Role of protein processing, intracellular trafficking and endocytosis in production of and immunity to yeast killer toxin.

Yeast strains harboring M1-dsRNA and its packaging virus ScV-L secrete a disulfide-linked, heterodimeric toxin which kills sensitive yeast cells by disrupting plasma membrane function. The mature toxin is derived from a precursor (preprotoxin) which undergoes post-translational processing steps during export via the established yeast secretory pathway. Cleavage by both the KEX1 and KEX2 endopeptidases is required for expression of killing activity. The same 1.0 kb open reading frame on M1-dsRNA directs the expression of immunity to toxin. Differentially processed derivatives of protoxin, as well as protoxin itself, have been proposed to serve as mediators of immunity. To understand the mechanisms by which the killing and immunity phenotypes can be derived from a common precursor, we have: 1) studied cellular processes implicated in expression of the phenotypes; and 2) developed a system to produce mutants defective in immunity, killing, or both. In the first approach, the role played by both endocytosis and vesicular traffiking in expression of killing and immunity was examined. Strains defective in endocytosis (end1, end2) or vacuolar protein localization (vpl3, vpl6) were transformed with a plasmid encoding killer toxin under control of the pho5 promoter. When induced by phosphate starvation, both end mutants and all vpl mutants expressed killing activity. Immunity to exogenous toxin, however, was significantly decreased in strains carrying both vpl mutant alleles and in one of the endocytosis mutants (end1]. This suicidal phenotype (rex for resistance expression) has been described previously in M1-containing strains as a leaky phenocopy.(ABSTRACT TRUNCATED AT 250 WORDS)