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Nov 22, 2004
Jacksonville, Florida
This is from Nature (magazine). I found it very interestiing that Auxin can interact directly with SCF^TIR1 which was not thought to be able to before.
Oh and if you need a brush up http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Auxin.html
This explains all about Auxin which is very important to plant life and CPs.

[b said:
Close encounters

Arianne Heinrichs

The plant hormone auxin controls many different aspects of plant growth and development. It mediates its downstream effects through changes in gene expression by targeting the Aux/IAA transcriptional repressors for ubiquitin-ligase-mediated degradation. Other aspects of auxin signalling, such as auxin perception, are less well understood. But the groups of Ottoline Leyser and Mark Estelle now report the remarkable finding that the F-box protein TIR1, which is part of the ubiquitin ligase complex SCFTIR1, is an auxin receptor.

The two groups had previously shown that auxin promotes the interaction between Aux/IAA proteins and SCFTIR1. Now, using pull-down assays, they have shown that the immunoprecipitated SCFTIR1 complexes retain the ability to interact with tagged Aux/IAA protein or peptide in an auxin-dependent manner — indicating that the auxin receptor must be present in the precipitate.

The Estelle group found that the effect of auxin was temperature independent. This, together with the continuous requirement of auxin for the Aux/IAA–SCFTIR1 interaction, argued against the possibility that auxin regulates the interaction by causing a stable enzymatic modification of TIR1 or Aux/IAA. Instead, might auxin modulate this interaction by binding SCFTIR1 directly? Both groups carried out further pull-down assays, but in the presence of radioactive auxin. Radioactivity was retained in the presence of wild-type Aux/IAA protein, but not with a mutant that was unable to bind SCFTIR1. Competitive binding studies with auxin and related compounds showed that auxin binding to SCFTIR1 was specific and dose dependent. They also confirmed that the auxin receptor must be localized to the SCFTIR1 complex.

As TIR1 is the only known auxin-specific subunit of the SCFTIR1 complex, the two groups tested whether auxin interacted directly with TIR1 — which it did. Leyser, Estelle and co-workers expressed TIR1 in two non-auxin-responsive systems (Xenopus laevis and insect cells), prepared cell extracts and used pull-down assays to show that heterologously expressed TIR1 and Aux/IAA proteins interacted in an auxin-specific and dose-dependent manner. However, a TIR1 mutant that lacked the F-box motif — which is required for binding the SCF protein SKP1 (ASK1 in Arabidopsis thaliana) — did not respond to auxin. So, auxin-induced binding of TIR1 to Aux/IAA requires the F-box motif. Whether this means that the deleted sequence directly mediates binding, or that the function of TIR1 as an auxin receptor requires its assembly into an SCF complex (or at least its interaction with SKP1), remains an open question.

The identification of TIR1 as an auxin receptor identifies a new mode of SCF regulation that might apply to some of the many uncharacterized SCFs in plants and animals. How auxin stimulates the interaction between SCFTIR1 and Aux/IAA is not clear — it could promote a conformational change in SCFTIR1 that favours Aux/IAA binding, or auxin might cooperatively bind both TIR1 and Aux/IAA. The authors also suggest that there are additional auxin receptors, including TIR1-related proteins and possibly an extracellular receptor.