Characterization and localization of fusicoccin-binding sites in leaf tissues of Vicia faba L. probed with a novel radioligand.

Tritiated 9'-nor-fusicoccin-8'-alcohol provides a highly bioactive radioligand of high specific activity which is easily prepared by oxidation of fusicoccin and subsequent reduction with tritiated sodium borohydride. Using this radioligand, we have identified and characterized a selective binding site for fusicoccin (Ka for [(3)H]-9'-nor-fusicoccin-8'-alcohol=0.20·10(9) M(-1); Ka, apparent for fusicoccin=0.21·10(9) M(-1)) located at the plasmalemma of Vicia faba leaf tissue. The site is thermolabile, readily degraded by trypsin and located at the apoplastic face of the plasmalemma based on results obtained using right-side-out plasmalemma vesicles prepared by aqueous two-phase partitioning and macromolecular fusicoccin-derivatives. The binding-protein is present in guard cells of Vicia faba, as shown by the use of purified guard-cell protoplasts.