THE PLANT CELL, Vol 2, Issue 2 153-161, Copyright © 1990 by American Society of Plant Biologists
Fungal Toxins Bind to the URF13 Protein in Maize Mitochondria and Escherichia coli
C. J. Braun, J. N. Siedow and C. S. Levings III
Department of Genetics, North Carolina State University, Raleigh, North Carolina 27695-7614
Expression of the maize mitochondrial T-urf13 gene results in a sensitivity
to a family of fungal pathotoxins and to methomyl, a structurally unrelated
systemic insecticide. Similar effects of pathotoxins and methomyl are
observed when T-urf13 is cloned and expressed in Escherichia coli. An
interaction between these compounds and the membrane-bound URF13 protein
permeabilizes the inner mitochondrial and bacterial plasma membranes. To
understand the toxin-URF13 effects, we have investigated whether toxin
specifically binds to the URF13 protein. Our studies indicate that toxin
binds to the URF13 protein in maize mitochondria and in E. coli expressing
URF13. Binding analysis in E. coli reveals cooperative toxin binding. A low
level of specific toxin binding is also demonstrated in cms-T and
cms-T-restored mitochondria; however, binding does not appear to be
cooperative in maize mitochondria. Competition and displacement studies in
E. coli demonstrate that toxin binding is reversible and that the toxins
and methomyl compete for the same, or for overlapping, binding sites. Two
toxin-insensitive URF13 mutants display a diminished capability to bind
toxin in E. coli, which identifies residues of URF13 important in toxin
binding. A third toxin-insensitive URF13 mutant shows considerable toxin
binding in E. coli, demonstrating that toxin binding can occur without
causing membrane permeabilization. Our results indicate that toxin-mediated
membrane permeabilization only occurs when toxin or methomyl is bound to
URF13.
