Ustilago maydis KP6 killer toxin: structure, expression in Saccharomyces cerevisiae, and relationship to other cellular toxins

The revenge of Zygosaccharomyces yeasts in food biotechnology and applied microbiology

Non-conventional yeasts refer to a huge and still poorly explored group of species alternative to the well-known model organism Saccharomyces cerevisiae. Among them, Zygosaccharomyces rouxii and the sister species Zygosaccharomyces bailii are infamous for spoiling food and beverages even in presence of several food preservatives. On the other hand, their capability to cope with a wide range of process conditions makes these yeasts very attractive factories (the so-called "ZygoFactories") for bio-converting substrates poorly permissive for the growth of other species. In balsamic vinegar Z. rouxii is the main yeast responsible for converting highly concentrated sugars into ethanol, with a preference for fructose over glucose (a trait called fructophily). Z. rouxii has also attracted much attention for the ability to release important flavor compounds, such as fusel alcohols and the derivatives of 4-hydroxyfuranone, which markedly contribute to fragrant and smoky aroma in soy sauce. While Z. rouxii was successfully proposed in brewing for producing low ethanol beer, Z. bailii is promising for lactic acid and bioethanol production. Recently, several research efforts exploited omics tools to pinpoint the genetic bases of distinctive traits in "ZygoFactories", like fructophily, tolerance to high concentrations of sugars, lactic acid and salt. Here, I provided an overview of Zygosaccharomyces industrially relevant phenotypes and summarized the most recent findings in disclosing their genetic bases. I suggest that the increasing number of genomes available for Z. rouxii and other Zygosaccharomyces relatives, combined with recently developed genetic engineering toolkits, will boost the applications of these yeasts in biotechnology and applied microbiology.

Propeptide genesis by Kex2-dependent cleavage of yeast wall protein 1 (Ywp1) of Candida albicans

Candida albicans is a prevalent fungal resident and opportunistic pathogen of humans, exhibiting a variety of ovoid and filamentous morphologies. Anchored within the cell wall of the ovoid yeast form of C. albicans is an abundant glycoprotein termed yeast wall protein 1 (Ywp1). Ywp1 has an antiadhesive effect that may facilitate yeast cell dispersal; it also contributes to the masking of the glucan matrix of the yeast cell wall, potentially providing shielding from recognition by the human immune system. Mature Ywp1 consists of an O-glycosylated core of 378 amino acids associated with an N-glycosylated propeptide that originates from an N-terminal segment of Ywp1. A tribasic (-RRR-) sequence in the immature Ywp1 polypeptide is separated by 8 amino acids from a dibasic (-KR-) sequence that is a canonical site for cleavage by the intracellular endopeptidase Kex2, and cleavage occurs at both of these sites to generate an 11 kilodalton (kDa) propeptide that remains strongly associated with the mature core of Ywp1. Previous studies demonstrated an absence of the 11 kDa propeptide in strains lacking Kex2, but the presence of lesser amounts of a 12 kDa propeptide ostensibly (and paradoxically) arising from cleavage at the dibasic site. Subsequent studies of wild type strains, however, suggested that post-secretion cleavages were carried out in vitro by acid proteases in unbuffered cultures to generate the 12 kDa propeptide. Here, intact and Gfp-tagged Ywp1 are utilized to show that neither of the two multibasic sites is normally cleaved in the absence of Kex2, but that uncleaved Ywp1 is still N-glycosylated and subsequently anchored to the cell wall. This furthers our understanding of the multistep cleavage of this highly conserved sequence, as well as the possible contributions of the cleaved propeptide to the maturation and functioning of Ywp1.

Fungal Secretome Analysis via PepSAVI-MS: Identification of the Bioactive Peptide KP4 from Ustilago maydis

Fungal secondary metabolites represent a rich and largely untapped source for bioactive molecules, including peptides with substantial structural diversity and pharmacological potential. As methods proceed to take a deep dive into fungal genomes, complimentary methods to identify bioactive components are required to keep pace with the expanding fungal repertoire. We developed PepSAVI-MS to expedite the search for natural product bioactive peptides and herein demonstrate proof-of-principle applicability of the pipeline for the discovery of bioactive peptides from fungal secretomes via identification of the antifungal killer toxin KP4 from Ustilago maydis P4. This work opens the door to investigating microbial secretomes with a new lens, and could have broad applications across human health, agriculture, and food safety. Graphical Abstract.

Enemies with benefits: mutualistic interactions of viruses with lower eukaryotes

Viruses represent some of the deadliest pathogens known to science. Recently they have been reported to have mutualistic interactions with their hosts, providing them direct or indirect benefits. The mutualism and symbiogenesis of such viruses with lower eukaryotic partners such as fungi, yeast, and insects have been reported but the full mechanism of interaction often remains an enigma. In many instances, these viral interactions provide resistance against several biotic and abiotic stresses, which could be the prime reason for the ecological success and positive selection of the hosts. These viruses modulate host metabolism and behavior, so both can obtain maximum benefits from the environment. They bring about micro- and macro-level changes in the hosts, benefiting their adaptation, reproduction, development, and survival. These virus-host interactions can be bilateral or tripartite with a variety of interacting partners. Exploration of these interactions can shed light on one of the well-coordinated biological phenomena of co-evolution and can be highly utilized for various applications in agriculture, fermentation and the pharmaceutical industries.

