Dissecting toxin immunity in virus-infected killer yeast uncovers an intrinsic strategy of self-protection.

Toxin-secreting "killer" yeasts were initially identified >40 years ago in Saccharomyces cerevisiae strains infected with a double-stranded RNA "killer" virus. Despite extensive research conducted on yeast killer toxins, the mechanism of protecting immunity by which toxin-producing cells evade the lethal activities of these proteins has remained elusive. Here, we identify the mechanism leading to protecting immunity in a killer yeast secreting a viral alpha/beta protein toxin (K28) that enters susceptible cells by receptor-mediated endocytosis and, after retrograde transport into the cytosol, blocks DNA synthesis, resulting in both cell-cycle arrest and caspase-mediated apoptosis. We demonstrate that toxin immunity is effected within the cytosol of a toxin-secreting yeast and occurs via the formation of complexes between reinternalized toxin and unprocessed precursor moieties that are subsequently ubiquitinated and proteasomally degraded, eliminating the active form of the toxin. Interference with cellular ubiquitin homeostasis, either through overexpression of mutated ubiquitin (Ub-RR(48/63)) or by blocking deubiquitination, prevents ubiquitination of toxin and results in an impaired immunity and the expression of a suicidal phenotype. The results presented here reveal the uniquely elegant and efficient strategy that killer cells have developed to circumvent the lethal effects of the toxin they produce.

Retrotranslocation of a viral A/B toxin from the yeast endoplasmic reticulum is independent of ubiquitination and ERAD.

K28 is a viral A/B toxin that traverses eukaryotic cells by endocytosis and retrograde transport through the secretory pathway. Here we show that toxin retrotranslocation from the endoplasmic reticulum (ER) requires Kar2p/BiP, Pdi1p, Scj1p, Jem1p, and proper maintenance of Ca(2+) homeostasis. Neither cytosolic chaperones nor Cdc48p/Ufd1p/Npl4p complex components or proteasome activity are required for ER exit, indicating that K28 retrotranslocation is mechanistically different from classical ER-associated protein degradation (ERAD). We demonstrate that K28 exits the ER in a heterodimeric but unfolded conformation and dissociates into its subunits as it emerges into the cytosol where beta is ubiquitinated and degraded. ER export and in vivo toxicity were not affected in a lysine-free K28 variant nor under conditions when ubiquitination and proteasome activity was blocked. In contrast, toxin uptake from the plasma membrane required Ubc4p (E2) and Rsp5p (E3) and intoxicated ubc4 and rsp5 mutants accumulate K28 at the cell surface incapable of toxin internalization. We propose a model in which ubiquitination is involved in the endocytic pathway of the toxin, while ER-to-cytosol retrotranslocation is independent of ubiquitination, ERAD and proteasome activity.