Characterization of a novel broad-spectrum antifungal protein from virus-infected Helminthosporium (Cochliobolus) victoriae

A broad-spectrum anti-fungal protein of approximately 10 kDa, designated victoriocin, was purified from culture filtrates of a virus-infected isolate of the plant-pathogenic fungus Helminthosporium victoriae (teleomorph: Cochliobolus victoriae) by a multistep procedure involving ultrafiltration and reverse-phase high-performance liquid chromatography (RP-HPLC). Amino acid sequences, obtained by automated Edman degradation sequencing of RP-HPLC-purified victoriocin-derived peptides, were used to design primers for degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR) amplification from H. victoriae DNA and cDNA templates. An open reading frame coding for a victoriocin precursor of 183 amino acids with calculated molecular mass of approximately 20 kDa was amplified by PCR from H. victoriae genomic DNA but not from the control fungus Penicillium chrysogenum. Southern hybridization analysis confirmed the presence of the victoriocin gene in all H. victoriae strains tested. Sequence analysis indicated that victoriocin has a sequence motif similar to that found in scorpion short toxin/charybdotoxin and a consensus sequence similar to that found in defensins. Victoriocin, like some other antifungal proteins, including the totivirus-encoded killer proteins, is predicted to be expressed in vivo as a preprotoxin precursor consisting of a hydrophobic N-terminal secretion signal followed by a pro-region and terminating in a classical Kex2p endopeptidase cleavage site that generates the N terminus of the mature victoriocin. A putative cell wall protein of approximately 30 kDa (P30) co-purified with victoriocin from cultural filtrates. The potential role of P30 in the antifungal activity of H. victoriae culture filtrates is discussed.