The Biology of Pichia membranifaciens Killer Toxins

The killer phenomenon is defined as the ability of some yeast to secrete toxins that are lethal to other sensitive yeasts and filamentous fungi. Since the discovery of strains of Saccharomyces cerevisiae capable of secreting killer toxins, much information has been gained regarding killer toxins and this fact has substantially contributed knowledge on fundamental aspects of cell biology and yeast genetics. The killer phenomenon has been studied in Pichia membranifaciens for several years, during which two toxins have been described. PMKT and PMKT2 are proteins of low molecular mass that bind to primary receptors located in the cell wall structure of sensitive yeast cells, linear (1→6)-β-d-glucans and mannoproteins for PMKT and PMKT2, respectively. Cwp2p also acts as a secondary receptor for PMKT. Killing of sensitive cells by PMKT is characterized by ionic movements across plasma membrane and an acidification of the intracellular pH triggering an activation of the High Osmolarity Glycerol (HOG) pathway. On the contrary, our investigations showed a mechanism of killing in which cells are arrested at an early S-phase by high concentrations of PMKT2. However, we concluded that induced mortality at low PMKT2 doses and also PMKT is indeed of an apoptotic nature. Killer yeasts and their toxins have found potential applications in several fields: in food and beverage production, as biocontrol agents, in yeast bio-typing, and as novel antimycotic agents. Accordingly, several applications have been found for P. membranifaciens killer toxins, ranging from pre- and post-harvest biocontrol of plant pathogens to applications during wine fermentation and ageing (inhibition of Botrytis cinerea, Brettanomyces bruxellensis, etc.).

KP4 to control Ustilago tritici in wheat: Enhanced greenhouse resistance to loose smut and changes in transcript abundance of pathogen related genes in infected KP4 plants

Ustilago tritici causes loose smut, which is a seed-borne fungal disease of wheat, and responsible for yield losses up to 40%. Loose smut is a threat to seed production in developing countries where small scale farmers use their own harvest as seed material. The killer protein 4 (KP4) is a virally encoded toxin from Ustilago maydis and inhibits growth of susceptible races of fungi from the Ustilaginales. Enhanced resistance in KP4 wheat to stinking smut, which is caused by Tilletia caries, had been reported earlier. We show that KP4 in genetically engineered wheat increased resistance to loose smut up to 60% compared to the non-KP4 control under greenhouse conditions. This enhanced resistance is dose and race dependent. The overexpression of the transgene kp4 and its effect on fungal growth have indirect effects on the expression of endogenous pathogen defense genes.

The atomic structure of the virally encoded antifungal protein, KP6

Killer toxins are produced by several genera of yeast and filamentous fungi. A small proportion of Ustilago maydis strains produce killer toxins, to which they are resistant, but sensitive strains are the majority in the wild populations. There are three killer types (P1, P4 and P6) that secrete KP1, KP4 and KP6 toxins, respectively, which are produced only by strains persistently infected with double-stranded RNA viruses (UmV) in the cell cytoplasm. Unlike nearly all other viruses, UmV are only transmitted through mitosis or meiosis. As shown here, KP6 is different from any other known cytotoxic protein. KP6 is neutral protein composed of two subunits: KP6α and KP6β. KP6α is responsible for targeting while KP6β is cytotoxic. Neither subunit is homologous in either sequence or structure to any other toxin, but they have highly similar structures to each other. The major difference between the two subunits is a hydrophobic helix at the N-terminus of KP6α and is likely key to target recognition. Unlike any other toxin, KP6 is translated as a single polypeptide with a 31-residue linker region in the middle of the protein. From structural prediction studies, this linker likely makes for a more compact KP6 structure that sequesters the hydrophobic helix of KP6α. A model whereby the protoxin undergoes a conformational activation process that exposes this helix immediately prior to secretion is presented.

Subtilases - versatile tools for protein turnover, plant development, and interactions with the environment

Subtilases (SBTs) constitute a large family of serine peptidases. They are commonly found in Archaea, Bacteria and Eukarya, with many more SBTs in plants as compared to other organisms. The expansion of the SBT family in plants was accompanied by functional diversification, and novel, plant-specific physiological roles were acquired in the course of evolution. In addition to their contribution to general protein turnover, plant SBTs are involved in the development of seeds and fruits, the manipulation of the cell wall, the processing of peptide growth factors, epidermal development and pattern formation, plant responses to their biotic and abiotic environment, and in programmed cell death. Plant SBTs share many properties with their bacterial and mammalian homologs, but the adoption of specific roles in plant physiology is also reflected in the acquisition of unique biochemical and structural features that distinguish SBTs in plants from those in other organisms. In this article we provide an overview of the earlier literature on the discovery of the first SBTs in plants, and highlight recent findings with respect to their physiological relevance, structure and function.

Molecular cloning, structural analysis and modelling of the AcAFP antifungal peptide from Aspergillus clavatus

An abundantly secreted thermostable peptide (designed AcAFP) with a molecular mass of 5777 Da was isolated and purified in a previous work from a local strain of A. clavatus (VR1). Based on the N-terminal amino acid (aa) sequence of the AcAFP peptide, an oligonucleotide probe was derived and allowed the amplification of the encoding cDNA by RT-PCR. This cDNA fragment encodes a pre-pro-protein of 94 aa which appears to be processed to a mature product of 51 aa cys-rich protein. The deduced aa sequence of the pre-pro-sequence reveals high similarity with ascomycetes antifungal peptide. Comparison of the nucleotide sequence of the genomic fragment and the cDNA clone revealed the presence of an open reading frame of 282 bp interrupted by two small introns of 89 and 56 bp with conserved splice site. The three-dimensional (3D) structure modeling of AcAFP exhibits a compact structure consisting of five anti-parallel beta barrel stabilized by four internal disulfide bridges. The folding pattern revealed also a cationic site and spatially adjacent hydrophobic stretch. The antifungal mechanism was investigated by transmission and confocal microscopy. AcAFP cause cell wall altering in a dose-dependent manner against the phytopathogenic fungus Fusarium oxysporum.

