AUX/IAA Proteins Are Active Repressors, and Their Stability and Activity Are Modulated by Auxin

Abstract

Aux/IAA genes are early auxin response genes that encode short-lived nuclear proteins with four conserved domains, referred to as I, II, III, and IV. Arabidopsis Aux/IAA proteins repressed transcription on auxin-responsive reporter genes in protoplast transfection assays. Mutations in domain II resulted in increased repression, whereas mutations in domains I and III partially relieved repression. Aux/IAA proteins fused to a heterologous DNA binding domain were targeted to promoters of constitutively expressed reporter genes and actively repressed transcription in an auxinresponsive and dose-dependent manner. In comparison with an unfused luciferase protein, luciferase fused to Aux/IAA proteins displayed less luciferase activity, which further decreased in the presence of auxin in transfected protoplasts. Domain II mutations increased and domain I mutations decreased luciferase activity with the fusion proteins. These results suggested that Aux/IAA proteins function as active repressors by dimerizing with auxin response factors bound to auxin response elements and that early auxin response genes are regulated by auxin-modulated stabilities of Aux/ IAA proteins.

Plant tropisms: providing the power of movement to a sessile organism

“…Dominant mutations in IAA19 that stabilize the resultant protein lead to an aphototropic phenotype reminiscent of nph4 . This is in agreement with biochemical results that suggest dominant mutations in Aux/IAA family members lead to a decrease in auxin-stimulated transcription (Tiwari et al, 2001). By increasing the stability of IAA19, protein turnover through the proteosome is decreased and IAA19 remains bound to ARF7, leaving the complex inactive in an increased auxin environment.…”

“…Liscum and Reed (2002) have presented a relatively simple model to explain auxinregulated ARF function. First, ARFS are thought to bind to AuxREs of target genes as inactive heterodimers with Aux/IAA proteins (Tiwari et al, 2001Figure 2). Next, as the auxin concentration rises, turnover of the IAA proteins occurs via SCF TIR1 -dependent proteoelysis (Gray et al, 2001;Ramos et al, 2001;Zenser et al, 2001Zenser et al, , 2003Kepinski and Leyser, 2004), allowing ARF-ARF heterodimers to form resulting in active complex ( Figure 2).…”

Peptidyl-prolyl isomerization targets rice Aux/IAAs for proteasomal degradation during auxin signalling

“…The auxin-dependent formation of the SCF TIR1 -Aux/IAA complex results in the ubiquitination of Aux/IAAs, which are subsequently subjected to degradation through the 26S proteasomal degradation machinery. The proteasomal destruction of Aux/ IAAs relieves ARFs from the repressive complex and the active ARFs directly bind to the promoters to activate or repress the transcription of the downstream target genes [7][8][9][10][11] .…”

“…Of those domains, domain II, termed as the degron motif, is a key determinant of auxin-dependent degradation of Aux/IAAs by mediating interaction with the auxin receptors TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEINS (TIR1/ AFBs). Genetic studies reveal that various gain-of-function mutations in domain II, largely located in the highly conserved central Gly-Trp-Pro-Pro-Val (GWPPV) motif, render Aux/ IAAs resistant to the auxin-induced proteasomal degradation, resulting in a dominant auxin insensitive phenotype [7][8][9][10][12][13][14] . Notably, the auxin-dependent interaction between TIR1 and Aux/IAAs is reduced or abolished by mutations in the GWPPV motif, illustrating the critical role of the domain II-mediated degradation of Aux/IAAs in auxin signalling 4,5,7,13,15 .…”

Context, Specificity, and Self-Organization in Auxin Response

“…Aux/IAA proteins do not appear to bind DNA themselves but can affect the transcription of ARF-regulated genes by dimerising with ARFs (Tiwari et al 2001;Guilfoyle and Hagen 2007). This is because Aux/IAAs possess a potent transcriptional repression activity conferred by domain I of the protein .…”