Structure and heterologous expression of the Ustilago maydis viral toxin KP4

Killer toxins are polypeptides secreted by some fungal species that kill sensitive cells of the same or related species. In the best-characterized cases, they function by creating new pores in the cell membrane and disrupting ion fluxes. Immunity or resistance to the toxins is conferred by the preprotoxins (or products thereof) or by nuclear resistance genes. In several cases, the toxins are encoded by one or more genomic segments of resident double-stranded RNA viruses. The known toxins are composed of one to three polypeptides, usually present as multimers. We have further characterized the KP4 killer toxin from the maize smut fungus Ustilago maydis. This toxin is also encoded by a single viral double-stranded RNA but differs from other known killer toxins in several respects: it has no N-linked glycosylation either in the precursor or in the mature polypeptide, it is the first killer toxin demonstrated to be a single polypeptide, and it is not processed by any of the known secretory proteinases (other than the signal peptidase). It is efficiently expressed in a heterologous fungal system.