Two E3 Ubiquitin-protein Ligases, AIP2 and AIP6, Differentially Regulate the Transcriptional Activity of ABI3 from Arabidopsis

Although ubiquitylation is generally assumed to regulate protein degradation via the 26S proteasome pathway, ubiquitin conjugation to transcription proteins and histones has alternative roles, such as increasing the transcriptional activity of acidic transcription activation domains (TADs). In the plant Arabidopsis thaliana the acidic transcription factor ABI3 interacts with and is regulated by many proteins, including two RING-H2 ubiquitin protein ligases, ABI3 interacting protein 2 and 6 (AIP2 and AIP6). We used heterologous expression in the yeast S. cerevisiae to ascertain the functions of AIP2 and AIP6 separate from other ABI3 interacting factors present in planta. AIP2 and AIP6 bound to both the N- and C-terminal halves of ABI3, and both ubiquitylated only the N-terminal half of ABI3 that contains an acidic TAD. Although dispensable for ABI3 binding, the RING-H2 domain of AIP6 was essential for ABI3 ubiquitylation. Ubiquitin conjugation to ABI3 by either AIP2 or AIP6 triggered proteolysis via the 20S proteasome pathway. When expressed alone, the transcriptional activity of ABI3 was less than 1.5% of the level detected in the presence of AIP6, suggesting that AIP6 could potentially regulate ABI3 activity in Arabidopsis both positively and negatively. Interestingly, AIP2 increased the activity of the ABI3-TAD to a much smaller extent. The effect of AIP6 on ABI3 is reminiscent of the licensing of transcription factor activity by ubiquitylation that has been described in yeast and animals, and to our knowledge, it is the first report that this type of regulation operates on a plant transcription activation domain.