1. Department of Pharmacology, University of Washington, School of Medicine, Box 357280, Seattle, Washington 98195, USA
2. Department of Biology, Indiana University, Bloomington, Indiana 47405, USA
3. Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK

Correspondence to: Ning Zheng¹ Correspondence and requests for materials should be addressed to N.Z. (Email: nzheng@u.washington.edu).

Abstract

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.

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