Role of the Arginyl-Glycyl-Aspartic Motif in the Action of Ptr Toxa Produced by Pyrenophora tritici-repentis

• Published in: Plant physiology (Bethesda) 2002-11, Vol.130 (3), p.1545-1551
• Main Authors: Meinhardt, Steven W, Cheng, Weijun, Kwon, Chil Y, Donohue, Christine M, Rasmussen, Jack B
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Amino acids; Apoptosis - drug effects; Cell death; Cloning, Molecular; Complementary DNA; DNA, Complementary - genetics; E coli; Electrolyte leakage; Electrolytes; Electrolytes - metabolism; Escherichia coli - genetics; Fungal Proteins - genetics; Fungal Proteins - pharmacology; Fungal Proteins - physiology; Genetic mutation; Integrins; Molecular Sequence Data; Mortality; Mutation; Mycotoxins - genetics; Mycotoxins - pharmacology; Mycotoxins - physiology; Oligopeptides - genetics; Oligopeptides - pharmacology; Oligopeptides - physiology; Pathogens; Plant Leaves - cytology; Plant Leaves - drug effects; Plant Leaves - metabolism; Plants; Plants Interacting with Other Organisms; Polymerase chain reaction; Receptors; Toxins; Triticum - cytology; Triticum - drug effects; Wheat
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.006684

A fundamental problem of plant science is to understand the biochemical basis of plant/pathogen interactions. The foliar disease tan spot of wheat (Triticum aestivum), caused by Pyrenophora tritici-repentis, involves Ptr ToxA, a proteinaceous host-selective toxin that causes host cell death. The fungal gene ToxA encodes a 17.2-kD pre-pro-protein that is processed to produce the mature 13.2-kD toxin. Amino acids 140 to 142 of the pre-pro-protein form an arginyl-glycyl-aspartic (RGD) sequence, a motif involved in the binding of some animal proteins and pathogens to transmembrane receptor proteins called integrins. Integrin-like proteins have been identified in plants recently, but their role in plant biology is unclear. Our model for Ptr ToxA action predicts that toxin interacts with a putative host receptor through the RGD motif. Mutant clones of a ToxA cDNA, created by polymerase chain reaction such that the RGD in the pro-toxin was changed to arginyl-alanyl-aspartic or to arginyl-glycyl-glutamic, were expressed in Escherichia coli. Extracts containing mutated forms of toxin failed to cause host cell death, but extracts from E. coli expressing both a wild-type pro-protein cDNA and a control mutation away from RGD were active in cell death development. In competition experiments, 2 mM RGD tripeptide reduced the level of electrolyte leakage from wheat leaves by 63% when co-infiltrated with purified Ptr ToxA (15 μg mL-1) obtained from the fungus, but the control peptide arginyl-glycyl-glutamyl-serine provided no protection. These experiments indicate that the RGD motif of Ptr ToxA is involved with toxin action, possibly by interacting with a putative integrin-like receptor in the host.