A killer yeast is a yeast, such as Saccharomyces cerevisiae, which is able to secrete one of a number of toxic proteins which are lethal to susceptible cells. These "killer toxins" are polypeptides that kill sensitive cells of the same or related species, often functioning by creating pores in target cell membranes. These yeast cells are immune to the toxic effects of the protein due to an intrinsic immunity. Killer yeast strains can be a problem in commercial processing because they can kill desirable strains. The killer yeast system was first described in 1963. Study of killer toxins helped to better understand the secretion pathway of yeast, which is similar to those of more complex eukaryotes. It also can be used in treatment of some diseases, mainly those caused by fungi.
The best characterized toxin system is from yeast (Saccharomyces cerevisiae), which was found to spoil brewing of beer. In S. cerevisiae are toxins encoded by a double-stranded RNA virus, translated to a precursor protein, cleaved and secreted outside of the cells, where they may affect susceptible yeast. There are other killer systems in S. cerevisiae, such as KHR1  and KHS1  genes encoded on chromosomes IX and V, respectively.
The virus, L-A, is an icosahedral virus of S. cerevisiae comprising a 4.6 kb genomic segment and several satellite double-stranded RNA sequences, called M dsRNAs. The genomic segment encodes for the viral coat protein and a protein which replicates the viral genomes. The M dsRNAs encode the toxin, of which there are at least three variants in S. cerevisiae, and many more variants across all species.
L-A virus uses yeast Ski complex (super killer) and MAK (maintenance of killer) chromosomal genes for its preservation in the cell. The virus is not released into the environment. It spreads between cells during yeast mating.
The initial protein product from translation of the M dsRNA is called the preprotoxin, which is targeted to the yeast secretory pathway. The preprotoxin is processed and cleaved to produce an α/β dimer, which is the active form of the toxin, and is released into the environment.
The two most studied variant toxins in S. cerevisiae are K1 and K28.
K1 binds to the β-1,6-D-glucan receptor on the target cell wall, moves inside, and then binds to the plasma membrane receptor Kre1p. It forms a cation-selective ion channel in the membrane, which is lethal to the cell.
K28 uses the α-1,6-mannoprotein receptor to enter the cell, and utilizes the secretory pathway in reverse by displaying the endoplasmic reticulum HDEL signal. From the ER, K28 moves into the cytoplasm and shuts down DNA synthesis in the nucleus, triggering apoptosis.
Sesti, Shih, Nikolaeva and Goldstein (2001) claimed that K1 inhibits the TOK1 membrane potassium channel before secretion, and although the toxin reenters through the cell wall it is unable to reactivate TOK1. However Breinig, Tipper and Schmitt (2002) showed that the TOK1 channel was not the primary receptor for K1, and that TOK1 inhibition does not confer immunity. Vališ, Mašek, Novotná, Pospíšek and Janderová (2006) experimented with mutants which produce K1 but do not have immunity to it, and suggested that cell membrane receptors were being degraded in the secretion pathway of immune cells, apparently due to the actions of unprocessed α chains.
Breinig, Sendzik, Eisfeld and Schmitt (2006) showed that K28 toxin is neutralized in toxin-expressing cells by the α chain in the cytosol, which has not yet been fully processed and still contains part of a γ chain attached to the C terminus. The uncleaved α chain neutralizes the K28 toxin by forming a complex with it.
Killer properties of Kluyveromyces lactis are associated with linear DNA plasmids, which have on their 5'end associated proteins, which enable them to replicate themselves, in a way similar to adenoviruses. It is an example of protein priming in DNA replication. MAK genes are not known. The toxin consists of three subunits, which are matured in golgi complex by signal peptidase and glycosylated.
The mechanism of action appears to be the inhibition of adenylate cyclase in sensitive cells. Affected cells are arrested in G1 phase and lose viability.
Other toxin systems are found in other yeasts:
- Pichia and Williopsis
- Hanseniaspora uvarum
- Zygosaccharomyces bailii
- Ustilago maydis: the smut fungus produces killer toxin Kp4 family fungal killer toxins.
- Debaryomyces hansenii
Use of toxins
The susceptibility to toxins varies greatly between yeast species and strains. Several experiments have made use of this to reliably identify strains. Morace, Archibusacci, Sestito and Polonelli (1984) used the toxins produced by 25 species of yeasts to differentiate between 112 pathogenic strains, based on their sensitivity to each toxin. This was extended by Morace et al. (1989) to use toxins to differentiate between 58 bacterial cultures. Vaughan-Martini, Cardinali and Martini (1996) used 24 strains of killer yeast from 13 species to find a resistance signature for each of 13 strains of S. cerevisiae which were used as starters in wine-making. Buzzini and Martini (2001) showed that sensitivity to toxins could be used to discriminate between 91 strains of Candida albicans and 223 other Candida strains.
