Specific abscisic acid-binding sites in mesocarp of grape berry: Properties and subcellular localization

The subcellular fractions were isolated from the mesocarp of grape berry (V itis vinifera x Vitis labrusca, cv. Kyoho). The high specific ABA-binding s ites in the microsomes were identified by the microvolume radioligand bindi ng assay. The binding was shown to be saturable, of high affinity and of lo w concentration. The binding sites possessed a protein nature. Phaseic acid and trans-ABA had no affinity to the binding proteins, indicating the high specificity of the ABA binding proteins to ABA. The binding was pH depende nt, with an optimum pH at 6.0, and this binding was higher at 25 degrees C than at 0 degrees C. The maximum binding was attained for 30 min, and there after the binding decreased along with the incubation time. According to th e Scatchard plots for each developmental phase [I, II, veraison (the onset of ripening) and III] of grape berry, the dissociation constants (Kd) are 1 7.5, 50.0, 6.3 and 13.3 nmol/L, and the maximum binding values (Bmax) were 98.6, 523.0, 41.6 and 85.4 pmol/g protein, respectively. Each Scatchard plo t from ABA-binding assays for a particular developmental phase appeared to be linear, indicating that one same type of ABA-binding proteins may exist for a particular developmental phase. The ABA-binding proteins were shown t o be mainly localized in the endomembranes but not in the plasma membranes nor in the cytoplasm. The possible functions of the ABA-binding proteins fo r fruit development were discussed. We hypothesized that the detected ABA-b inding proteins may be either putative ABA-receptors or ABA-transporters th at mediate ABA signals during grape berry development.