K66 Killer System Evidences Expanded Assortment of Helper and Satellite Viruses.

The yeast family recently became recognized for expanding of the repertoire of different dsRNA-based viruses, highlighting the need for understanding of their cross-dependence. We isolated the AML-15-66 killer strain from spontaneous fermentation of serviceberries and identified helper and satellite viruses of the family , which are responsible for the killing phenotype. The corresponding full dsRNA genomes of viruses have been cloned and sequenced. Sequence analysis of SpV-LA-66 identified it to be most similar to LA-28 type viruses, while SpV-M66 was mostly similar to the SpV-M21 virus. Sequence and functional analysis revealed significant differences between the K66 and the K28 toxins. The structural organization of the K66 protein resembled those of the K1/K2 type toxins. The AML-15-66 strain possesses the most expressed killing property towards the K28 toxin-producing strain. A genetic screen performed on YKO library strains revealed 125 gene products important for the functioning of the K66 toxin, with 85% of the discovered modulators shared with K2 or K1 toxins. Investigation of the K66 protein binding to cells and different polysaccharides implies the β-1,6 glucans to be the primary receptors of K66 toxin. For the first time, we demonstrated the coherent habitation of different types of helper and satellite viruses in a wild-type strain.