title: Solution NMR structures of Pyrenophora tritici-repentis ToxB and its inactive homolog reveal potential determinants of toxin activity.
abstract: Pyrenophora tritici-repentis Ptr ToxB (ToxB) is a proteinaceous host-selective toxin produced by Pyrenophora tritici-repentis (P. tritici-repentis), a plant pathogenic fungus that causes the disease tan spot of wheat. One feature that distinguishes ToxB from other host-selective toxins is that it has naturally occurring homologs in non-pathogenic P. tritici-repentis isolates that lack toxic activity. There are no high-resolution structures for any of the ToxB homologs, or for any protein with >30% sequence identity, and therefore what underlies activity remains an open question. Here, we present the NMR structures of ToxB and its inactive homolog Ptr toxb. Both proteins adopt a β-sandwich fold comprising three strands in each half that are bridged together by two disulfide bonds. The inactive toxb, however, shows higher flexibility localized to the sequence-divergent β-sandwich half. The absence of toxic activity is attributed to a more open structure in the vicinity of one disulfide bond, higher flexibility, and residue differences in an exposed loop that likely impacts interaction with putative targets. We propose that activity is regulated by perturbations in a putative active site loop and changes in dynamics distant from the site of activity. Interestingly, the new structures identify AvrPiz-t, a secreted avirulence protein produced by the rice blast fungus, as a structural homolog to ToxB. This homology suggests that fungal proteins involved in either disease susceptibility such as ToxB or resistance such as AvrPiz-t may have a common evolutionary origin.
authors: From the Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331.
authors: Afua
authors: A
authors: Nyarko
authors: the Center for NMR and Structural Chemistry, Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India, and.
authors: Kiran K
authors: KK
authors: Singarapu
authors: the Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, Oregon 97331.
authors: Melania
authors: M
authors: Figueroa
authors: the Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, Oregon 97331.
authors: Viola A
authors: VA
authors: Manning
authors: the Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, Oregon 97331.
authors: Iovanna
authors: I
authors: Pandelova
authors: the Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, Oregon 97331.
authors: Thomas J
authors: TJ
authors: Wolpert
authors: the Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, Oregon 97331 ciuffetL@science.oregonstate.edu.
authors: Lynda M
authors: LM
authors: Ciuffetti
authors: From the Department of Biochemistry and Biophysics, Oregon State University, Corvallis, Oregon 97331, barbare@science.oregonstate.edu.
authors: Elisar
authors: E
authors: Barbar
mesh_terms: D018360:Crystallography, X-Ray; D019143:Evolution, Molecular; D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism / Q000633:toxicity; D054884:Host-Pathogen Interactions / Q000235:genetics*; D009682:Magnetic Resonance Spectroscopy; D010935:Plant Diseases / Q000382:microbiology*; D017510:Protein Folding; D017433:Protein Structure, Secondary; D012996:Solutions / Q000737:chemistry; D014908:Triticum / Q000235:genetics / Q000382:microbiology*
chemical_list: D005656:Fungal Proteins; D012996:Solutions; C434209:ToxB protein, Pyrenophora tritici-repentis
keywords: Host-Pathogen Interaction; Nuclear Magnetic Resonance (NMR); Pathogenesis; Protein Dynamic; Protein Structure; Toxin