Increased field resistance to Tilletia caries provided by a specific antifungal virus gene in genetically engineered wheat

The field performance of a viral gene in two Swiss wheat (Triticum aestivum) varieties showed 10% increased fungal resistance against Tilletia caries (stinking smut). To the best of our knowledge, this is the first report of improved resistance against any fungus in the field achieved by genetic engineering in wheat. The genetically modified wheat lines previously showed a c. 30% decrease in symptoms of T. caries in the glasshouse (Clausen, M., Kräuter, R., Schachermayr, G., Potrykus, I. and Sautter, C. (2000) Antifungal activity of a virally encoded gene in transgenic wheat. Nat. Biotechnol. 18, 446-449), depending on the fungal strain inoculated. A glasshouse experiment run in parallel to the field test, and using the same collection of T. caries, gave the same results. In a dose-response experiment with isolated fungal strains, in which the infection pressure was varied via the spore concentration, the transgene behaved as a quantitative resistance gene and shifted the S-shaped dose-response curve towards higher resistance. The transgene was shown to be highly specific for fungi of the order Ustilaginales. Tests of the transgene using cell cultures of eukaryotes, including hamster and human, showed no significant side-effects with respect to biosafety. Endogenous pathogen-related genes were also activated on fungal infection in the presence of the kp4 transgene.

Proper folding of the antifungal protein PAF is required for optimal activity

The Penicillium chrysogenumantifungal protein PAF is secreted into the supernatant after elimination of a preprosequence. PAF is actively internalized into the hyphae of sensitive molds and provokes growth retardation as well as changes in morphology. Thus far, no information is available on the exact mode of action of PAF, nor on the function of its prosequence in protein activity. Therefore, we sought to investigate the effects of secreted PAF as well as of intracellularly retained pro-PAF and mature PAF on the sensitive ascomycete Aspergillus nidulans, and transformed this model organism by expression vectors containing 5'-sequentially truncated paf-coding sequences under the control of the inducible P. chrysogenum-derived xylanase promoter. Indirect immunofluorescence staining revealed the localization of recombinant PAF predominantly in the hyphal tips of the transformant Xylpaf1 which expressed prepro-PAF, whereas the protein was found to be distributed intracellularly within all segments of hyphae of the transformants Xylpaf2 and Xylpaf3 which expressed pro-PAF and mature PAF, respectively. Growth retardation of Xylpaf1 and Xylpaf3 hyphae was detected by proliferation assays and by light microscopy analysis. Using transmission electron microscopy of ultrathin hyphal sections a marked alteration of the mitochondrial ultrastructure in Xylpaf1 was observed and an elevated amount of carbonylated proteins pointed to severe oxidative stress in this strain. The effects induced by secreted recombinant PAF resembled those evoked by native PAF. The results give evidence that properly folded PAF is a prerequisite for its activity.

The Zygosaccharomyces bailii antifungal virus toxin zygocin: cloning and expression in a heterologous fungal host

Zygocin, a monomeric protein toxin secreted by a virus-infected killer strain of the osmotolerant spoilage yeast Zygosaccharomyces bailii, kills a broad spectrum of human and phytopathogenic yeasts and filamentous fungi by disrupting cytoplasmic membrane function. The toxin is encoded by a double-stranded (ds)RNA killer virus (ZbV-M, for Z. bailii virus M) that stably persists within the yeast cell cytosol. In this study, the protein toxin was purified, its N-terminal amino acid sequence was determined, and a full-length cDNA copy of the 2.1 kb viral dsRNA genome was cloned and successfully expressed in a heterologous fungal system. Sequence analysis as well as zygocin expression in Schizosaccharomyces pombe indicated that the toxin is in vivo expressed as a 238-amino-acid preprotoxin precursor (pptox) consisting of a hydrophobic N-terminal secretion signal, followed by a potentially N-glycosylated pro-region and terminating in a classical Kex2p endopeptidase cleavage site that generates the N-terminus of the mature and biologically active protein toxin in a late Golgi compartment. Matrix-assisted laser desorption mass spectrometry further indicated that the secreted toxin is a monomeric 10.4 kDa protein lacking detectable post-translational modifications. Furthermore, we present additional evidence that in contrast with other viral antifungal toxins, zygocin immunity is not mediated by the toxin precursor itself and, therefore, heterologous pptox expression in a zygocin-sensitive host results in a suicidal phenotype. Final sequence comparisons emphasize the conserved pattern of functional elements present in dsRNA killer viruses that naturally infect phylogenetically distant hosts (Saccharomyces cerevisiae and Z. bailii) and reinforce models for the sequence elements that are in vivo required for viral RNA packaging and replication.

The viral killer system in yeast: from molecular biology to application

Since the initial discovery of the yeast killer system almost 40 years ago, intensive studies have substantially strengthened our knowledge in many areas of biology and provided deeper insights into basic aspects of eukaryotic cell biology as well as into virus-host cell interactions and general yeast virology. Analysis of killer toxin structure, synthesis and secretion has fostered understanding of essential cellular mechanisms such as post-translational prepro-protein processing in the secretory pathway. Furthermore, investigation of the receptor-mediated mode of toxin action proved to be an effective means for dissecting the molecular structure and in vivo assembly of yeast and fungal cell walls, providing important insights relevant to combating infections by human pathogenic yeasts. Besides their general importance in understanding eukaryotic cell biology, killer yeasts, killer toxins and killer viruses are also becoming increasingly interesting with respect to possible applications in biomedicine and gene technology. This review will try to address all these aspects.

