Abscisic acid binds to an Arabidopsis thaliana phosphodiesterase and tunes its activity

Growing evidence suggests that plant proteomes contain numerous proteins that specifically bind abscisic acid (ABA). Many of them are complex multidomain proteins where specific ABA-binding can cause biochemical and physiological changes. Here we show that the Arabidopsis thaliana K⁺ transporter AtKUP5 contains both a functional cytoplasmic N-terminal adenylate cyclase (AC) enabling the synthesis of 3',5'-cAMP from ATP and a C-terminal phosphodiesterase (PDE) that hydrolyses 3',5'-cAMP to 5'-AMP. We found that ABA binds in a ligand-specific manner to the catalytic center of the PDE thereby causing a reduction of 3',5'-cAMP hydrolysis in vitro. The hydrolytic activity of the PDE is ABA concentration-dependent, biphasic and requires the presence of an intact ABA-binding site similar to the one in the canonical Pyrabactin resistance 1/PYR-like/Abscisic acid receptors, with V_max of 1.19 pmole min^-1 μg^-1 in the absence of ABA, increasing to 1.58 pmole min^-1 μg^-1 at 2 nM ABA, and decreasing to 0.75 pmole min^-1 μg^-1 at 50 nM ABA. These findings are therefore consistent with a direct role of ABA in PDE activity modulations and form a functional link between 3',5'-cAMP signaling and K⁺ flux. Furthermore, we predict that a growing number of such receptor-like proteins that specifically and directly interact with ABA will be discovered thereby uncovering complex and ancient layers of signaling and metabolic regulation.