STRUCTURE AND HETEROLOGOUS EXPRESSION OF THE USTILAGO-MAYDIS VIRAL TOXIN KP4

Killer toxins are polypeptides secreted by some fungal species that ki ll sensitive cells of the same or related species. In the best-charact erized cases, they function by creating new pores in the cell membrane and disrupting ion fluxes. Immunity or resistance to the toxins is co nferred by the preprotoxins (or products thereof) or by nuclear resist ance genes. In several cases, the toxins are encoded by one or more ge nomic segments of resident double-stranded RNA viruses. The known toxi ns are composed of one to three polypeptides, usually present as multi mers. We have further characterized the KP4 killer toxin from the maiz e smut fungus Ustilago maydis. This toxin is also encoded by a single viral double-stranded RNA but differs from other known killer toxins i n several respects: it has no N-linked glycosylation either in the pre cursor or in the mature polypeptide, it is the first killer toxin demo nstrated 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.