Mutational analysis of K28 preprotoxin processing in the yeast Saccharomyces cerevisiae

K28 killer strains of Saccharomyces cerevisiae are permanently infected with a cytoplasmic persisting dsRNA virus encoding a secreted alpha/beta heterodimeric protein toxin that kills sensitive cells by cell-cycle arrest and inhibition of DNA synthesis. In vivo processing of the 345 aa toxin precursor (preprotoxin; pptox) involves multiple internal and carboxy-terminal cleavage events by the prohormone convertases Kex2p and Kex1p. By site-directed mutagenesis of the preprotoxin gene and phenotypic analysis of its in vivo effects it is now demonstrated that secretion of a biological active virus toxin requires signal peptidase cleavage after Gly(36) and Kex2p-mediated processing at the alpha subunit N terminus (after Glu-Arg(49)), the alpha subunit C terminus (after Ser-Arg(149)) and at the beta subunit N terminus (after Lys-Arg(245)). The mature C terminus of the beta subunit is trimmed by Kex1p, which removes the terminal Arg(345) residue, thus uncovering the toxin's endoplasmic reticulum targeting signal (HDEL) which--in a sensitive target cell--is essential for retrograde toxin transport. Interestingly, both toxin subunits are covalently linked by a single disulfide bond between alpha-Cys(56) and beta-Cys(340), and expression of a mutant toxin in which beta-Cys(340) had been replaced by Ser(340) resulted in the secretion of a non-toxic alpha/beta heterodimer that is blocked in retrograde transport and incapable of entering the yeast cell cytosol, indicating that one important in vivo function of beta-Cys(340) might be to ensure accessibility of the toxin's beta subunit C terminus to the HDEL receptor of the target cell.

The Ustilaginales as plant pests and model systems

The Ustilaginales are a vast and diverse group of fungi, which includes the plant pathogenic smuts that cause significant losses to crops worldwide. Members of the Ustilaginales are also valuable models for the unraveling of fundamental mechanisms controlling important biological processes. Ustilago maydis is an important fungal model system and has been well studied with regard to mating, morphogenesis, pathogenicity, signal transduction, mycoviruses, DNA recombination, and, recently, genomics. In this review we discuss the life cycles of members of the Ustilaginales and provide background on their economic impact as agricultural pests. We then focus on providing a summary of the literature with special attention to topics not well covered in recent reviews such as the use of U. maydis in mycovirus research and as a model for understanding the molecular mechanisms of fungicide resistance and DNA recombination and repair.

Antifungal activity of a virally encoded gene in transgenic wheat

The cDNA encoding the antifungal protein KP4 from Ustilago maydis-infecting virus was inserted behind the ubiquitin promoter of maize and genetically transferred to wheat varieties particularly susceptible to stinking smut (Tilletia tritici) disease. The transgene was integrated and inherited over several generations. Of seven transgenic lines, three showed antifungal activity against U. maydis. The antifungal activity correlated with the presence of the KP4 transgene. KP4-transgenic, soil-grown wheat plants exhibit increased endogenous resistance against stinking smut.

Structure of Ustilago maydis killer toxin KP6 alpha-subunit. A multimeric assembly with a central pore

Ustilago maydis is a fungal pathogen of maize, some strains of which secrete killer toxins. The toxins are encoded by double-stranded RNA viruses in the cell cytoplasm. The U. maydis killer toxin KP6 contains two polypeptide chains, alpha and beta, having 79 and 81 amino acids, respectively, both of which are necessary for its killer activity. The crystal structure of the alpha-subunit of KP6 (KP6alpha) has been determined at 1.80-A resolution. KP6alpha forms a single domain structure that has an overall shape of an ellipsoid with dimensions 40 A x 26 A x 21 A and belongs to the alpha/beta-sandwich family. The tertiary structure consists of a four-stranded antiparallel beta-sheet, a pair of antiparallel alpha-helices, a short strand along one edge of the sheet, and a short N-terminal helix. Although the fold is reminiscent of toxins of similar size, the topology of KP6alpha is distinctly different in that the alpha/beta-sandwich motif has two right-handed betaalphabeta split crossovers. Monomers of KP6alpha assemble through crystallographic symmetries, forming a hexamer with a central pore lined by hydrophobic N-terminal helices. The central pore could play an important role in the mechanism of the killing action of the toxin.

Functional analysis of a Golgi-localized Kex2p-like protease in tobacco suspension culture cells

Kex2p is the prototype of a Golgi-resident protease responsible for the processing of prohormones in yeast and mammalian cells. A Kex2p-like pathway was shown to be responsible for processing the fungal KP6 protoxin in transgenic tobacco plants. We previously described a chimeric integral membrane reporter protein that traffics through Golgi to the lytic prevacuole where it was proteolytically processed. As a first step to isolate and clone the Kex2p-like protease in plant cells, we designed and used a similar chimeric reporter protein containing Kex2 cleavage sites to assay the Kex2p-like activity and to determine its substrate specificity in tobacco cells. Here we demonstrate that the Kex2 cleavage sites of the reporter were specifically processed by a protease activity with a substrate specificity characteristic of yeast Kex2p. This Kex2p-like protease in tobacco cells is also a Golgi-resident enzyme. Thus, the reporter protein provides a biochemical marker for studying protein traffic through the Golgi in plant cells. These results additionally should allow the design of synthetic substrates for use in biochemical purification of the plant enzyme.

Integral membrane protein sorting to vacuoles in plant cells: evidence for two pathways

Plant cells may contain two functionally distinct vacuolar compartments. Membranes of protein storage vacuoles (PSV) are marked by the presence of alpha-tonoplast intrinsic protein (TIP), whereas lytic vacuoles (LV) are marked by the presence of gamma-TIP. Mechanisms for sorting integral membrane proteins to the different vacuoles have not been elucidated. Here we study a chimeric integral membrane reporter protein expressed in tobacco suspension culture protoplasts whose traffic was assessed biochemically by following acquisition of complex Asn-linked glycan modifications and proteolytic processing, and whose intracellular localization was determined with confocal immunofluorescence. We show that the transmembrane domain of the plant vacuolar sorting receptor BP-80 directs the reporter protein via the Golgi to the LV prevacuolar compartment, and attaching the cytoplasmic tail (CT) of gamma-TIP did not alter this traffic. In contrast, the alpha-TIP CT prevented traffic of the reporter protein through the Golgi and caused it to be localized in organelles separate from ER and from Golgi and LV prevacuolar compartment markers. These organelles had a buoyant density consistent with vacuoles, and alpha-TIP protein colocalized in them with the alpha-TIP CT reporter protein when the two were expressed together in protoplasts. These results are consistent with two separate pathways to vacuoles for membrane proteins: a direct ER to PSV pathway, and a separate pathway via the Golgi to the LV.