Others experimented with using killer yeasts to control undesirable yeasts. Palpacelli, Ciani and Rosini (1991) found that Kluyveromyces phaffii was effective against Kloeckera apiculata, Saccharomycodes ludwigii and Zygosaccharomyces rouxii – all of which cause problems in the food industry. Polonelli et al. (1994) used a killer yeast to vaccinate against C. albicans in rats. Lowes et al. (2000) created a synthetic gene for the toxin HMK normally produced by Williopsis mrakii, which they inserted into Aspergillus niger and showed that the engineered strain could control aerobic spoilage in maize silage and yoghurt. Ciani and Fatichenti (2001) used a toxin-producing strain of Kluyveromyces phaffii to control apiculate yeasts in wine-making. Da Silvaa, Caladoa, Lucasa and Aguiar (2007) found a toxin produced by Candida nodaensis was effective at preventing spoilage of highly salted food by yeasts.
Several experiments suggest that antibodies that mimic the biological activity of killer toxins have application as antifungal agents.
Young and Yagiu (1978) experimented with methods of curing killer yeasts. They found that using a cycloheximine solution at 0.05 ppm was effective in eliminating killer activity in one strain of S. cerevisiae. Incubating the yeast at 37 °C eliminated activity in another strain. The methods were not effective at reducing toxin production in other yeast species. Many toxins are sensitive to pH levels; for example K1 is permanently inactivated at pH levels over 6.5.
The greatest potential for control of killer yeasts appears to be the addition of the L-A virus and M dsRNA, or an equivalent gene, into the industrially desirable variants of yeast, so they achieve immunity to the toxin, and also kill competing strains.
- Young TW, Yagiu M (1978). "A comparison of the killer character in different yeasts and its classification". Antonie van Leeuwenhoek. 44 (1): 59–77. doi:10.1007/BF00400077. PMID 655699. S2CID 20931283.
- Breinig F, Sendzik T, Eisfeld K, Schmitt MJ (March 2006). "Dissecting toxin immunity in virus-infected killer yeast uncovers an intrinsic strategy of self-protection". Proceedings of the National Academy of Sciences of the United States of America. 103 (10): 3810–5. Bibcode:2006PNAS..103.3810B. doi:10.1073/pnas.0510070103. PMC 1533781. PMID 16505373.
- Wickner RB (1986). "Double-stranded RNA replication in yeast: the killer system". Annual Review of Biochemistry. 55: 373–95. doi:10.1146/annurev.bi.55.070186.002105. PMID 3527047.
- Bevan, E. A., and M. Makower. (1963). "The physiological basis of the killer character in yeast". Proc. XIth Int. Congr. Genet. 1:202–203.
- Goto K, Iwatuki Y, Kitano K, Obata T, Hara S (April 1990). "Cloning and nucleotide sequence of the KHR killer gene of Saccharomyces cerevisiae". Agricultural and Biological Chemistry. 54 (4): 979–84. doi:10.1271/bbb1961.54.979. PMID 1368554.
- Goto K, Fukuda H, Kichise K, Kitano K, Hara S (August 1991). "Cloning and nucleotide sequence of the KHS killer gene of Saccharomyces cerevisiae". Agricultural and Biological Chemistry. 55 (8): 1953–8. doi:10.1271/bbb1961.55.1953. PMID 1368726.
- Ribas JC, Wickner RB (April 1998). "The Gag domain of the Gag-Pol fusion protein directs incorporation into the L-A double-stranded RNA viral particles in Saccharomyces cerevisiae". The Journal of Biological Chemistry. 273 (15): 9306–11. doi:10.1074/jbc.273.15.9306. PMID 9535925.
- Wickner RB, Tang J, Gardner NA, Johnson JE (2008). "The Yeast dsRNA Virus L-A Resembles Mammalian dsRNA Virus Cores". In Patton JT (ed.). Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. p. 105. ISBN 978-1-904455-21-9.
- Tipper DJ, Bostian KA (June 1984). "Double-stranded ribonucleic acid killer systems in yeasts". Microbiological Reviews. 48 (2): 125–56. doi:10.1128/MMBR.48.2.125-156.1984. PMC 373216. PMID 6377033.
- Bussey H (October 1991). "K1 killer toxin, a pore-forming protein from yeast". Molecular Microbiology. 5 (10): 2339–43. doi:10.1111/j.1365-2958.1991.tb02079.x. PMID 1724277.
- Breinig F, Tipper DJ, Schmitt MJ (February 2002). "Kre1p, the plasma membrane receptor for the yeast K1 viral toxin". Cell. 108 (3): 395–405. doi:10.1016/S0092-8674(02)00634-7. PMID 11853673. S2CID 16889563.
