic subjects (6/7) experienced typical oral allergy syndrome symptoms, and allergen-specific IgE to the purified protein was positive. The allergen has a pI ≥9.5 and high contents of arginine, threonine, glycine, and cysteine. Circular dichroism revealed a secondary structure rich in beta sheets/turns. Based on its N-terminal sequence of A-T-F-D-I-Q-N-N-C-X-Y-, the allergen (21 578 Da) was identified as a thaumatin-like protein by homology.

CONCLUSION

The causative allergen in sapodilla plum has been identified and characterized as a highly basic thaumatin-like protein belonging to the pathogenesis-related protein (PR-5) family, which has been recognized as a new family of conserved, cross-reactive plant allergens.

Properties and mechanisms of action of naturally occurring antifungal peptides

Antimicrobial peptides are a vital component of the innate immune system of all eukaryotic organisms and many of these peptides have potent antifungal activity. They have potential application in the control of fungal pathogens that are a serious threat to both human health and food security. Development of antifungal peptides as therapeutics requires an understanding of their mechanism of action on fungal cells. To date, most research on antimicrobial peptides has focused on their activity against bacteria. Several antimicrobial peptides specifically target fungal cells and are not active against bacteria. Others with broader specificity often have different mechanisms of action against bacteria and fungi. This review focuses on the mechanism of action of naturally occurring antifungal peptides from a diverse range of sources including plants, mammals, amphibians, insects, crabs, spiders, and fungi. While antimicrobial peptides were originally proposed to act via membrane permeabilization, the mechanism of antifungal activity for these peptides is generally more complex and often involves entry of the peptide into the cell.

Dissimilar sweet proteins from plants: oddities or normal components?

The fruits of a few tropical plants contain intensely sweet proteins. Their common property points to a protein family. Generally, proteins belonging to the same family share similar folds, similar sequences and, at least in part, similar function but sweet proteins constitute an exception to this rule. Apart from sharing the rather unusual taste function, they show no obvious similarities either in their sequences or in three-dimensional structures. In this review we describe the nature, structure and mechanism of action of the best known sweet tasting proteins, including two taste modifying proteins. Sweet proteins stand out among sweet molecules because their volume is not compatible with an interaction with orthosteric active sites of the sweet taste receptor. The best explanation of their mechanism of action is the interaction with the external surface of the sweet taste receptor, according to a model that has been named "wedge model". It is hypothesized that this mode of action may be related to the ability of other members of their protein families to inhibit different enzymes.

Single-drop optimization of protein crystallization

A completely new crystal-growth device has been developed that permits charting a course across the phase diagram to produce crystalline samples optimized for diffraction experiments. The utility of the device is demonstrated for the production of crystals for the traditional X-ray diffraction data-collection experiment, of microcrystals optimal for data-collection experiments at a modern microbeam insertion-device synchrotron beamline and of nanocrystals required for data collection on an X-ray laser beamline.

Influence of drying on the secondary structure of intrinsically disordered and globular proteins

Circular dichroism (CD) spectroscopy of five Arabidopsis late embryogenesis abundant (LEA) proteins constituting the plant specific families LEA_5 and LEA_6 showed that they are intrinsically disordered in solution and partially fold during drying. Structural predictions were comparable to these results for hydrated LEA_6, but not for LEA_5 proteins. FTIR spectroscopy showed that verbascose, but not sucrose, strongly affected the structure of the dry proteins. The four investigated globular proteins were only mildly affected by drying in the absence, but strongly in the presence of sugars. These data highlight the larger structural flexibility of disordered compared to globular proteins and the impact of sugars on the structure of both disordered and globular proteins during drying.

New insights into the characteristics of sweet and bitter taste receptors

Understanding the molecular bases of taste is of primary importance for the field of human senses as well as for translational medical science. This chapter describes the complexity of the mechanism of action of sweet, bitter, and umami receptors. Most molecular weight sweeteners interact with orthosteric sites of the sweet receptor. The mechanism of action of sweet proteins is more difficult to interpret. In the only general mechanism proposed for the action of sweet proteins, the "wedge model," it is hypothesized that proteins bind to an external active site of the active conformation of the sweet receptor. This model can be updated by building topologically correct complexes of proteins with the receptor. Among the recent advances that will be described here are the discovery of taste modulators and the possibility that certain bitter compounds are recognized by the umami receptor.

