Structural and Physical Properties of a Necrosis-Inducing Toxin from Pyrenophora tritici-repentis

Cultivar-specific toxic metabolites of Pyrenophora tritici-repentis are involved in the appearance of necrotic and chlorotic foliar lesions characteristic of tan spot. A P. tritici-repentis necrosis-inducing toxin, Ptr necrosis toxin, was purified from isolate 86-124, sequenced by gas-phase amino acid microsequencing, and characterized by circular dichroism (CD) spectroscopy and isoelectric focusing. The purified protein had a similar amino acid composition and molecular weight as previously reported. Analysis of the CD spectrum from 178 to 250 nm indicated a protein consisting of 13% alpha-helix, 36% antiparallel beta-sheet, 25% turns, and 25% other structures. The Ptr necrosis toxin from isolate 86-124 has an isoelectric point near pH 10. Using overlapping proteolytic fragments obtained from the toxin, a sequence of 101 continuous amino acids was obtained, but the amino terminus was blocked and 9 to 16 amino acids could not be sequenced. Secondary structure prediction based on the amino acid sequence indicated a beta-sheet protein with little alpha-helix, which is in agreement with the structure determined by CD spectroscopy. Sequence analysis indicated the presence of a possible membrane adhesion site and several possible phosphorylation sites that may be involved in phytotoxicity.