The novel acidophilic structure of the killer toxin from halotolerant yeast demonstrates remarkable folding similarity with a fungal killer toxin

BACKGROUND

Several strains of yeasts and fungi produce proteinous substances, termed killer toxins, which kill sensitive strains. The SMK toxin, secreted by the halotolerant yeast Pichia farinosa KK1 strain, uniquely exhibits its maximum killer activity under conditions of acidic pH and high salt concentration. The toxin is composed of two distinct subunits, alpha and beta, which tightly interact with each other under acidic conditions. However, they are easily dissociated under neutral conditions and lose the killer activity. The three-dimensional structure of the SMK toxin will provide a better understanding of the mechanism of toxicity of this protein and the cause of its unique pH-dependent stability.

RESULTS

Two crystal structures of the SMK toxin have been determined at 1.8 A resolution in different ionic strength conditions. The two subunits, alpha and beta, are jointly folded into an ellipsoidal, single domain structure belonging to the alpha/beta-sandwich family. The folding topology of the SMK toxin is essentially the same as that of the fungal killer toxin, KP4. This shared topology contains two left-handed split betaalphabeta motifs, which are rare in the other proteins. Many acidic residues are clustered at the bottom of the SMK toxin molecule. Some of the carboxyl sidechains interact with each other through hydrogen bonds. The ionic strength difference induces no evident structural change of the SMK toxin except that, in the high ionic strength crystal, a number of sulfate ions are electrostatically bound near the basic residues which are also locally distributed at the bottom of the toxin molecule.

CONCLUSIONS

The two killer toxins, SMK and KP4, share a unique folding topology which contains a rare structural motif. This observation may suggest that these toxins are evolutionally and/or functionally related. The pH-dependent stability of the SMK toxin is a result of the intensive interactions between the carboxyl groups. This finding is important for protein engineering, for instance, towards stabilization of the toxin molecule in a broader pH range. The present crystallographic study revealed that the structure of the SMK toxin itself is hardly affected by the ionic strength, implying that a high salt concentration affects the sensitivity of the cell against the toxin.

The Ustilago maydis virally encoded KP1 killer toxin

Some strains of the plant-pathogenic fungus Ustilago maydis secrete toxins (killer toxins) that are lethal to susceptible strains of the same fungus. There are three well-characterized killer toxins in U. maydis-KP1, KP4, and KP6-which are secreted by the P1, P4, and P6 subtypes, respectively. These killer toxins are small polypeptides encoded by segments of an endogenous, persistent double-stranded RNA (dsRNA) virus in each U. maydis subtype. In P4 and P6, the M2 dsRNA segment encodes the toxin. In this work, the KP1 killer toxin was purified for internal amino acid sequence analysis, and P1M2 was identified as the KP1 toxin-encoding segment by sequence analysis of cDNA clones. The KP1 toxin is a monomer with a predicted molecular weight of 13.4kDa and does not have extensive sequence similarity with other viral anti-fungal toxins. The P1M2 segment is different from the P4 and P6 toxin-encoding dsRNA segments in that the 3' non-coding region of its plus strand has no sequence homology to the 3' ends of the plus strands of P1M1, P4M2, or P6M2.

High-level secretion of a virally encoded anti-fungal toxin in transgenic tobacco plants

Ustilago maydis killer toxins are small polypeptides (7-14 kDa) which kill susceptible cells of closely related fungal species. The KP4 toxin is a single polypeptide subunit with a molecular weight of 11.1 kDa. In this work, a transgenic tobacco plant was constructed which secretes the KP4 toxin at a high level. The KP4 toxin expressed in this transgenic plant was of the same size and specificity as the authentic Ustilago KP4 toxin. The expression level was at least 500 times higher than that of the KP6 toxin expressed in plants. Transgenic crop plants producing the KP4 toxin could be rendered resistant to KP4-susceptible fungal pathogens.

Cloning, structural organization and regulation of expression of the Penicillium chrysogenum paf gene encoding an abundantly secreted protein with antifungal activity

An abundantly secreted, highly basic 12-kDa protein (PAF) was purified from the culture medium of Penicillium chrysogenum (Pc). Based on the N-terminal amino acid (aa) sequence of the protein, an oligodeoxyribonucleotide probe was derived and used for amplification of the encoding cDNA by PCR. This cDNA fragment encodes a Cys-rich preproprotein of 92 aa which appears to be processed to a mature product of 55 aa. The deduced aa sequence of the preproprotein reveals 42.6% identity to an antifungal protein (AFP) of Aspergillus giganteus. Agar diffusion tests confirmed that the Pc protein exhibits antifungal activity. In order to investigate the promoter region and the structural organization of the paf gene, a genomic 6-kb fragment was isolated and partially sequenced. Comparison of the nucleotide sequence of the genomic fragment and the cDNA clone revealed the presence of a coding region of 279 bp which is interrupted by two introns of 76 and 68 bp in length. In the promoter region, a typical TATA box, a motif resembling the fungal carbon catabolite repression element, as well as several putative GATA factor binding motifs, were found. Northern blot analysis indicated that the regulation of paf expression occurs at the level of mRNA transcription and is under control of carbon catabolite and nitrogen metabolite repression regulatory circuits.