- Reiter J, Herker E, Madeo F, Schmitt MJ (January 2005). "Viral killer toxins induce caspase-mediated apoptosis in yeast". The Journal of Cell Biology. 168 (3): 353–8. doi:10.1083/jcb.200408071. PMC 2171720. PMID 15668299.
- Eisfeld K, Riffer F, Mentges J, Schmitt MJ (August 2000). "Endocytotic uptake and retrograde transport of a virally encoded killer toxin in yeast". Molecular Microbiology. 37 (4): 926–40. doi:10.1046/j.1365-2958.2000.02063.x. PMID 10972812.
- Sesti F, Shih TM, Nikolaeva N, Goldstein SA (June 2001). "Immunity to K1 killer toxin: internal TOK1 blockade". Cell. 105 (5): 637–44. doi:10.1016/S0092-8674(01)00376-2. PMID 11389833. S2CID 16673130.
- Valis K, Masek T, Novotná D, Pospísek M, Janderová B (2006). "Immunity to killer toxin K1 is connected with the Golgi-to-vacuole protein degradation pathway". Folia Microbiologica. 51 (3): 196–202. doi:10.1007/BF02932122. PMID 17004650. S2CID 22496847.
- Sturley SL, Elliot Q, LeVitre J, Tipper DJ, Bostian KA (December 1986). "Mapping of functional domains within the Saccharomyces cerevisiae type 1 killer preprotoxin". The EMBO Journal. 5 (12): 3381–9. doi:10.1002/j.1460-2075.1986.tb04654.x. PMC 1167337. PMID 3545818.
- Morace G, Archibusacci C, Sestito M, Polonelli L (February 1984). "Strain differentiation of pathogenic yeasts by the killer system". Mycopathologia. 84 (2–3): 81–5. doi:10.1007/BF00436517. PMID 6371541. S2CID 27061681.
- Morace G, Manzara S, Dettori G, Fanti F, Conti S, Campani L, et al. (September 1989). "Biotyping of bacterial isolates using the yeast killer system". European Journal of Epidemiology. 5 (3): 303–10. doi:10.1007/BF00144830. PMID 2676582. S2CID 30871936.
- Vaughan-Martini A, Cardinali G, Martini A (August 1996). "Differential killer sensitivity as a tool for fingerprinting wine-yeast strains of Saccharomyces cerevisiae". Journal of Industrial Microbiology. 17 (2): 124–7. doi:10.1007/BF01570055. PMID 8987896. S2CID 11095134.
- Buzzini P, Martini A (September 2001). "Discrimination between Candida albicans and other pathogenic species of the genus Candida by their differential sensitivities to toxins of a panel of killer yeasts". Journal of Clinical Microbiology. 39 (9): 3362–4. doi:10.1128/JCM.39.9.3362-3364.2001. PMC 88347. PMID 11526179.
- Palpacelli V, Ciani M, Rosini G (November 1991). "Activity of different 'killer' yeasts on strains of yeast species undesirable in the food industry". FEMS Microbiology Letters. 68 (1): 75–8. doi:10.1111/j.1574-6968.1991.tb04572.x. PMID 1769559.
- Polonelli L, De Bernardis F, Conti S, Boccanera M, Gerloni M, Morace G, et al. (March 1994). "Idiotypic intravaginal vaccination to protect against candidal vaginitis by secretory, yeast killer toxin-like anti-idiotypic antibodies". Journal of Immunology. 152 (6): 3175–82. PMID 8144911.
- Lowes KF, Shearman CA, Payne J, MacKenzie D, Archer DB, Merry RJ, Gasson MJ (March 2000). "Prevention of yeast spoilage in feed and food by the yeast mycocin HMK". Applied and Environmental Microbiology. 66 (3): 1066–76. doi:10.1128/AEM.66.3.1066-1076.2000. PMC 91944. PMID 10698773.
- Ciani M, Fatichenti F (July 2001). "Killer toxin of Kluyveromyces phaffii DBVPG 6076 as a biopreservative agent to control apiculate wine Yeasts". Applied and Environmental Microbiology. 67 (7): 3058–63. doi:10.1128/AEM.67.7.3058-3063.2001. PMC 92981. PMID 11425722.
- da Silva S, Calado S, Lucas C, Aguiar C (2008). "Unusual properties of the halotolerant yeast Candida nodaensis Killer toxin, CnKT". Microbiological Research. 163 (2): 243–51. doi:10.1016/j.micres.2007.04.002. PMID 17761407.
- Magliani W, Conti S, Salati A, Vaccari S, Ravanetti L, Maffei DL, Polonelli L (October 2004). "Therapeutic potential of yeast killer toxin-like antibodies and mimotopes". FEMS Yeast Research. 5 (1): 11–8. doi:10.1016/j.femsyr.2004.06.010. PMID 15381118.