Hydrogen peroxide is a regulator of ABI1, a protein phosphatase 2C from Arabidopsis

Protein phosphatases 2C (PP2Cs) exhibit diverse regulatory functions in sig nalling pathways of animals, yeast and plants. ABI1 is a PP2C of Arabidopsi s that exerts negative control on signalling of the phytohormone abscisic a cid (ABA). Characterisation of the redox sensitivity of ABI1 revealed a str ong enzymatic inactivation by hydrogen peroxide (H2O2) which has recently b een implicated as a secondary messenger of ABA signalling. H2O2 reversibly inhibited ABI1 activity in vitro with an IC50 of approximately 140 muM in t he presence of physiological concentrations of glutathione. In addition, AB I1 was highly susceptible to inactivation by phenylarsine oxide (IC50 = 3-4 muM) indicative for the facile oxidation of vicinal cysteine residues. Thu s, H2O2 generated during ABA signalling seems to inactivate the negative re gulator of the ABA response. (C) 2001 Federation of European Biochemical So cieties. Published by Elsevier Science B.V. All rights reserved.