The virally encoded killer proteins from Ustilago maydis

► KP4 is made by a dsRNA virus that persistently infects the P4 strain of Ustilago maydis. ► KP4 affects calcium channels in mammalian, fungal, and plant tissue. ► When expressed in maize transgenically, KP4 is highly efficacious in protecting the plant against Ustilago maydis infection. ► KP6 is also virally encoded, but bears no similarity to KP4 in sequence, structure, or mode of action.