Pichia membranifaciens CYC 1086 secretes a killer toxin (PMKT2) that is inhibitory to a variety of spoilage yeasts and fungi of agronomical interest. The killer toxin in the culture supernatant was concentrated by ultrafiltration and purified to homogeneity by two successive steps, including native electrophoresis and HPLC gel filtration. Biochemical characterization of the toxin showed it to be a protein with an apparent molecular mass of 30 kDa and an isoelectric point of 3.7. At pH 4.5, optimal killer activity was observed at temperatures up to 20 6C. Above approximately this pH, activity decreased sharply and was barely noticeable at pH 6. The toxin concentrations present in the supernatant during optimal production conditions exerted a fungicidal effect on a variety of fungal and yeast strains. The results obtained suggest that PMKT2 has different physico-chemical properties from PMKT as well as different potential uses in the biocontrol of spoilage yeasts. PMKT2 was able to inhibit Brettanomyces bruxellensis while Saccharomyces cerevisiae was fully resistant, indicating that PMKT2 could be used in wine fermentations to avoid the development of the spoilage yeast without deleterious effects on the fermentative strain. In small-scale fermentations, PMKT2, as well as P. membranifaciens CYC 1086, was able to inhibit B. bruxellensis, verifying the biocontrol activity of PMKT2 in simulated winemaking conditions. INTRODUCTIONWorldwide, microbial growth destroys large amounts of various products, causing yield losses in the agronomical and biotechnological industries. Traditionally, biocides have been used to deal with these problems but different disadvantages such as establishment of resistant strains and suppression of natural competitors have made alternatives such as biological control necessary (Beever et al., 1989;Raposo et al., 2000). Biological control strategies include natural plant-and animal-derived compounds, as well as antagonistic micro-organisms (Ciani & Fatichenti, 2001).During recent decades, microbiological control of spoilage micro-organisms has evolved as a possibility. Many yeast strains and other micro-organisms inhibiting plant pathogens have been reported, especially within the fruit-and vegetable-producing sector, and several new products have reached the commercial market (Janisiewicz & Korsten, 2002). The suggested modes of action of biocontrol yeasts are not likely to constitute any hazard for the consumer (Janisiewicz et al., 2001;Masih & Paul, 2002;Comitini et al., 2004;Santos & Marquina, 2004a).The food and beverage industries were among the first to explore the application of killer-toxin-producing yeasts to kill spoilage micro-organisms (Lowes et al., 2000). Yeast strains often achieve competitive advantage by producing killer toxins, which kill off competing sensitive cells belonging to either the same or a different species (Young, 1987;Ciani & Fatichenti, 2001). The most thoroughly studied examples are the Saccharomyces cerevisiae toxins K1, K2 and K28; producers of these toxi...
SummaryPMKT is a channel-forming killer toxin secreted by Pichia membranifaciens. To identify novel genes that mediate cellular resistance to PMKT we screened a collection of 288 deletion mutants. We found 29 open reading frames (ORFs) that, when deleted, confer resistance to PMKT. In addition, the deletion of 15 ORFs was observed to increase protoplast resistance, in agreement with the initial assumption that a plasma membrane receptor for PMKT exists. Whole cells and protoplasts of a cwp2D mutant were found to be completely resistant to PMKT and were unable to bind PMKT, indicating that Cwp2p interacts with it. A protein with a molecular mass of 11.7 kDa was purified from PMKT-affinity columns. This protein was sequenced and identified as Cwp2p. Glycosylphosphatidylinositol (GPI) anchoring-defective mutants were much less sensitive to PMKT, as were wild-type protoplasts pretreated with phosphatidylinositolspecific phospholipase C to remove GPI-anchored proteins, indicating that the GPI-anchored precursor of Cwp2p is also necessary for PMKT activity. Carboxyfluorescein-entrapped liposomes containing a purified GFP-Cwp2p fusion protein in their membranes were much more sensitive to PMKT than protein-free liposomes. Cwp2p and its GPI-anchored precursor are proposed for the first time to be involved as PMKT secondary receptors.
The transcriptional response of Saccharomyces cerevisiae to Pichia membranifaciens killer toxin (PMKT) was investigated. We explored the global gene expression responses of the yeast S. cerevisiae to PMKT using DNA microarrays, real time quantitative PCR, and Northern blot. We identified 146 genes whose expression was significantly altered in response to PMKT in a non-random functional distribution. The majority of induced genes, most of them related to the high osmolarity glycerol (HOG) pathway, were core environmental stress response genes, showing that the coordinated transcriptional response to PMKT is related to changes in ionic homeostasis. Hog1p was observed to be phosphorylated in response to PMKT implicating the HOG signaling pathway. Individually deleted mutants of both up-(99) and down-regulated genes (47) were studied for altered sensitivity; it was observed that the deletion of up-regulated genes generated hypersensitivity (82%) to PMKT. Deletion of down-regulated genes generated wild-type (36%), resistant (47%), and hypersensitive (17%) phenotypes. This is the first study that shows the existence of a transcriptional response to the poisoning effects of a killer toxin.Killer phenomena are widespread in yeasts. Killer toxins are proteins or glycoproteins that are lethal to sensitive strains of the same species and a different variety of other yeast genera. In this line, attention has focused mainly on the characterization of killer toxins from Saccharomyces cerevisiae (K1, K2, and K28) followed more recently by the investigation of yeasts such as Kluyveromyces lactis, Zygosaccharomyces bailii, Hanseniaspora uvarum, Pichia membranifaciens, Debaryomyces hansenii, Schwanniomyces occidentalis,.P. membranifaciens CYC 1106 is a strain originally isolated from fermenting olive brines with pronounced killer activity against a variety of yeast species (8) and fungi (9). Once the protein nature of the toxin produced was established, the secreted protein was purified from the supernatant of growing cultures of P. membranifaciens in the early stationary phase. Previous biochemical studies on the PMKT 2 mechanism of killing of sensitive yeast cells indicated that PMKT is an 18-kDa protein that interacts with the (1 3 6)--D-glucans of the cell wall of sensitive yeasts (6, 10). Recently the killing mechanism of this killer toxin has been elucidated (10). Regardless of certain possible additional effects, the killer toxin of P. membranifaciens CYC 1106 acts by disrupting plasma membrane electrochemical gradients. The death of sensitive cells in the presence of killer toxin is characterized by a leakage of common physiological ions through non-regulated ion channels in the plasma membrane causing a discharge of cellular membrane potential and changes in ionic homeostasis in a way comparable to that of certain killer toxins (K1) (11). Non-selective channel formation has been suggested to be the cytotoxic mechanism of action of PMKT (10).Yeasts must cope with different adverse environmental conditions, including hea...
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