Mechanism of auxin perception by the TIR1 ubiquitin ligase

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.

Peptidyl-prolyl isomerization targets rice Aux/IAAs for proteasomal degradation during auxin signalling

“…Notably, the auxin-dependent interaction between TIR1 and Aux/IAAs is reduced or abolished by mutations in the GWPPV motif, illustrating the critical role of the domain II-mediated degradation of Aux/IAAs in auxin signalling 4,5,7,13,15 . Consistent with these observations, the analysis of the crystal structure of the Arabidopsis TIR1-auxin-IAA7 peptide complex reveals that the GWPPV motif of domain II is crucial for the formation of the coreceptor complex 6 . These studies demonstrate that the domain IImediated degradation of Aux/IAAs is a key step in auxin signalling.…”

“…When the intracellular concentration of auxin is low, Aux/IAAs directly interact with AUXIN RESPONSE FACTORS (ARFs), a class of transcription factors that control the expression of downstream auxin-responsive genes, and thereby inhibit the transcriptional activity of ARFs. In response to specific developmental or environmental cues, auxin promotes the formation of a co-receptor complex consisting of a ubiquitin-ligase SCF TIR1/AFB and Aux/IAAs (refs [4][5][6]. The auxin-dependent formation of the SCF TIR1 -Aux/IAA complex results in the ubiquitination of Aux/IAAs, which are subsequently subjected to degradation through the 26S proteasomal degradation machinery.…”

Recognition of substrate degrons by E3 ubiquitin ligases and modulation by small-molecule mimicry strategies

“…Interestingly, plants have evolved two analogous induced protein degradation mechanisms by phytohormones auxin and jasmonate as part of their signalosome [50,51]. Transport inhibitor response 1 (TIR1) is the F-box substrate recognition subunit of a SCF TIR1 ubiquitin ligase (Fig.…”

“…4a), which targets transcriptional repressors known as Aux/IAA (indole-3-acetic acid) proteins for proteosomal degradation. By binding auxin, TIR1 increases affinity for its targets and triggers their rapid ubiquitination and proteosomal degradation [50,52]. Crystal structures of Arabidopsis TIR1 in complex with auxin and an Aux/IAA degron peptide derived from the IAA7 protein elucidated the structural basis of how auxin binding directs TIR1:substrate interactions (Fig.…”

“…Crystal structures of Arabidopsis TIR1 in complex with auxin and an Aux/IAA degron peptide derived from the IAA7 protein elucidated the structural basis of how auxin binding directs TIR1:substrate interactions (Fig. 4b) [50].…”