Positively selected disease response orthologous gene sets in the cereals identified using Sorghum bicolor L. Moench expression profiles and comparative genomics

Disease response genes (DRGs) diverge under recurrent positive selection as a result of a molecular arms race between hosts and pathogens. Most of these studies were conducted in animals, and few defense genes have been shown to evolve adaptively in plants. To test for adaptation in the molecules mediating disease resistance in the cereals, we first combined information from the expression pattern of Sorghum bicolor genes and from divergence to the full genome of rice to identify candidate DRGs. We then used evolutionary analyses of orthologous gene sets from several grass species, to determine whether the DRGs show signals of positive selection and the residues targeted. We found 140 divergent genes upregulated under biotic stress in S. bicolor by evaluating the relative abundance of expressed sequence tags in different libraries and comparing them with rice genes. For 10 of these genes, we found sets of orthologs including sequences from rice and three other cereals; six genes showed a pattern of substitution that was consistent with positive selection. Three of these genes, a thaumatin, a peroxidase, and a barley mlo homolog, are known antifungal proteins. The other three genes with evidence of positive selection were a MCM-1 agamous deficiens SRF- (MADS) box transcription factor, an eIF5 translation initiation factor, and a gene of unknown function but with evidence of expression during stress. Permutation analyses, using different ortholog and paralog sequences, consistently identified five positively selected codons in the peroxidase, a member of a cluster of genes and a large gene family. We mapped the positively selected residues onto the structure of the peroxidase and thaumatin and found that all sites are on the surface of these proteins and several are close to biochemically determined active sites. Identifying new positively selected plant disease resistance genes and the critical amino acid sites provides a basis for functional studies that may increase our understanding of their underlying molecular mechanisms of action. Additionally, it may lead to the identification of individuals having variation at functionally important sites, as well as eventually using this information in the rational design and engineering of proteins involved in plant disease resistance.

A preliminary neutron crystallographic study of thaumatin

A preliminary neutron crystallographic study of the sweet protein thaumatin is presented. Large hydrogenated crystals were prepared in deuterated crystallization buffer using the gel-acupuncture method. Data were collected to a resolution of 2 A on the LADI-III diffractometer at the Institut Laue Langevin (ILL). The results demonstrate the feasibility of a full neutron crystallographic analysis of this structure aimed at providing relevant information on the location of H atoms, the distribution of charge on the protein surface and localized water in the structure. This information will be of interest for understanding the specificity of thaumatin-receptor interactions and will contribute to further understanding of the molecular mechanisms underlying the perception of taste.

Different Structural Behaviors Evidenced in Thaumatin-Like Proteins: A Spectroscopic Study

Three proteins belonging to the thaumatin-like proteins family were compared in this study from a structural point of view: zeamatin, a new recently isolated PR-5 from Cassia didymobotrya and the commercial sweet-thaumatin. The former two proteins possess antifungal activities while commercial thaumatin is well known to be a natural sweetener. Intrinsic fluorescence studies have evidenced that the three proteins behave differently in unfolding experiments showing different structural rigidity. All the three proteins are more stable at slight acidic buffers, but sweet-thaumatin has a major tendency to destructurate itself. Similar observations were made from circular dichroism studies where a structural dependence relationship from the pH and the solvent used confirmed a hierarchic scale of stability for the three proteins. These structural differences should be considered to be significant for a functional role.

The sweet taste receptor: a single receptor with multiple sites and modes of interaction

Elucidation of the molecular bases of sweet taste is very important not only for its intrinsic biological significance but also for the design of new artificial sweeteners. Up to few years ago design was complicated by the common belief that different classes of sweet compounds, notably sweet proteins, might interact with different receptors altogether. The recent identification and functional expression of the receptor for sweet taste have shown that there is but one receptor, drastically changing our approach to the development of new sweeteners. The explanation of how the sweet receptor can bind several different classes of molecules is that rather than multiple receptors there are, apparently, multiple sites on the single sweet taste receptor. In this chapter, the mechanisms of interaction of small and macromolecular sweet molecules will be examined, with particular emphasis on sweet proteins. Systematic homology modeling yields reliable models of all possible heterodimers of the human T1R2 and T1R3 sequences with the closed (A) and open (B) conformations of one of the metabotropic glutamate receptors (mGluR1), used as template. The most important result of these studies is the "wedge model," the first explanation of the taste of sweet proteins. In addition, it was shown that simultaneous binding to the A and B sites is not possible with two large sweeteners but is possible with a small molecule in site A and a large one in site B. This observation accounted for the first time for the peculiar phenomenon of synergy between some sweeteners.

