PROTEIN PHOSPHATASE-ACTIVITY OF ABSCISIC-ACID INSENSITIVE-1 (ABI1) PROTEIN FROM ARABIDOPSIS-THALIANA

Mutations at the ABI1 (abscisic acid insensitive 1) locus of the plant Arabidopsis thaliana cause a reduction in sensitivity to the plant ho rmone abscisic acid. The sequence of ABI1 predicts a protein composed of an N-terminal domain that contains motifs for an EF-hand Ca2+-bindi ng site, and a C-terminal domain with similarities to protein serine/t hreonine phosphatases 2C. We report here two sets of experimental evid ence that indicate that ABI1 has typical protein phosphatase 2C activi ty. First, expression of the ABI1 c-terminal domain partially compleme nted the temperature-sensitive growth defect of a Saccharomyces cerevi siae protein phosphatase 2C mutant. Second, recombinant proteins that contained the ABI1 C-terminal domain displayed in vitro phosphatase ac tivity towards P-32-labelled casein, and this activity displayed Mg2or Mn2+ dependence and okadaic acid insensitivity typical of protein p hosphatases 2C. Characterisation of recombinant proteins that containe d various portions of ABI1 indicated that the putative EF-hand motif i s unlikely to mediate Ca2+ regulation of the ABI1 phosphatase activity at physiological Ca2+ concentrations, and may represent an EF-hand an alogue rather than an EF-hand homologue. The abil-1 mutation appeared to cause significant reduction in the phosphatase activity of ABI1. Th ese results are discussed in relation to the dominant phenotype of abi l-1 over the wild-type allele in plants, and to the possible role of A BI1 in abscisic acid signalling.