Processing and secretion of a virally encoded antifungal toxin in transgenic tobacco plants: Evidence for a Kex2p pathway in plants

Ustilago maydis is a fungal pathogen of maize. Some strains of U. maydis encode secreted polypeptide toxins capable of killing other susceptible strains of U. maydis. We show here that one of these toxins, the KP6 killer toxin, is synthesized by transgenic tobacco plants containing the viral toxin cDNA under the control of a cauliflower mosaic virus promoter. The two components of the KP6 toxin, designated α and β, with activity and specificity identical to those found in toxin secreted by U. maydis cells, were isolated from the intercellular fluid of the transgenic tobacco plants. The β polypeptide from tobacco was identical in size and N-terminal sequence to the U. maydis KP6 β polypeptide. Processing of the KP6 preprotoxin in U. maydis requires a subtilisin-like processing protease, Kex2p, which is present in both animal and fungal cells and is required for processing of (among other things) small secreted polypeptide hormones and secreted toxins. Our findings present evidence for Kex2p-like processing activity in plants. The systemic production of this viral killer toxin in crop plants may provide a new method of engineering biological control of fungal pathogens in crop plants.