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.