ABSTRACT
Farnesylated proteins undergo a series of post-translational modifications, including carboxyl terminal isoprenylation, proteolysis, and methylation. In Arabidopsis thaliana, protein farnesylation has been shown to be necessary for negative regulation of ABA signaling. However, the role of post-isoprenylation protein processing in ABA signal transduction has not been described. Here, we show that the A. thaliana genome contains two distinct genes on chromosome V, AtSTE14A and AtSTE14B, which encode functional prenylcysteine alpha-carboxyl methyltransferases. AtSTE14B encodes a methyltransferase with lower apparent Kms for prenylcysteine substrates and higher specific activities than the previously described AtSTE14A-encoded methyltransferase. Furthermore, whereas AtSTE14A transcription is restricted to root and shoot tips, young leaves, and vascular tissue, AtSTE14B transcription is observed in all organs except hypocotyls and petioles. Pharmacological inhibitors of prenylcysteine alpha-carboxyl methyltransferase activity cause increased ABA sensitivity, seed dormancy, and stomatal closure, consistent with the hypothesis that prenylcysteine alpha-carboxyl methylation is necessary for negative regulation of ABA signaling. These results suggest that carboxyl methylation, which is a reversible and potentially regulated step in the processing, targeting, and function of isoprenylated plant proteins, may be an important biochemical target for introducing altered ABA sensitivity and drought tolerance into plants.
