2p1q

Auxin is a pivotal plant hormone that controls many aspects of plant growth and development. Perceived by a small family of F-box proteins including transport inhibitor response 1 (TIR1), auxin regulates gene expression by promoting SCF ubiquitin-ligase-catalysed degradation of the Aux/IAA transcription repressors, but how the TIR1 F-box protein senses and becomes activated by auxin remains unclear. Here we present the crystal structures of the Arabidopsis TIR1-ASK1 complex, free and in complexes with three different auxin compounds and an Aux/IAA substrate peptide. These structures show that the leucine-rich repeat domain of TIR1 contains an unexpected inositol hexakisphosphate co-factor and recognizes auxin and the Aux/IAA polypeptide substrate through a single surface pocket. Anchored to the base of the TIR1 pocket, auxin binds to a partially promiscuous site, which can also accommodate various auxin analogues. Docked on top of auxin, the Aux/IAA substrate peptide occupies the rest of the TIR1 pocket and completely encloses the hormone-binding site. By filling in a hydrophobic cavity at the protein interface, auxin enhances the TIR1-substrate interactions by acting as a 'molecular glue'. Our results establish the first structural model of a plant hormone receptor.

Mechanism of auxin perception by the TIR1 ubiquitin ligase., Tan X, Calderon-Villalobos LI, Sharon M, Zheng C, Robinson CV, Estelle M, Zheng N, Nature. 2007 Apr 5;446(7136):640-5. PMID:17410169

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.

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Categories: Arabidopsis thaliana | Auxin and TIR1 Ubiquitin Ligase | RCSB PDB Molecule of the Month | Calderon-Villalobos, L I.A. | Estelle, M. | Robinson, C V. | Sharon, M. | Tan, X. | Zheng, N. | F-box | Leucine rich repeat | Signaling protein