Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.

Sci. STKE, 31 May 2005
Vol. 2005, Issue 286, p. tw208
[DOI: 10.1126/stke.2862005tw208]


AUXIN A New Route to Degradation

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]

Citation: A New Route to Degradation. Sci. STKE 2005, tw208 (2005).

To Advertise     Find Products

Science Signaling. ISSN 1937-9145 (online), 1945-0877 (print). Pre-2008: Science's STKE. ISSN 1525-8882