The history of sweet taste: not exactly a piece of cake

Understanding the molecular bases of sweet taste is of crucial importance not only in biotechnology but also for its medical implications, since an increasing number of people is affected by food-related diseases like, diabetes, hyperlipemia, caries, that are more or less directly linked to the secondary effects of sugar intake. Despite the interest paid to the field, it is only through the recent identification and functional expression of the receptor for sweet taste that new perspectives have been opened, drastically changing our approach to the development of new sweeteners. We shall give an overview of the field starting from the early days up to discussing the newest developments. After a review of early models of the active site, the mechanisms of interaction of small and macromolecular sweet molecules will be examined in the light of accurate modeling of the sweet taste receptor. The analysis of the homology models of all possible dimers allowed by combinations of the human T1R2 and T1R3 sequences of the sweet receptor and the closed (A) and open (B) conformations of the mGluR1 glutamate receptor shows that only 'type B' sites, either T1R2(B) and T1R3(B), can host the majority of small molecular weight sweeteners. Simultaneous binding to the A and B sites is not possible with two large sweeteners but is possible with a small molecule in site A and a large one in site B. This observation accounted for the first time for the peculiar phenomenon of synergy between some sweeteners. In addition to these two sites, the models showed an external binding site that can host sweet proteins.

Phylogenetic and structural relationships of the PR5 gene family reveal an ancient multigene family conserved in plants and select animal taxa

Pathogenesis-related group 5 (PR5) plant proteins include thaumatin, osmotin, and related proteins, many of which have antimicrobial activity. The recent discovery of PR5-like (PR5-L) sequences in nematodes and insects raises questions about their evolutionary relationships. Using complete plant genome data and discovery of multiple insect PR5-L sequences, phylogenetic comparisons among plants and animals were performed. All PR5/PR5-L protein sequences were mined from genome data of a member of each of two main angiosperm groups-the eudicots (Arabidoposis thaliana) and the monocots (Oryza sativa)-and from the Caenorhabditis nematode (C. elegans and C. briggsase). Insect PR5-L sequences were mined from EST databases and GenBank submissions from four insect orders: Coleoptera (Diaprepes abbreviatus and Biphyllus lunatus), Orthoptera (Schistocerca gregaria), Hymenoptera (Lysiphlebus testaceipes), and Hemiptera (Toxoptera citricida). Parsimony and Bayesian phylogenetic analyses showed that the PR5 family is paraphyletic in plants, likely arising from 10 genes in a common ancestor to monocots and eudicots. After evolutionary divergence of monocots and eudicots, PR5 genes increased asymmetrically among the 10 clades. Insects and nematodes contain multiple sequences (seven PR5-Ls in nematodes and at least three in some insects) all related to the same plant clade, with nematode and insect sequences separating as two clades. Protein structural homology modeling showed strong similarity among animal and plant PR5/PR5-Ls, with divergence only in surface-exposed loops. Sequence and structural conservation among PR5/PR5-Ls suggests an important and conserved role throughout the evolutionary divergence of the diverse organisms from which they reside.

Lessons from crystals grown in the Advanced Protein Crystallisation Facility for conventional crystallisation applied to structural biology

The crystallographic quality of protein crystals that were grown in microgravity has been compared to that of crystals that were grown in parallel on earth gravity under otherwise identical conditions. A goal of this comparison was to assess if a more accurate 3D-structure can be derived from crystallographic analysis of the former crystals. Therefore, the properties of crystals prepared with the Advanced Protein Crystallisation Facility (APCF) on earth and in orbit during the last decade were evaluated. A statistical analysis reveals that about half of the crystals produced under microgravity had a superior X-ray diffraction limit with respect of terrestrial controls. Eleven protein structures could be determined at previously unachieved resolutions using crystals obtained in the APCF. Microgravity induced features of the most relevant structures are reported. A second goal of this study was to identify the cause of the crystal quality enhancement useful for structure determination. No correlations between the effect of microgravity and other system-dependent parameters, such as isoelectric point or crystal solvent content, were found except the reduced convection during the crystallisation process. Thus, crystal growth under diffusive regime appears to be the key parameter explaining the beneficial effect of microgravity on crystal quality. The mimicry of these effects on earth in gels or in capillary tubes is discussed and the practical consequences for structural biology highlighted.

