Solution NMR Structures of Active Pyrenophora tritici-repentis ToxB and its Inactive Homolog Reveal Potential Determinants of Toxin Activity [Plant Biology]

July 26th, 2014 by Nyarko, A., Singarapu, K. K., Figueroa, M., Manning, V. A.., Pandelova, I., Wolpert, T. J., Ciuffetti, L. M., Barbar, E.

Ptr ToxB (ToxB) is a proteinaceous host-selective toxin produced by Pyrenophora tritici-repentis (Ptr), 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 Ptr isolates that lack toxic activity. There are no high-resolution structures for any of the ToxB homologs, or for any protein with greater than 30% sequence identity, and therefore what underlies activity remains an open question. Here we present the NMR structures of ToxB and its inactive homolog 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.
  • Posted in Journal of Biological Chemistry, Publications
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