Protein secretion in yeast: Two chromosomal mutants that oversecrete killer toxin in Saccharomyces cerevisiae.

Two chromosomal mutations in yeast that result in oversecretion of the K1 killer toxin protein were examined. A recessive mutation in gene ski5 appears to lead to toxin oversecretion through a defect in a cell surface, PMSF-inhibited protease. A wild type killer strain degraded toxin following synthesis, and degradation could be partially prevented by addition of PMSF to the growth medium. The ski5 mutation caused an approximate ten fold oversecretion of toxin, similar to that seen in a PMSF-treated wild type culture, and no increased oversecretion in the presence of PMSF. The ski5 mutation caused oversecretion of other low molecular weight secreted proteins and appeared to oversecrete the α-factor pheromone, as judged by activity tests. The ski5 mutation was complemented by mutations in ski genes 1-4, and the mutant was not supersensitive to mating pheromones or K2 killer toxin.We also examined killer strains with a mutation in the nuclear gene krel which results in a defective (1→6)-β-D-glucan cell wall receptor for killer toxin. Such strains oversecrete toxin into the growth medium, but also, unexpectedly, oversecrete most other secreted proteins. The defect in (1→6)-β-D-glucan in these mutants appears to perturb the partitioning of secreted proteins between the cell wall and the medium.