Getting a grasp on domain III/IV responsible for Auxin Response Factor–IAA protein interactions

Auxin Response Factors (ARFs) and Indole Acetic Acid (IAA) proteins contain a similar carboxyl-terminal domain (domain III/IV) that facilitates interactions among these transcription factors as well as other proteins. The specificity of these interactions is controversial, and the mechanisms involved in these interactions have not been investigated. Here, we review some of the controversies about the specificities and requirements for ARF and IAA interactions and discuss some of the technical problems that might contribute to differences reported for these interactions. We make some preliminary conclusions that ARF activator–IAA, ARF activator–ARF activator, and ARF repressor–ARF repressor interactions are favored over ARF repressor–IAA and ARF repressor–ARF activator interactions, and we suggest that IAA–IAA interactions are largely indiscriminant. Based upon the predicted secondary structure of domain III/IV, we introduce a model for how ARF and IAA proteins might interact with one another through a ubiquitin-like β-grasp fold.

► The domains III/IV of ARF and IAA proteins function in protein–protein interactions.
► Specificity of ARF and IAA protein interactions is controversial.
► Domain III/IV resembles a PB1 domain with a β-grasp fold.