URF13, A LIGAND-GATED, PORE-FORMING RECEPTOR FOR T-TOXIN IN THE INNERMEMBRANE OF CMS-T MITOCHONDRIA

URF13 is the product of a mitochondrial-encoded gene (T-urf13) found o nly in maize plants containing the Texas male-sterile cytoplasm (cms-T ), and it is thought to be responsible for both cytoplasmic male steri lity and the susceptibility of cms-T maize to the fungal pathogens Bip olaris maydis race T and Phyllosticta maydis. Mitochondria isolated fr om cms-T maize are uniquely sensitive to pathotoxins (T-toxin) produce d by these fungi and to methomyl (a commercial insecticide). URF13 act s as a receptor that specifically binds T-toxin to produce hydrophilic pores in the inner mitochondrial membrane. When expressed in Escheric hia coil cells, URF13 also forms hydrophilic pores in the plasma membr ane if exposed to T-toxin or methomyl. Topological studies established that URF13 contains three membrane-spanning alpha-helices, two of whi ch are amphipathic and can contribute to pore formation. Chemical cros slinking of URF13 was used to demonstrate the existence of URF13 oligo mers in cms-T mitochondria and E. coil cells. The ability of the carbo xylate-specific reagent N,N'-dicyclohexycarbodiimide, to cross-link UR F13 was used in conjunction with site-directed mutagenesis to establis h that the URF13 tetramer has a central core consisting of a four-alph a-helical bundle which undergoes a conformational change after interac tion with T-toxin or methomyl. Overall, the experimental evidence indi cates that URF13 functions as a ligand-gated, pore-forming T-toxin rec eptor in cms-T mitochondria.