Binding affinity of the methyl ester of AK-toxin I to membrane fractions from Japanese pear leaves

The binding site of AK-toxin, a host-specific toxin against Japanese pear, was searched for in the membrane fractions of the pear leaves, using H-3-la beled AK-toxin I methyl ester. Binding activity, which was displaceable by the unlabeled ligand, was observed for microsomal fraction from a toxin-sus ceptible cultivar, Nijisseiki. However, the binding was also observed for t hose from toxin-resistant cultivars, Kosui and Hosui, Detection of the spec ific binding failed for the plasma membrane fraction which was prepared fro m microsomal fraction of the toxin-susceptible cultivar by aqueous two-phas e separation, and the hitherto presumed model of the AK-toxin receptor in t he plasma membrane could not be verified.