Abstract
Pore-forming toxins are ubiquitous in biology, functioning as virulence factors, immune defense proteins, and in species competition. K62 is an understudied antifungal killer toxin from Saccharomyces paradoxus, encoded by a double-stranded RNA satellite, and has a pH and temperature optima similar to other well-studied Saccharomyces killer toxins. Tertiary structure predictions and molecular dynamics simulations revealed the striking similarity of K62 to the five beta-strand motifs characteristic of the pore-forming aerolysin family of toxins. Recombinant K62 assembled into membrane-associated high molecular weight (>250 kDa) oligomers that were heat- and detergent-resistant. Structural prediction of K62 oligomers yielded a circular complex and beta-barrel with structural and biochemical similarities to aerolysin pre-pores and pores. K62 has >1,000 uncharacterized sequence homologs mostly found in the Ascomycota, but also fungi of the Chytridiomycota and Basidiomycota, as well as plants and bacteria. Homologs were found in pathogenic species, including human and plant pathogens from the Candida and Fusarium genera. Together, these findings identify K62 as the founding member of a large, previously unrecognized family of aerolysin-like proteins with potential roles in fungal pathogenesis and interspecies competition.