Recombinant curculin heterodimer exhibits taste-modifying and sweet-tasting activities

Curculin from Curculigo latifolia is a unique sweet protein that exhibits both sweet-tasting and taste-modifying activities. We isolated a gene that encodes a novel protein highly homologous to curculin. Using cDNAs of the previously known curculin (designated as curculin1) and the novel curculin isoform (curculin2), we produced a panel of homodimeric and heterodimeric recombinant curculins by Escherichia coli expression systems. It was revealed that sweet-tasting and taste-modifying activities were exhibited solely by the heterodimer of curculin1 and curculin2.

Cloning, expression and characterization of recombinant sweet-protein thaumatin II using the methylotrophic yeastPichia pastoris

Thaumatin, an intensely sweet-tasting protein, was secreted by the methylotrophic yeast Pichia pastoris. The mature thaumatin II gene was directly cloned from Taq polymerase-amplified PCR products by using TA cloning methods and fused the pPIC9K expression vector that contains Saccharomyces cerevisiae prepro alpha-mating factor secretion signal. Several additional amino acid residues were introduced at both the N- and C-terminal ends by genetic modification to investigate the role of the terminal end region for elicitation of sweetness in the thaumatin molecule. The secondary and tertiary structures of purified recombinant thaumatin were almost identical to those of the plant thaumatin molecule. Recombinant thaumatin II elicited a sweet taste as native plant thaumatin II; its threshold value of sweetness to humans was around 50 nM, which is the same as that of plant thaumatin II. These results demonstrate that the functional expression of thaumatin II was attained by Pichia pastoris systems and that the N- and C-terminal regions of the thaumatin II molecule do not -play an important role in eliciting the sweet taste of thaumatin.

Purification of an osmotin-like protein from the seeds of Benincasa hispida and cloning of the gene encoding this protein

A pathogenesis-related (PR) protein was purified from the seeds of Benincasa hispida, which is a medicinal plant and a member of the Cucurbitaceae family. Purification was achieved by using a procedure consisting of an acid treatment step followed by two chromatography steps. The protein is a basic protein with molecular mass of approximately 28 kDa. The sequences of the N-terminal 30 amino acids and four peptides generated from protease digestion were determined. These sequences indicated that the protein is an osmotin-like protein (OLP). Osmotin and OLPs are members of the thaumatin-like, PR-5 family of the PR proteins. A genomic clone of the gene encoding the protein was isolated and sequenced. The predicted protein has a signal peptide of 18 amino acids, and the mature protein has a molecular mass of 24.8 kDa with an isoelectric point of 7.67. The protein has 17 cysteine residues, of which 16 appear in the same positions as those appear in the sweet-tasting protein thaumatin and several other thaumatin-like proteins. Southern hybridization analysis indicated that the gene encoding the protein is a single copy gene. A computer-generated, three-dimensional model of the protein is presented.

Heterotrimeric G-proteins of a filamentous fungus regulate cell wall composition and susceptibility to a plant PR-5 protein

Membrane permeabilizing plant defensive proteins first encounter the fungal cell wall that can harbor specific components that facilitate or prevent access to the plasma membrane. However, signal transduction pathways controlling cell wall composition in filamentous fungi are largely unknown. We report here that the deposition of cell wall constituents that block the action of osmotin (PR-5), an antifungal plant defense protein, against Aspergillus nidulans requires the activity of a heterotrimeric G-protein mediated signaling pathway. The guanidine nucleotide GDPbetaS, that locks G-proteins in a GDP-bound inactive form, inhibits osmotin-induced conidial lysis. A dominant interfering mutation in FadA, the alpha-subunit of a heterotrimeric G-protein, confers resistance to osmotin. A deletion mutation in SfaD, the beta-subunit of a heterotrimeric G-protein also increases osmotin resistance. Aspergillus nidulans strains bearing these mutations also have increased tolerance to SDS, reduced cell wall porosity and increased chitin content in the cell wall.

