A New Route to Degradation

Science's STKE  31 May 2005:
Vol. 2005, Issue 286, pp. tw208
DOI: 10.1126/stke.2862005tw208

Two groups have identified the Arabidopsis auxin receptor and shown that auxin promotes degradation of target proteins through a novel mechanism. Auxins, plant hormones that play a critical role in plant growth and development, stimulate the transcription of target genes by promoting the degradation of members of a family of transcriptional repressors, the Aux/IAA proteins. Aux/IAA proteins are targeted for degradation by SCFTIR1, a ubiquitin ligase complex in which the F-box protein (the component that recognizes and recruits the target) is TIR1 (transport inhibitor response protein 1). Auxins enhance the association of Aux/IAA proteins with SCFTIR1; however, the underlying mechanism and the identity of the auxin receptor have been unclear (see Callis).

Dharmasiri et al. found that the auxin-induced association between Aux/IAA and SCFTIR1 was temperature independent, arguing against the requirement for an enzyme-based modification underlying this association. Association between Aux/IAA and SCFTIR1 depended on the continuous presence of auxin, which also promoted binding of partially purified (immunoprecipitated) TIR1 to IAA7. Similarly, Kepinski and Leyser showed auxin-dependent binding of immunoprecipitated TIR1 to a 17-amino acid peptide from a conserved Aux/IAA domain whose mutation disrupts the Aux/IAA-SCFTIR1 interaction (Aux/IAA domain II peptide) and found that this depended on the continuous presence of auxin. Competitive binding assays performed by both groups indicated that auxins bound to a component of SCFTIR1. Dharmasiri et al. showed that TIR1 synthesized in insect cells exhibited auxin-dependent binding to IAA7 and that saturable auxin binding was lost in extracts from plant lines that lacked TIR1 and related F-box proteins. Kepinski and Leyser showed that TIR1 synthesized in Xenopus embryos exhibited auxin-dependent binding to Aux/IAA domain II peptide. Thus, both groups identified TIR1 as the auxin receptor and concluded that auxin promoted association of SCFTIR1 to Aux/IAA proteins by directly binding to TIR1 rather than by instigating a posttranslational modification of the target protein.

N. Dharmasiri, S. Dharmasiri, M. Estelle, The F-box protein TIR1 is an auxin receptor. Nature 435, 441-445 (2005). [PubMed]

S. Kepinski, O. Leyser, The Arabidopsis F-box protein TIR1 is an auxin receptor. Nature 435, 446-451 (2005). [PubMed]

J. Callis, Auxin action. Nature 435, 436-437 (2005). [PubMed]