Identification of the 100-kD victorin binding protein from oats

The fungus Cochliobolus victoriae, the causal agent of victoria blight of oats, produces the host-specific toxin victorin. Sensitivity of oats to victorin, and thus susceptibility to the fungus, is controlled by a single dominant gene. This gene is believed to also confer resistance to the crown rust pathogen Puccinia coronata. In the case of victoria blight, the gene has been hypothesized to condition susceptibility by encoding a toxin receptor. A 100-kD victorin binding protein (VBP) has been identified; it binds radiolabeled victorin derivatives in a ligand-specific manner and in a genotype-specific manner in vivo. The VBP may function as a toxin receptor. In vitro translation coupled with indirect immunoprecipitation was used to identify the mRNA for the 100-kD VBP, and fractionated mRNAs were used to prepare cDNA libraries enriched in the relative abundance of cDNA for the 100-kD VBP. A 3.4-kb cDNA clone was isolated that, when subjected to a 400-bp 5' deletion, was capable of directing the synthesis of a protein in Escherichia coli, which reacted to an antibody specific for the 100-kD VBP. Peptide mapping, by limited proteolysis, indicated that the protein directed by the cDNA is the 100-kD VBP. Nucleotide sequence analysis of the cDNA revealed extensive homology to a previously cloned cDNA for the P protein component of the multienzyme complex glycine decarboxylase. Glycine decarboxylase is a nuclear-encoded, mitochondrial enzyme complex. Protein gel blot analysis indicated that the 100-kD VBP copurifies with mitochondria. Based on analysis of in vitro translation products, nucleotide sequence homology, mitochondrial localization, and the widespread species distribution of the 100-kD VBP, we concluded that the 100-kD VBP is the P protein component of glycine decarboxylase.