Pearl millet cysteine protease inhibitor. Evidence for the presence of two distinct sites responsible for anti-fungal and anti-feedent activities

Recently, pearl millet cysteine protease inhibitor (CPI) was, for the first time, shown to possess anti-fungal activity in addition to its anti-feedent (protease inhibitory) activity [Joshi, B.N. et al. (1998) Biochem. Biophys. Res. Commun. 246, 382-387]. Characterization of CPI revealed that it has a reversible mode of action for protease inhibition. The CD spectrum exhibited a 35% alpha helix and 65% random coil structure. The intrinsic fluorescence spectrum was typical of a protein devoid of tryptophan residues. Demetallation of Zn2+ resulted in a substantial change in the secondary and tertiary structure of CPI accompanied by the complete loss of anti-fungal and inhibitory activity indicating that Zn2+ plays an important role in maintaining both structural integrity and biological function. The differential response of anti-fungal and inhibitory activities to specific modifiers showed that there are two different reactive sites associated with anti-fungal and anti-feedent activity in CPI located on a single protein as revealed from its N-terminal sequence data (AGVCYGVLGNNLP). Modification of cysteine, glutamic/aspartic acid or argnine resulted in abolition of the anti-fungal activity of CPI, whereas modification of arginine led to an enhancement of the inhibitory activity in solution. Modification of histidine resulted in a twofold increase in the protease inhibitory activity without affecting the anti-fungal activity, whereas modification of serine led to selective inhibition of the protease inhibitory activity. The differential nature of the two activities was further supported by differences in the temperature stabilities of the anti-fungal (60 degrees C) and inhibitory (40 degrees C) activities. Binding of papain to CPI did not abolish the anti-fungal activity of CPI, supporting the presence of two active sites on CPI. The differential behavior of CPI towards anti-fungal and anti-feedent activity cannot be attributed to changes in conformation, as assessed by their CD and fluorescence spectra. The interaction of CPI modified for arginine or histidine with papain resulted in an enhancement of CPI activity accompanied by a slight decrease in fluorescence intensity of 15-20% at 343 nm. In contrast, modification of serine resulted in inhibition of CPI activity with a concomitant increase of 20% in the fluorescence intensity when complexed by the enzyme. This implies the involvement of enzyme-based tryptophan in the formation of a biologically active enzyme-inhibitor complex. The presence of anti-fungal and anti-feedent activity on a single protein, as evidenced in pearl millet CPI, opens up a new possibility of raising a transgenic plant resistant to pathogens, as well as pests, by transfer of a single CPI gene.

Crystal structure of tobacco PR-5d protein at 1.8 A resolution reveals a conserved acidic cleft structure in antifungal thaumatin-like proteins

The crystal structure of tobacco PR-5d, an antifungal thaumatin-like protein isolated from cultured tobacco cells, was determined at the resolution of 1.8 A. The structure consists of 208 amino acid residues and 89 water molecules with a crystallographic R-factor of 0.169. The model has good stereochemistry, with respective root-mean-square deviations from the ideal values for bond and angle distances of 0.007 A and 1.542 degrees. Of the homologous PR-5 proteins, only those with antifungal activity had a common motif, a negatively charged surface cleft. This cleft is at the boundary between domains I and II, with a bottom part consisting of a three-stranded antiparallel beta-sheet in domain I. The acidic residues located in the hollow of the cleft form the beta-sheet region. Sequence and secondary structure analyses showed that the amino acid residues comprising the acidic cleft of PR-5d are conserved among other antifungal PR-5 proteins. This is the first report on the high-resolution crystal structure of an antifungal PR-5 protein. This structure provides insight into the function of pathogenesis-related proteins.

Osmotin, a plant antifungal protein, subverts signal transduction to enhance fungal cell susceptibility

The plant pathogenesis-related protein osmotin is an antifungal cytotoxic agent that causes rapid cell death in the yeast S. cerevisiae. We show here that osmotin uses a signal transduction pathway to weaken defensive cell wall barriers and increase its cytotoxic efficacy. The pathway activated by osmotin includes the regulatory elements of the mating pheromone response STE4, STE18, STE20, STE5, STE11, STE7, FUS3, KSS1, and STE12. Neither the pheromone receptor nor its associated G protein alpha subunit GPA1 are required for osmotin action. However, mutation of SST2, a negative regulator of G alpha proteins, resulted in supersensitivity to osmotin. Phosphorylation of STE7 was rapidly stimulated by osmotin preceding any changes in cell vitality or morphology. These results demonstrate that osmotin subverts target cell signal transduction as part of its mechanism of action.

