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Functional characterization of tomato Sl-IAA3 and Sl-hls genes. Role ...

Functional characterization of tomato Sl-IAA3 and Sl-hls genes. Role ...

ChapitreI: Bibliographic

ChapitreI: Bibliographic review (Woodward and Bartel, 2005) Figure 4. A Model for auxin response through the TIR1 auxin receptor pathway. Transcriptionally activating ARFs (green) are bound to auxin-responsive promoter elements but are counteracted (blunt arrows) by heterodimerization with Aux/IAA transcriptional repressors (dark red) via two domains (III and IV) conserved between ARF and Aux/IAA proteins. Auxin (pink) binds to TIR1 (blue) or a TIR1-Aux/IAA complex to promote or stabilize TIR1-Aux/ IAA domain II interaction. TIR1 tethers the Aux/IAA protein to an SCF complex (purple) that is thought to catalyze attachment of multiple ubiquitin (Ub) moieties to the Aux/IAA target protein. The ubiquitinated Aux/IAA protein is then degraded by the 26S proteasome (orange). Increased Aux/IAA degradation in response to auxin frees the activating ARF proteins from repression, allowing auxin-responsive transcription. Among the auxin-induced transcripts are those encoding the Aux/IAA repressors themselves, creating a negative feedback regulatory system. See text for references. Tan et al. (2007) expressed TIR1 complexed with ASK1 (a SCF TIR1 adaptor) and demonstrated that auxin enhanced the binding of an Aux/IAA protein to the complex, they obtained crystal structures for the complex alone and for complexes bound to IAA and the two synthetic auxins (1-NAA and 2,4-D) along with an Aux/IAA peptide. Figure 5. The main molecular players in the work of Tan et al (2007). The three auxin ligands that the authors crystallizedin association with the TIR1 auxin receptor. IAA itself, and the synthetic auxins 1-NAA and 2,4-D, bind to a ‘promiscuous’ cavity in the receptor with different affinities, but all of them stabilize the interaction between the Aux/IAA repressor and the receptor. The crystal structures showed that the TIR1–ASK1 complex had a mushroom shape, with the leucine-rich-repeat domain of TIR1 forming the cap, and the F- 8

ChapitreI: Bibliographic review box of TIR1 along with ASK1 forming the stem. A pocket on the top of the TIR1 leucine-rich-repeat domain functions in both auxin binding and substrate recruitment. It turns out that auxin binds to the bottom of the pocket that tolerates moderately different planar ring structures (that is, natural and synthetic auxins). (Tan et al., 2007) Figure 6. Schematic diagram of auxin functioning as a ‘molecular glue’ to enhance TIR1– substrate interactions. In contrast to an allosteric mechanism, auxin binds to the same TIR1 pocket that docks the Aux/IAA substrate. Without inducing significant conformational changes in its receptor, auxin increases the affinity of two proteins by simultaneously interacting with both in a cavity at the protein interface. The Aux/IAA peptide binds in close proximity to the auxin-binding site in the upper part of the pocket (Figure6). The GWPPV motif of Aux/IAA proteins (Figure7) is packed directly against auxin and covers the auxin binding site. This is thought to trap auxin in the binding pocket until the Aux/IAA peptide is released and moved along the degradation pathway. (Tan et al., 2007) Figure 7. Depiction of the four conserved domains of Aux/IAA repressors. The synthetic peptide from domain II is sufficient for targeting Aux/IAA to the TIR1 auxin receptor, the core sequence being GWPPV (G, glycine; W, tryptophan; P, proline; V, valine). The authors compared TIR1 structures that had IAA or the two synthetic auxins in the ligand binding site. IAA binds to TIR1 with the greatest affinity of the three auxins (Figure 5). The synthetic auxins bind to TIR1 in a manner similar to IAA, but with affinities determined by how effectively their ring structures fit into and interact with the promiscuous cavity of the receptor. Interestingly, the binding of auxin does not induce significant conformational changes in TIR1. Instead, auxin enhances the binding of Aux/IAA substrate to TIR1 by occupying a cavity between substrate and receptor, thus forming a 9

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