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A combinatorial TIR1/AFB–Aux/IAA co-receptor system for differential sensing of auxin

Abstract

The plant hormone auxin regulates virtually every aspect of plant growth and development. Auxin acts by binding the F-box protein transport inhibitor response 1 (TIR1) and promotes the degradation of the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) transcriptional repressors. Here we show that efficient auxin binding requires assembly of an auxin co-receptor complex consisting of TIR1 and an Aux/IAA protein. Heterologous experiments in yeast and quantitative IAA binding assays using purified proteins showed that different combinations of TIR1 and Aux/IAA proteins form co-receptor complexes with a wide range of auxin-binding affinities. Auxin affinity seems to be largely determined by the Aux/IAA. As there are 6 TIR1/AUXIN SIGNALING F-BOX proteins (AFBs) and 29 Aux/IAA proteins in Arabidopsis thaliana, combinatorial interactions may result in many co-receptors with distinct auxin-sensing properties. We also demonstrate that the AFB5–Aux/IAA co-receptor selectively binds the auxinic herbicide picloram. This co-receptor system broadens the effective concentration range of the hormone and may contribute to the complexity of auxin response.

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Figure 1: The auxin receptor is a co-receptor system.
Figure 2: Differences in auxin-dependent TIR1/AFB–Aux/IAA interaction are not exclusively determined by the degron domain.
Figure 3: Aux/IAA proteins determine the affinity of the co-receptor complex for auxin and, together with TIR1, form a series of co-receptor complexes with a range of auxin-sensing properties.
Figure 4: Mutations at auxin, Aux/IAA and InsP6 binding sites impair auxin-dependent TIR1–Aux/IAA interaction and compromise TIR1 function in vivo.
Figure 5: Auxin agonists differentially stabilize TIR1–Aux/IAA complexes.
Figure 6: TIR1–IAA7 and AFB5–IAA7 co-receptor complexes have differential auxin-binding affinities.

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Acknowledgements

We thank E.J. Chapman for helpful discussions and comments to the manuscript and R. Shao and I. Kim for technical assistance. We also thank A. McCammon for hosting part of the computational analyses. We gratefully acknowledge financial support from the US National Institutes of Health (NIH) (R01 CA107134 to N.Z. and T32 GM07270 to L.B.S.), HHMI (M.E. and N.Z.) and the UK Biotechnology and Biological Sciences Research Council (BB/F013981/1 to S.K. and BB/F014651/1 to R.N.). The McCammon group, including A.I. and C.D.O., is supported by the NIH, US National Science Foundation and HHMI. We dedicate this work to the memory of L.B. Sheard, a talented young scientist who made key contributions at the early stage of this work. Her life was tragically cut short while this manuscript was under review.

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L.I.A.C.V. and M.E. prepared the manuscript. L.I.A.C.V. designed and performed the experiments and analyzed the data. X.T. and H.M. purified TIR1–ASK1 complex. L.I.A.C.V. expressed and purified AFB5–ASK1 complex. G.P. contributed to the generation of Y2H clones; C.D.O., A.I. and W.B. carried out homology modeling of AFB5 and docking experiments of picloram; and IAA. S.L., L.A., R.N. and S.K. expressed TIR1–ASK1 and AFB5–ASK1 constructs. S.L., S.K. and R.N. designed and performed SPR experiments. L.B.S. and N.Z. helped in the expression and purification of TIR1–ASK1 and AFB5–ASK1 complexes and the initial radioligand binding experiments. S.K., R.N., L.S. and N.Z. provided input to the manuscript. M.E. oversaw the project and approved the intellectual content.

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Correspondence to Mark Estelle.

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Calderón Villalobos, L., Lee, S., De Oliveira, C. et al. A combinatorial TIR1/AFB–Aux/IAA co-receptor system for differential sensing of auxin. Nat Chem Biol 8, 477–485 (2012). https://doi.org/10.1038/nchembio.926

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