The Helminthosporoside-binding Protein of Sugarcane

Clones of sugarcane susceptible to the eyespot disease caused by Helminthosporium sacchari possess a membrane protein that binds helminthosporoside, the host-specific toxin produced by this fungus pathogen (Steiner, G. W., and Strobel, G. A. (1971) J. Biol. Chem. 246, 4350). Membrane preparations from clones resistant to the toxin do not possess binding activity. Clones that are slightly affected by the toxin, likewise possess only an intermediate amount of toxin-binding activity. That the binding of the toxin to membranes occurs in vivo in susceptible clones and not resistant ones was established by labeling experiments. Furthermore, treatment of leaves with one of any number of α-galactosides inhibits symptom expression in them after application of the toxin. These same α-galactosides also lower the amount of binding of helminthosporoside by membrane preparations. The binding protein was purified from Triton X-100-treated membrane preparations of the susceptible clone 51 NG 97. The protein has a molecular weight of about 49,000 as determined by gel electrophoresis and 45,000 by gel filtration. It consists of four subunits each with a molecular weight of about 11,700. It has a Km in the order of 10-5, at least two binding sites for helminthosporoside, and its amino acid composition and isoelectric point have been established.