THE PLANT CELL, Vol 7, Issue 4 463-471, Copyright © 1995 by American Society of Plant Biologists
Inhibition of the Glycine Decarboxylase Multienzyme Complex by the Host-Selective Toxin Victorin
D. A. Navarre and T. J. Wolpert
Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon 97331-2902
Victoria blight of oats is caused by the fungus Cochliobolus victoriae.
This fungus is pathogenic due to its ability to produce the host-selective
toxin victorin. We previously identified a 100-kD protein that binds
victorin in vivo only in susceptible genotypes and a 15-kD protein that
binds victorin in vivo in both susceptible and resistant genotypes.
Recently, we determined that the oat 100-kD victorin binding protein is the
P protein of the glycine decarboxylase complex (GDC). In this study, we
examined the effect of victorin on glycine decarboxylase activity (GDA).
Victorin was a potent in vivo inhibitor of GDA. Leaf slices pretreated for
2 hr with victorin displayed an effective concentration for 50% inhibition
(EC50) of 81 pM for GDA. Victorin inhibited the glycine-bicarbonate
exchange reaction in vitro with an EC50 of 23 [mu]M. We also identified a
15-kD mitochondrial protein that bound victorin in a ligand-specific
manner. Based on amino acid sequence analysis, we concluded that the 15-kD
mitochondrial protein is the H protein component of the GDC. Thus, victorin
specifically binds to two components of the GDC. GDA in resistant tissue
treated with 100 [mu]g/mL victorin for 5 hr was inhibited 26%, presumably
as a consequence of the interaction of victorin with the H protein.
Victorin had no detectable effect on GDA in isolated mitochondria,
apparently due to the inability of isolated mitochondria to import
victorin. These results suggest that the interaction of victorin with the
GDC is central to victorin's mode of action.