Structure and function of a virally encoded fungal toxin from Ustilago maydis: a fungal and mammalian Ca2+ channel inhibitor

BACKGROUND

The P4 strain of the corn smut fungus, Ustilago maydis, secretes a fungal toxin, KP4, encoded by a fungal virus (UMV4) that persistently infects its cells. UMV4, unlike most other (non-fungal) viruses, does not spread to uninfected cells by release into the extracellular milieu during its normal life cycle and is thus dependent upon host survival for replication. In symbiosis with the host fungus, UMV4 encodes KP4 to kill other competitive strains of U. maydis, thereby promoting both host and virus survival. KP4 belongs to a family of fungal toxins and determining its structure should lead to a better understanding of the function and evolutionary origins of these toxins. Elucidation of the mechanism of toxin action could lead to new anti-fungal agents against human pathogens.

RESULTS

We have determined the atomic structure of KP4 to 1.9 A resolution. KP4 belongs to the alpha/beta-sandwich family, and has a unique topology comprising a five-stranded antiparallel beta-sheet with two antiparallel alpha-helices lying at approximately 45 degrees to these strands. The structure has two left-handed beta alpha beta cross-overs and a basic protuberance extending from the beta-sheet. In vivo experiments demonstrated abrogation of toxin killing by Ca2+ and, to a lesser extent, Mg2+. These results led to experiments demonstrating that the toxin specifically inhibits voltage-gated Ca2+ channels in mammalian cells.

CONCLUSIONS

Similarities, although somewhat limited, between KP4 and scorpion toxins led us to investigate the possibility that the toxic effects of KP4 may be mediated by inhibition of cation channels. Our results suggest that certain properties of fungal Ca2+ channels are homologous to those in mammalian cells. KP4 may, therefore, be a new tool for studying mammalian Ca2+ channels and current mammalian Ca2+ channel inhibitors may be useful lead compounds for new anti-fungal agents.

Molecular characterization and heterologous expression of an endo-beta-1,6-glucanase gene from the mycoparasitic fungus Trichoderma harzianum

Hydrolytic enzymes from the filamentous fungus Trichoderma harzianum have been described as critical elements of the mycoparasitic action of Trichoderma against fungal plant pathogens. In this report we describe the first genomic and cDNA clones encoding a beta-1,6-endoglucanase gene. The deduced protein sequence has limited homology with other beta-glucanases. Northern experiments show a marked repression of mRNA accumulation by glucose. The protein has been successfully produced in Saccharomyces cerevisiae upon construction of a transcriptional fusion of the cDNA with a yeast promoter. This S. cerevisiae recombinant strain shows a strong lytic action on agar plates containing beta-1,6-glucan.

Processing and secretion of a virally encoded antifungal toxin in transgenic tobacco plants: evidence for a Kex2p pathway in plants

Ustilago maydis is a fungal pathogen of maize. Some strains of U. maydis encode secreted polypeptide toxins capable of killing other susceptible strains of U. maydis. We show here that one of these toxins, the KP6 killer toxin, is synthesized by transgenic tobacco plants containing the viral toxin cDNA under the control of a cauliflower mosaic virus promoter. The two components of the KP6 toxin, designated alpha and beta, with activity and specificity identical to those found in toxin secreted by U. maydis cells, were isolated from the intercellular fluid of the transgenic tobacco plants. The beta polypeptide from tobacco was identical in size and N-terminal sequence to the U. maydis KP6 beta polypeptide. Processing of the KP6 preprotoxin in U. maydis requires a subtilisin-like processing protease, Kex2p, which is present in both animal and fungal cells and is required for processing of (among other things) small secreted polypeptide hormones and secreted toxins. Our findings present evidence for Kex2p-like processing activity in plants. The systemic production of this viral killer toxin in crop plants may provide a new method of engineering biological control of fungal pathogens in crop plants.

The characterization and crystallization of a virally encoded Ustilago maydis KP4 toxin

KP4 is a virally encoded and highly specific toxin that kills fungi closely related to the fungus Ustilago maydis. The toxin was purified and crystals were formed using ammonium sulfate as precipitant. The crystals belong to the space group P6(1)(5)22 and diffracted to approximately 2.2 A resolution. Circular dicroism spectroscopy suggests that the protein is predominantly comprised of beta-strands.

Molecular cloning, sequence analysis and expression of the gene encoding an antifungal-protein from Aspergillus giganteus

The gene encoding the precursor of a small secretory protein with antifungal activity was isolated from A. giganteus and characterized by restriction mapping, hybridization and nucleotide sequencing. The promoter contains a typical TATA-box at a distance of 135 bp upstream of the open reading frame. The open reading frame is interrupted by two small introns with conserved splice sites. The precursor of the antifungal protein (AFP) consists of 94 amino acids and appears to be processed to the mature AFP of 51 amino acids by a two-step process. Transfer of the gene into A. niger yielded only transformants with a very low expression level, probably because high-expression transformants were counterselected by the antifungal activity of the recombinant protein.

Virus-associated down-regulation of the gene encoding cryparin, an abundant cell-surface protein from the chestnut blight fungus, Cryphonectria parasitica

We report here the cloning and analysis of the gene (Crp) encoding cryparin (CRP) and the suppression of the accumulation of its mRNA by a double-stranded RNA virus. CRP belongs to a group of fungal hydrophobic cell surface proteins and has sequence similarity to cerato-ulmin, a phytotoxin produced by Ophiostoma ulmi, the Dutch elm disease pathogen. Transcripts of the Crp gene represent almost 25% of the mRNA of the fungal cell during the peak of CRP production.

