IDENTIFICATION OF THE 100-KD VICTORIN BINDING-PROTEIN FROM OATS

The fungus Cochliobolus victorias, the causal agent of victoria blight of oats, produces the host-specific toxin victorin. Sensitivity of oa ts to victorin, and thus susceptibility to the fungus, is controlled b y a single dominant gene. This gene is believed to also confer resista nce to the crown rust pathogen Puccinia coronata. In the case of victo ria 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 wit h indirect immunoprecipitation was used to identify the mRNA for the 1 00-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-k b cDNA clone was isolated that, when subjected to a 400-bp 5' deletion , was capable of directing the synthesis of a protein in Escherichia c oli, 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 cDN A revealed extensive homology to a previously cloned cDNA for the P pr otein component of the multienzyme complex glycine decarboxylase. Glyc ine decarboxylase is a nudear-encoded, mitochondrial enzyme complex. P rotein gel blot analysis indicated that the 100-kD VBP copurifies with mitochondria. Based on analysis of in vitro translation products, nuc leotide sequence homology, mitochondrial localization, and the widespr ead species distribution of the 100-kD VBP, we concluded that the 100- kD VBP is the P protein component of glycine decarboxylase.