Killer yeasts

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Killer yeasts are yeasts, such as Saccharomyces cerevisiae, which can carry a double-stranded RNA virus, causing them to secrete a number of toxic proteins which are lethal to receptive cells.[1] These yeast cells are immune to the toxic effects of the protein due to an intrinsic immunity.[2] Killer yeast strains can be a problem in commercial processing because they kill desirable strains.[3]

Contents

[edit] RNA virus

Main article: Saccharomyces cerevisiae virus L-A

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, which are called M dsRNAs. The genomic segment encodes for the viral coat protein and a protein which replicates the viral genomes.[4] The M dsRNAs encode the toxin, of which there are at least three variants in S. cerevisiae,[2][5] and many more variants across all species.[1][6]

L-A is not released into the environment. It spreads between cells during yeast mating.[5]

[edit] Toxins

The K1 preprotoxin, showing the α and β chains which make up the K1 toxin. The numbers count amino acid residues.
The K1 preprotoxin, showing the α and β chains which make up the K1 toxin. The numbers count amino acid residues.

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.[2][7]

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.[7][8]

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.[9][10]

[edit] Immunity

Sestia, Shiha, Nikolaevaa 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.[11] 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.[8] 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.[12][13]

The K28 preprotoxin forms a complex with the K28 α/β dimer, neutralizing it.
The K28 preprotoxin forms a complex with the K28 α/β dimer, neutralizing it.

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.[2]

[edit] 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.[14] This was extended by Morace et al (1989) to use toxins to differentiate between 58 bacterial cultures.[15] 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 used as starters in wine-making.[16] 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.[17]

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.[18] Polonelli et al (1994) used a killer yeast to vaccinate against C. albicans in rats.[19] 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.[20] Ciani and Fatichenti (2001) used a toxin-producing strain of Kluyveromyces phaffii to control apiculate yeasts in wine-making.[21] 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.[22]

Several experiments suggest that antibodies that mimic the biological activity of killer toxins have application as antifungal agents.[23]

[edit] Control methods

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.[1] Many toxins are sensitive to pH levels; for example K1 is permanently inactivated at pH levels over 6.5.[6]

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.[3]