Curculin, a sweet-tasting and taste-modifying protein, is a non-functional mannose-binding lectin

A three-dimensional model of curculin, a sweet-tasting and taste-modifying protein from the fruits of Curculigo latifolia, was built from the X-ray coordinates of GNA, a mannose-binding lectin from snowdrop (Galanthus nivalis). The three mannose-binding sites present in GNA were found in curculin but are devoid of mannose-binding activity as shown by docking experiments performed with mannose. Some regions well exposed on the surface of the three-dimensional model of curculin could act as epitopes responsible for the sweet-tasting properties of this protein.

Issues and advances in the use of transgenic organisms for the production of thaumatin, the intensely sweet protein from Thaumatococcus danielli

The thaumatins are a class of intensely sweet proteins isolated from the fruit of the tropical plant Thaumatococcus danielli. Thaumatin is approved for use in many countries and has application as both a flavor enhancer and a high-intensity sweetener. The supply of naturally occurring thaumatin is limited, which has prompted extensive research into its synthesis via transgenic organisms. The gene encoding thaumatin has been introduced into various microorganisms under transcriptional control of heterologous promoters. Yields to date have been low, but the factors governing more efficient microbial production have been identified. Continued research should allow microbial yields to be improved to commercially viable levels. The unique properties of thaumatin as a food additive could well be exploited by the food industry. Alternatively, the thaumatin gene could be engineered directly into selected fruit and vegetable crops to improve their flavor and sweetness.

Osmotin overexpression in potato delays development of disease symptoms

Transgenic potato and tobacco plants carrying the osmotin gene under the control of the cauliflower mosaic virus 35S promoter constitutively overexpressed osmotin to a level of approximately 2% of total cellular protein. Leaves of transgenic potato plants exhibited delayed development of disease symptoms after inoculation with spore suspensions of Phytophthora infestans, which is the cause of late blight disease of potato. In contrast, transgenic tobacco plants did not display any change in the development of disease symptoms when challenged with either spore suspensions or fungal mycelia of Phytophthora parasitica var. nicotianae. Using in vitro assays, purified osmotin was found to be more effective against P. infestans. Some inhibition of P. parasitica also was observed in vitro even though no in vivo effect could be established.

Recognition of errors in three-dimensional structures of proteins

A major problem in the determination of the three-dimensional structure of proteins concerns the quality of the structural models obtained from the interpretation of experimental data. New developments in X-ray crystallography and nuclear magnetic resonance spectroscopy have accelerated the process of structure determination and the biological community is confronted with a steadily increasing number of experimentally determined protein folds. However, in the recent past several experimentally determined protein structures have been proven to contain major errors, indicating that in some cases the interpretation of experimental data is difficult and may yield incorrect models. Such problems can be avoided when computational methods are employed which complement experimental structure determinations. A prerequisite of such computational tools is that they are independent of the parameters obtained from a particular experiment. In addition such techniques are able to support and accelerate experimental structure determinations. Here we present techniques based on knowledge based mean fields which can be used to judge the quality of protein folds. The methods can be used to identify misfolded structures as well as faulty parts of structural models. The techniques are even applicable in cases where only the C alpha trace of a protein conformation is available. The capabilities of the technique are demonstrated using correct and incorrect protein folds.

Ultrastructural evidence for a binding substance to the sweet-tasting protein thaumatin inside taste bud pores of rhesus monkey foliate papillae

Thaumatin is a protein that tastes intensely sweet only to Old World monkeys and to higher primates, including man. Here we used pre-embedding ultrastructural methods to study the distribution of thaumatin in apical regions of Rhesus monkey foliate papillae, using thaumatin conjugated to 5 nm gold particles. With freeze-substitution we saw that gold-labeled thaumatin bound to an electron-opaque, sponge-like secretory substance inside the taste bud pores. Labeled thaumatin was found at the surface of the secretory substance even deep inside the pore, where other, unlabeled cellular structures surrounded the substance. With freeze-fracture deep-etching the secretory substance that bound the thaumatin-gold particles appeared coarsely granular. There was no labeling of any other taste bud pore structure, including microvilli and small membrane-lined vesicles. Pre-incubation with an excess of unlabeled thaumatin inhibited binding with gold-labeled thaumatin. The results suggest that the secretory substance had the greatest affinity of all taste pore structures to the sweet-tasting compound under our experimental conditions. Therefore, gustatory reception probably involves various taste compound binding structures, microvilli, and also secretory substances like the one described here which bound thaumatin. We speculate that the secretory substance may bind taste stimuli and serve as an intermediate between stimuli and receptors. It could be involved in stimulus removal or delivery or both.