Purification and Characterization of Extracellular and Intracellular Killer Toxin of Saccharomyces cerevisiae Strain 28

The extracellular killer toxin of Saccharomyces cerevisiae strain 28 was concentrated by ultrafiltration of culture supernatants and purified by ion-exchange chromatography. Polyacrylamide gradient gel electrophoresis in SDS indicated that the toxin is a glycoprotein with a molecular weight of about 16000. Amino acid analysis revealed that the killer toxin contains 111 amino acid residues, equivalent to a molecular weight of 14045; the ratio of protein to carbohydrate in the molecule is therefore about 9 to 1. The isoelectric point of the killer toxin was pH 4·4 to 4·5. The toxin was unaffected by heating at 40 °C for 1 h and its maximum activity against sensitive yeast cells was observed at pH 5v0. Cell-free extracts prepared from well-washed cells of S. cerevisiae strain 28 were toxic for sensitive yeasts. The toxin present in these extracts (intracellular toxin) was partially purified by ultrafiltration and ion-exchange chromatography. The isoelectric points of the extracellular and intracellular killer toxin were similar.