[edit] References

  1. ^ a b c Young, T. W. & Yagiu, M. (1978). "A comparison of the killer character in different yeasts and its classification". Antonie van Leeuwenhoek 44 (1): 59-77. 
  2. ^ a b c d Breinig, F. Sendzik, T., Eisfeld, K. & Schmitt, M. J. (2006). "Dissecting toxin immunity in virus-infected killer yeast uncovers an intrinsic strategy of self-protection". PNAS 103 (10): 3810-3815. 
  3. ^ a b Wickner, R.B. (1986). "Double-stranded RNA replication in yeast: The killer system". Annual Review of Biochemistry 55: pp. 373-395. 
  4. ^ Ribas, J. C. & Wickner, R. B. (April 10, 1998). "The Gag Domain of the Gag-Pol Fusion Protein Directs Incorporation into the L-A Double-stranded RNA Viral Particles in Saccharomyces cerevisiae". Journal of Biological Chemistry 273 (15): 9306-9311. 
  5. ^ a b Wickner, R. B., Jinghua Tang, Gardner, N. A. & Johnson, J. E. (2008). "The Yeast dsRNA Virus L-A Resembles Mammalian dsRNA Virus Cores", in John T. Patton: Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-21-9. 
  6. ^ a b Tipper, D.J. & Bostian, K.A. (1984). "Double-stranded ribonucleic acid killer systems in yeasts". Microbiological Reviews 48 (2): 125-156. 
  7. ^ a b Bussey, H. (1991). "K1 killer toxin, a pore-forming protein from yeast". Molecular Microbiology 5 (10): 2339-2343. 
  8. ^ a b Breinig, F., Tipper, D. J. & Schmitt, M. J. (2002). "Kre1p, the Plasma Membrane Receptor for the Yeast K1 Viral Toxin". Cell 108 (3): 395-405. 
  9. ^ Reiter, J., Herker, E., Madeo, F. & Schmitt, M. J. (2005). "Viral killer toxins induce caspase-mediated apoptosis in yeast". Journal of Cell Biology 168 (3): 353-358. 
  10. ^ "Endocytotic uptake and retrograde transport of a virally encoded killer toxin in yeast" (2000). Molecular Microbiology 37 (4): 926-940. 
  11. ^ Sestia, F., Shiha, T. M., Nikolaevaa, N. & Goldstein, S. (2001). "Immunity to K1 Killer Toxin: Internal TOK1 Blockade". Cell 105 (5): 637-644. 
  12. ^ Vališ, K., Mašek, T., Novotná, D., Pospíšek, M. & Janderová, B. (2006). "Immunity to killer toxin K1 is connected with the Golgi-to-vacuole protein degradation pathway". Folia Microbiologica (Praha) 51 (3): 196-202. 
  13. ^ Sturley, S. L., Elliot, Q, LeVitre. J. Tipper, D. J. & Bostian, K. A. (1986). "Mapping of functional domains within the Saccharomyces cerevisiae type 1 killer preprotoxin". The EMBO Journal 5 (12): 3381-3389. 
  14. ^ Morace, G., Archibusacci, C., Sestito, M. & Polonelli, L. (1984). "Strain differentiation of pathogenic yeasts by the killer system.". Mycopathologia 84 (2-3): 81-85. 
  15. ^ Morace, G., Manzara, S., Dettori, G., Fanti, F., Conti, S., Campani, L., Polonelli, L. & Chezzi, C. (1989). "Biotyping of bacterial isolates using the yeast killer system". European Journal of Epidemiology 5 (3): 303-310. 
  16. ^ Vaughan-Martini, A., Cardinali, G., & Martini, A. (1996). "Differential killer sensitivity as a tool for fingerprinting wine-yeast strains of Saccharomyces cerevisiae". Journal of Industrial Microbiology & Biotechnology 17 (2): 124-127. 
  17. ^ Buzzini, P. & Martini, A. (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): 3362-3364. 
  18. ^ Palpacelli, V., Ciani, M. & Rosini, G. (1991). "Activity of different ‘killer’ yeasts on strains of yeast species undesirable in the food industry". FEMS Microbiology Letters 84 (1): 75-78. 
  19. ^ Polonelli, L., De Bernardis, F., Conti, S., Boccanera, M., Gerloni, M., Morace, G. et al (1994). "Idiotypic intravaginal vaccination to protect against candidal vaginitis by secretory, yeast killer toxin-like anti-idiotypic antibodies". Journal of Immunology 152 (6): 3175-3182. 
  20. ^ Lowes, K.F., Shearman, C.A., Payne, J., MacKenzie, D., Archer, D.B., Merry, R.J. & Gasson, M.J. (2000). "Prevention of yeast spoilage in feed and food by the yeast mycocin HMK". Applied Environmental Microbiology 66 (3): 1066-1076. 
  21. ^ Ciani, M. & Fatichenti, F. (2001). "Killer Toxin of Kluyveromyces phaffii DBVPG 6076 as a Biopreservative Agent to Control Apiculate Wine Yeasts". Applied Environmental Microbiology 67 (7): 3058-3063. 
  22. ^ Da Silvaa, S., Caladoa, S., Lucasa, C. & Aguiar, C. (2007). "Unusual properties of the halotolerant yeast Candida nodaensis Killer toxin, CnKT". Microbiol Research 163 (2008): 243-251. 
  23. ^ Magliani, W., Conti, S., Salati, A., Vaccari, S., Ravanetti, L., Maffei, D. L. et al (2004). "Therapeutic potential of yeast killer toxin-like antibodies and mimotopes". FEMS Yeast Research 5 (1): 11-18.