Ustilago maydis Viruses and Their Killer Toxins

The dsRNA viruses that persistently infect Ustilago maydis are members of the family Totiviridae. In a symbiotic manner, the virus genomes encode ‘killer toxins’ that are secreted by the host and attack strains of U. maydis that are not infected by that viral strain. Three strains of U. maydis have been identified that secrete the KP1, KP4, and KP6 toxins. These toxins do not share significant sequence similarity amongst themselves or with the killer toxins from Saccharomyces cerevisiae. All three toxins appear to have distinct atomic structures and modes of action. KP4 blocks calcium channels while KP6 causes lysis by an unknown mechanism.