Abscisic acid-specific binding sites in the flesh of developing apple fruit

Abscisic acid (ABA) specific-binding sites localized in the cytosol were id entified and characterized in the flesh of developing apple (Malus pumila L . cv. Starkrimon) fruit. ABA binding activity was scarcely detectable in th e microsomes but high ABA binding activity in the cytosolic fraction was de tected. The ABA-binding sites possessed a protein nature with both active s erine residues and thiol-groups of cysteine residues in their functional bi nding sites. ABA binding was shown to be saturable, reversible and of high affinity. A Scatchard plot provided evidence for two different ABA binding proteins, one with higher affinity (K-d=2.3 nM) and the other with lower af finity (K-d=58.8 nM). Phaseic acid, trans-ABA and (-)-ABA had essentially n o affinity for the binding proteins, indicating their stereospecificity to bind physiologically active cis-(+)-ABA. The time-course, pH- and temperatu re-dependence of the ABA-binding proteins were determined. It is hypothesiz ed that the detected ABA-binding proteins may be putative ABA-receptors tha t mediate ABA signals during fruit development.