Abstract
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.).
Highlights
Killer yeast strains have the characteristic of secreting toxins of proteinaceous nature that are lethal to sensitive yeast cells and filamentous fungi
Killer yeasts are widespread in nature where they can be found in percentages that several fold exceed those found in laboratory strains, indicating the existence of a competitive advantage
Killer toxin production has been related in S. cerevisiae with the presence of two dsRNA viruses: L-A, the helper virus, and the M killer virus that encodes a killer toxin that determines its phenotype (K1, K2, K28 or Klus)
Summary
Killer yeast strains have the characteristic of secreting toxins of proteinaceous nature that are lethal to sensitive yeast cells and filamentous fungi. One of the mechanisms to gain advantage that have been described is shown by some killer yeast through the presence of dsRNA viruses These mycoviruses encode killer toxins that provide benefits to the producing cells by killing sensitive yeast cells. It was found that the majority of yeasts isolated from spontaneously fermenting olive brines possessed the killer character and that strains of Pichia membranifaciens, the dominant species, were active in the presence of salt, eventually influencing the development of some spontaneous fermentations [43,44] The reasons for such an increased killer activity was elucidated in the study of the mechanism of action of the first P. membranifaciens toxin analyzed, called PMKT (produced by the strain CYC 1106). The potential applications and future perspectives of killer toxins, with special focus in P. membranifaciens killer toxins, are described and discussed
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