New developments in fungal virology

Although viruses are widely distributed in fungi, their biological significance to their hosts is still poorly understood. A large number of fungal viruses are associated with latent infections of their hosts. With the exception of the killer-immune character in the yeasts, smuts, and hypovirulence in the chestnut blight fungus, fungal properties that can specifically be related to virus infection are not well defined. Mycoviruses are not known to have natural vectors; they are transmitted in nature intracellularly by hyphal anastomosis and heterokaryosis, and are disseminated via spores. Because fungi have a potential for plasmogamy and cytoplasmic exchange during extended periods of their life cycles and because they produce many types of propagules (sexual and asexual spores), often in great profusion, mycoviruses have them accessible to highly efficient means for transmission and spread. It is no surprise, therefore, that fungal viruses are not known to have an extracellular phase to their life cycles. Although extracellular transmission of a few fungal viruses have been demonstrated, using fungal protoplasts, the lack of conventional methods for experimental transmission of these viruses have been, and remains, an obstacle to understanding their biology. The recent application of molecular biological approaches to the study of mycoviral dsRNAs and the improvements in DNA-mediated fungal transformation systems, have allowed a clearer understanding of the molecular biology of mycoviruses to emerge. Considerable progress has been made in elucidating the genome organization and expression strategies of the yeast L-A virus and the unencapsidated RNA virus associated with hypovirulence in the chestnut blight fungus. These recent advances in the biochemical and molecular characterization of the genomes of fungal viruses and associated satellite dsRNAs, as they relate to the biological properties of these viruses and to their interactions with their hosts are the focus of this chapter.

Structure and heterologous expression of the Ustilago maydis viral toxin KP4

Killer toxins are polypeptides secreted by some fungal species that kill sensitive cells of the same or related species. In the best-characterized cases, they function by creating new pores in the cell membrane and disrupting ion fluxes. Immunity or resistance to the toxins is conferred by the preprotoxins (or products thereof) or by nuclear resistance genes. In several cases, the toxins are encoded by one or more genomic segments of resident double-stranded RNA viruses. The known toxins are composed of one to three polypeptides, usually present as multimers. We have further characterized the KP4 killer toxin from the maize smut fungus Ustilago maydis. This toxin is also encoded by a single viral double-stranded RNA but differs from other known killer toxins in several respects: it has no N-linked glycosylation either in the precursor or in the mature polypeptide, it is the first killer toxin demonstrated to be a single polypeptide, and it is not processed by any of the known secretory proteinases (other than the signal peptidase). It is efficiently expressed in a heterologous fungal system.

Mutants of Ustilago maydis defective in production of one of two polypeptides of KP6 toxin from the preprotoxin

Double-stranded RNA viruses of Ustilago maydis encode secreted killer toxins to which other cells of the same species and closely related species are sensitive. KP6 toxin consists of two polypeptides, alpha and beta, produced from a single precursor preprotoxin. In this work, we cloned complementary DNA for the toxin-encoding segment of two of the KP6 nonkiller mutants NK3 and NK13 that secrete the beta and alpha polypeptides, respectively. Both sequence analysis of the cDNA clones and in vitro translation of the toxin-encoding double-stranded RNAs showed that both mutants can produce full-length preprotoxins. Cys51 in alpha is converted to Arg in NK3 and Thr25 and Lys42 in beta are changed to Pro and Arg, respectively, in NK13. Although alpha and beta are encoded in a single prepropolypeptide, only the beta polypeptide is secreted by NK3 and only the alpha polypeptide is secreted by NK13. This differential expression of peptides from one precursor is a unique phenomenon. Neither of the nonsecreted polypeptides accumulated in the cytosol. The possible effects of these mutations on preprotoxin folding and their consequences for toxin secretion are discussed.

Neutralizing monoclonal antibodies against alpha and beta subunits of the Ustilago maydis virus encoded toxin

The toxins secreted by Ustilago maydis are encoded by dsRNA viruses. The KP6 toxin encoded by subtype P6 consists of two polypeptides alpha and beta, which are not covalently bound. Neutralizing monoclonal antibodies (MoAbs) were raised against each subunit. Some of the anti-beta MoAbs identify different epitopes in the antigen. The MoAbs were used to affinity purify alpha and beta polypeptides from culture media and to detect the precursor of the mature toxin.

Immunity and resistance to the KP6 toxin of Ustilago maydis

The KP6 toxin of Ustilago maydis, encoded by segmented double-stranded (ds) RNA viruses, is lethal to sensitive strains of the same species and related species. The toxin consists of two polypeptides, alpha and beta, synthesized as a single preprotoxin, which are not covalently linked. Neither polypeptide alone is toxic, but killer activity can be restored by in vitro and in vivo complementation. Killer-secreting strains are resistant to the toxin they produce. Resistance is conferred by a single recessive nuclear gene. This study describes a search for cytoplasmic factors that may confer resistance, also referred to as immunity. The approaches used to detect cytoplasmic immunity included transmission of dsRNA and transmission of virus particles to sensitive cells by cytoduction, cytoplasmic mixing in diploids and infection with viruses. An alternative approach was also used to express cloned cDNAs of the KP6 toxin-encoding dsRNA and of the alpha and beta polypeptides. The results indicated that no immunity to KP6 can be detected. While KP6, alpha and beta polypeptides were expressed by resistant cells, neither KP6 nor beta were expressed in sensitive strains. The alpha polypeptide was expressed in sensitive cells, but it did not confer immunity. These results suggest that neither the preprotoxin nor the alpha or beta polypeptides confer immunity and thus beta may be the toxic component of the binary toxin.

Kex2-dependent processing of yeast K1 killer preprotoxin includes cleavage at ProArg-44

The K1 killer toxin of Saccharomyces cerevisiae consists of 103- and 83-residue alpha and beta components whose derivation, from a 316-residue precursor preprotoxin, requires processing at the alpha N-terminus (after ProArg-44), the alpha C-terminus (after ArgArg-149) and at the beta N-terminus (after LysArg-233). These processing events occur after translocation to the Golgi and have been investigated using beta-lactamase fusions. Signal peptidase cleavage of the precursor, predicted to occur after Ala-26, was confirmed by N-terminal sequence analysis of Ala-34 and Ile-52 fusions. Cleavage at all of the other predicted processing sites, including ProArg-44, is dependent on activity of the Kex2 protease. A fourth Kex2-dependent cleavage occurs at LysArg-188. Implications for the specificity of Kex2 cleavage and preprotoxin processing are discussed.

