Family AUX/IAA

Aux/IAA genes are genes that code for small nuclear proteins, 18 to 35-kD. These proteins function as transcriptional repressors by dimerizing with Auxin response factors (ARFs) that reside on auxin responsive promoter elements (AuxREs). Most Aux/IAA proteins have four conserved domains, I-IV. Domain I is an active repression domain with a conserved LxLxL motif that is important for conferring repression. Domain II interacts with an F-box protein TIR1, a component of the SCFTIR1 ubiquitin ligase complex. Auxin increases this interaction in a dose-dependent manner, promoting the rapid degradation of Aux/IAA proteins through the ubiquitin-proteasome pathway. Mutations in this domain result in increased stability of the Aux/IAA protein and increased repression of the AuxRE. Domains III and IV have a similar amino acid sequence to motifs III and IV found in the C-terminal domains of ARF proteins, and these motifs mediate dimerization between Aux/IAA and ARF proteins (PMID:14742873).