The virally encoded killer proteins from Ustilago maydis

Several strains of Ustilago maydis, a causal agent of corn smut disease, exhibit a 'killer' phenotype that is due to persistent infection by double-stranded RNA Totiviruses. These viruses produce potent killer proteins that are secreted by the host. This is a rare example of virus/host symbiosis in that these viruses are dependent upon host survival and, to that end, produce antifungal proteins that kill competing, uninfected strains of U. maydis. Two of the best-studied examples of this killer phenomenon are U. maydis strains P4 and P6 that secrete killer proteins KP4 and KP6, respectively. The mature form of KP4 is comprised of 105 residues while KP6 consists of two subunits, a and b chains, 76 and 82 residues in length, respectively. KP6 is not homologous to any known protein, and only recently has KP4 been shown to have possible homologs in pathogenic fungi. While very little is known as to the mode of action of KP6, we have shown that KP4 blocks L-type Ca²⁺ channels in fungi and animal cells in a reversible and cytostatic manner. In contrast, preliminary results suggest that KP6 acts via a completely different mechanism and is a potent cytolytic antifungal protein. When KP4 is expressed in maize, the resulting transgenic lines are nearly immune to U. maydis infection. Therefore, a greater understanding of the modes of action of these potent antifungal proteins could lead to development of broad-spectrum antifungal agents.