Yeast dsRNA viruses: replication and killer phenotypes

The cytoplasmic L-A dsRNA virus of Saccharomyces cerevisiae consists of a 4.5 kb dsRNA and the two gene products it encodes; the capsid (cap) and at least one copy of the capsid-polymerase (cap-pol) fusion protein. Virion cap-pol catalyses transcription of the plus (sense)-strand; this is extruded from the virus and serves as messenger for synthesis of cap and cap-pol. Nascent cap-pol binds to a specific domain in the plus strand to initiate encapsidation and then catalyses minus-strand synthesis to complete the replication cycle. Products of at least three host genes are required for replication, and virus copy number is kept at tolerable levels by the SKI antivirus system. S. cerevisiae killer viruses are satellite dsRNAs that use a similar encapsidation domain to parasitize the L-A replication machinery. They encode precursors of secreted polypeptide toxins and immunity (specific resistance) determinants and are self-selecting. Three unique killer types, K1, K2 and K28, are currently recognized. They are distinguished by an absence of cross-immunity and by toxin properties and lethal mechanisms; while K1 and K2 toxins bind to cell-wall glucan and disrupt membrane functions, K28 toxin binds to mannoprotein and causes inhibition of DNA synthesis.

Sequences of three dsRNAs associated with La France disease of the cultivated mushroom (Agaricus bisporus)

La France disease of the cultivated mushroom, Agaricus bisporus, is known to be associated with the presence of a number of dsRNA segments. The nucleotide sequences of the dsRNAs M2 (1.3 kb), M1 (1.55 kb) and L3 (2.8 kb), invariably associated with the disease, were determined. Putative coding sequences for proteins with molecular weights of 38, 40 and 87 kDa were found for M2, M1 and L3 dsRNAs, respectively. The average G + C content of these dsRNAs was 43%, close to that of A. bisporus nuclear DNA. The nucleotide sequences, as well as the amino acid sequences, appear to be unique, as no matching sequences could be found among databases. S3 dsRNA (0.39 kb), which is occasionally found in large amounts in diseased mushrooms, is an internally deleted variant of M2 dsRNA and is largely composed of the non-coding ends of that dsRNA.

Expression in yeast of a cDNA copy of the K2 killer toxin gene

A cDNA copy of the M2 dsRNA encoding the K2 killer toxin of Saccharomyces cerevisiae was expressed in yeast using the yeast ADH1 promoter. This construct produced K2-specific killing and immunity functions. Efficient K2-specific killing was dependent on the action of the KEX2 endopeptidase and the KEX1 carboxypeptidase, while K2-specific immunity was independent of these proteases. Comparison of the K2 toxin sequence with that of the K1 toxin sequence shows that although they share a common processing pathway and are both encoded by cytoplasmic dsRNAs of similar basic structure, the two toxins are very different at the primary sequence level. Site-specific mutagenesis of the cDNA gene establishes that one of the two potential KEX2 cleavage sites is critical for toxin action but not for immunity. Immunity was reduced by an insertion of two amino acids in the hydrophobic amino-terminal region which left toxin activity intact, indicating an independence of toxin action and immunity.

Ustilago maydis virus P4 killer toxin: characterization, partial amino terminus sequence, and evidence for glycosylation

The toxin from Ustilago maydis virus P4 was purified to homogeneity and characterized. The native molecular mass, using size-exclusion HPLC was estimated to be 7.2 kDa. The purified toxin was composed of a single subunit. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis under reduced and nonreduced conditions resulted in estimated molecular masses of 8.4 and 7.4 kDa, respectively. The purified toxin was found to be glycosylated when tested for carbohydrates using the phenol-sulfuric acid method, Schiff's base reagent, and a Glycan detection kit and when probed against different biotinylated lectins. Partial amino acid sequence analysis of the purified toxin indicated a free N-terminus, 16% glycine, and 23% basic amino acid residues. No homology was found to either the alpha or the beta subunit of the toxin encoded by U. maydis infected with the P6 virus.

An expression vector for the phytopathogenic fungus, Ustilago maydis

We have constructed an expression vector for the phytopathogenic fungus Ustilago maydis. This vector, pUXV, expresses genes located downstream from a U. maydis glyceraldehyde-3-phosphate dehydrogenase promoter. Plasmid pUXV also contains a selective marker gene conferring resistance to the antibiotic hygromycin B and a U. maydis autonomously replicating sequence, UARS, allowing high transformation efficiency. Expression of a cDNA from the toxin-encoding region of the U. maydis virus P6 in pUXV resulted in as much killing activity as from viral particles when evaluated by killer plate assay. Plasmid pUXV preserves essential sequences from pUC12 and is therefore a shuttle vector for U. maydis and Escherichia coli.

Interfaces of the yeast killer phenomenon

A new prophylactic and therapeutic antimicrobial strategy based on a specific physiological target that is effectively used by killer yeasts in their natural ecological competition is theorized. The natural system exploited is the yeast killer phenomenon previously adopted as an epidemiological marker for intraspecific differentiation of opportunistic yeasts, hyphomycetes, and bacteria. Pathogenic microorganisms (Candida albicans) may be susceptible to the activity of yeast killer toxins due to the presence of specific cell wall receptors. On the basis of the idiotypic network, we report that antiidiotypic antibodies, produced against a monoclonal antibody bearing the receptor-like idiotype, are in vivo protecting animals immunized through idiotypic vaccination and in vitro mimicking the antimicrobial activity of yeast killer toxins, thus acting as antibiotics.