The Story of Auxin-Binding Protein 1 (ABP1)

Abstract

The auxin-binding protein 1 (ABP1) has endured a history of undulating prominence as a candidate receptor for this important phytohormone. Its capacity for binding auxin has not been in doubt, a feature adequately explained by its crystal structure, but any relevance of this to auxin signaling and plant development has been far more demanding to define. Over its research lifetime, it has been associated with many auxin-induced activities, including ion fluxes across the plasma membrane, rearrangement of the cytoskeleton and cell shape, and the abundance of PIN proteins at the plasma membrane via control of endocytosis, all of which required its presence in the apoplast. Yet, ABP1 has a KDEL sequence that targets it to the endoplasmic reticulum, where most of it remains. This mismatch has been more than adequately compensated for by the need for an auxin receptor to account for responses far too rapid to be executed through transcription and translation and the TIR1/AuxIAA coreceptor system. However, discoveries showing that abp1-null mutants are not compromised for auxin signaling or development, that TIR1 or AFB1 are necessarily involved with very rapid responses at the plasma membrane, and that these rapid responses are mediated with intracellular auxin all suggest that ABP1's auxin-binding capacity is not physiologically relevant. Nevertheless, ABP1 is ubiquitous in higher plants and throughout plant tissues. We need to complete its history by defining its function inside plant cells.

Figure 1.

Structural features of auxin-binding protein 1 (ABP1). In all panels, 1-NAA is shown in green and Zn²⁺ in purple. (A) The dimer of ABP1 with one monomer shown as a molecular surface (gray), the other as a ribbon (orange). The glycans are both shown as surfaces, with the glycan of the orange monomer (glycan 1) folded across the gray monomer, and vice versa. (B) The aperture to the auxin-binding site with the surface of 1-NAA also shaded in transparent green. (C) The Zn²⁺ ion is coordinated by a cluster of histidine residues and an aspartate, and when 1-NAA binds a water molecule is displaced (not shown) to allow the carboxylic acid group to hydrogen bond to the Zn²⁺ (dashed lines). Tryptophan151 is seen behind the auxin at the end of an α helix (receding into the background). (D) The conserved sequence boxes of ABP1: box A (magenta), box B (yellow), and box C (cyan), plus the conserved Trp151 (orange). PDB 1LRH visualized using Chimera (Pettersen et al. 2004).

Figure 2.

Expression maps for auxin-binding protein 1 (ABP1) in Arabidopsis (top, AT4G02980) and maize (bottom, GRMZM2FG116204). The Arabidopsis image was generated from data from Klepikova et al. (2016). The maize image is based on data from Downs et al. (2013). Both images were prepared by bar.utoronto.ca/eplant (Waese et al. 2017). (RSE) Relative standard error.