A structure for binding auxins

Abstract

Auxin is the name given to a specific group of compounds that control cell division, cell differentiation and cell elongation in plants. This group includes the natural phytohormone indole acetic acid (IAA), phenylacetic acid and several synthetic compounds frequently used to manipulate plant growth. IAA is a small molecule with a planar indole ring system and a carboxylic acidic side chain. It orchestrates a multitude of developmental and morphological phenomena in plants, most notably those involved in apical dominance, root initiation and a variety of tropisms. Ever since the action of hormones in animal cells was ascribed to specific receptor proteins, botanists have searched for a receptor for IAA. It is thought that the clue to the design of future agrochemicals lies in the structure of the receptor's IAA-binding site. At present, the best candidate for an auxin receptor is auxin binding protein 1 (ABP1), which appears to be necessary both for cell division and for auxin-induced cell elongation.Jim Warwicker1xModelling of auxin-binding protein 1 suggests that its C terminus and auxin could compete for a binding site that incorporates a metal ion and tryptophan residue 44. Warwicker, J. Planta. 2001; 212: 343–347Crossref | PubMed | Scopus (18)See all References1 has now predicted the three-dimensional structure of the auxin-binding site in ABP1 based on its similarity with proteins of the cupin and vicilin superfamily. He suggests that ABP1 forms a β-barrel structure, with a cluster of amino acids that could bind a metal ion at its centre. This metal ion could complement the carboxylic acid group of IAA and an adjacent tryptophan could provide a hydrophobic platform for its indole ring. Thus, an auxin-binding site is formed. The predicted geometry of this site can accommodate an indole or napthalene ring system with a carboxylic acid side-chain whose acidic group is perpendicular to the ring system. Such a structure is consistent with the specificity of ABP1 for binding ligands. Warwicker's proposed structure for ABP1 is also consistent with a physiological model for auxin perception and signal transduction, in which IAA displaces the C-terminus of ABP1 from the auxin-binding site to force a conformational change in the protein and initiate a physiological response.Elucidating the three-dimensional structure of the auxin-binding site of ABP1 provides an insight of great consequence. It can be used not only in the computer-aided design of auxin mimetics, such as the next generation of herbicides or plant growth regulators, but also in defining how ABP1 could be modified to alter the competition between auxin and the C-terminal domain or the geometry of the auxin-binding site. Such modifications could be useful in manipulating the responsiveness or specificity of cell division and cell elongation to auxins. However, although clues to the structure of the auxin-binding site of ABP1 obtained by modelling are exciting, they remain predictions. Fortuitously, Eui-Jeon Woo and colleagues2xCrystallisation and preliminary X-ray analysis of the auxin receptor ABP1. Woo, E-J. et al. Acta Crystallogr. Sect. D. 2000; 56: 1476–1478Crossref | PubMed | Scopus (14)See all References2 have recently succeeded in producing crystals of ABP1. With crystals diffracting to 1.9 A, it should not be long before a definitive structure is available against which such predictions can be tested.