Lethal effect of the expression of a killer gene SMK1 in Saccharomyces cerevisiae

Expression of the SMK1 gene which encodes the yeast killer toxin SMKT is le thal in Saccharomyces cerevisiae. Effects of deletion and site-directed mut agenesis of SMK1 on the lethality and the secretion of the gene products we re examined. Deletion of the interstitial gamma peptide or the C-terminal l oop from Ala208 to the C-terminal Asp222 had no effect on the lethality. Th ose SMK1 products that lacked either the gamma peptide or the C-terminal lo op were expressed in the cells but were not secreted into the culture mediu m, suggesting that these peptides may have a role in secretion or in protei n stability. On the other hand, deletion of the signal sequence resulted in complete loss of the lethal activity. Entering the secretory pathway may b e critical for the lethality. Further, deletion of the region from the C-te rminus to Leu207 resulted in loss of the lethal activity. Leu207 is located at the C-terminus of the central strand of the beta-sheet structure of SMK T and its side chain is thrust into a hydrophobic environment between the b eta-sheet and the alpha-helices, The result obtained upon substitutions of Ala, Ser or Glu for Leu207 suggested that the side chain of Leu207 stabiliz es the hydrophobic environment that contributes to the overall structure of the SMK1 product.