Regulation of ABI5 turnover by reversible post-translational modifications.

Post-translational modifications (PTMs) such as phosphorylation, ubiquitination, and sumoylation play significant roles in regulating abscisic acid (ABA) signaling. The targets for PTM are usually transcriptional regulators such as Abscisic acid Insensitive 5 (ABI5). PTM regulate ABI5 stability as well as activity. The abundance of ABI5 is tightly controlled by the ubiquitination-26S proteasome system. E3 ubiquitin ligases such as KEG negatively regulate ABA signaling by promoting ABI5 ubiquitination and subsequent degradation by the 26S proteasome. In our recent study we demonstrated that, in the absence of ABA, KEG-mediated turnover of ABI5 occurs within the cytoplasm. Whereas ubiquitination promotes ABI5 degradation, sumoylation prohibits degradation of the transcription factor. While phosphorylation has been shown to regulate ABI5 activity, our studies and others suggest that the phosphorylation status of ABI5 does not play a significant role in modulating ABI5 turnover.