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Investigation of cytocidal effect of K5 type yeast killer protein on sensitive microbial cells

Sertkaya, Abdullah
Some yeasts secrete polypeptide toxins, which are lethal to other sensitive yeast cells, gram-positive pathogenic bacteria and pathogenic fungi. Therefore these are designated as killer toxins. Killer toxins are suggested as potent antimicrobial agents especially for the protection of fermentation process against contaminating yeasts, biological control of undesirable yeasts in the preservation of foods. Moreover they are promising antimicrobial agents in the medical field; due to immune system suppressing diseases like AIDS, there is an increase in the incidence of fungal diseases and current antimycotics have low selectivity and severe side effects. In this study our aim was to explain the cytocidal effect and enzymatic properties of K5 type yeast killer protein, which is secreted by Pichia anomala NCYC 434 cells, and known to have a broad range of killing spectrum. Competitive inhibition of the toxin with cell wall polysaccharides showed that primary binding site of toxin is β-1,3-glucans of sensitive cells. Toxin showed exo-β-1,3-glucanase activity which causes loss of cell wall rigidity leading cell death. Km and Vmax were found to be 0,3 mg/ml and 372,3 æmol/min/mg for laminarin hydrolysis. The toxin exerted its cytocidal effect after 2 h contact with the target cells. Toxin production was found to be dependent on β-1,3-glucan content of the media. Toxin activity was completely inhibited by Hg+2 ,while several metal ions and DTT increased the activity to different extends. Our findings revealed the characteristics of K5 type killer toxin which will help for its possible uses in near future.