The auxin-binding pocket of auxin-binding protein 1 comprises the highly conserved boxes a and c

Planta (2009) 230:917924DOI 10.1007/s00425-009-0995-2

ORIGINAL ARTICLE

The auxin-binding pocket of auxin-binding protein 1 comprises the highly conserved boxes a and c

Renate I. Dahlke Hartwig Lthen Bianka SteVens

Received: 31 March 2009 / Accepted: 17 July 2009 / Published online: 11 August 2009 Springer-Verlag 2009

Abstract The auxin-binding protein 1 (ABP1) has already been proved to be an extracellular receptor of auxin in single cell systems. Protoplasts of maize coleoptiles respond to auxin with an increase in volume. The 2-naphthaleneacetic acid (2-NAA), an inactive auxin analog, acts as an anti-auxin in protoplast swelling, as it suppresses the eVect of indole-3-acetic acid (IAA). Antibodies raised against box a of ABP1 induce protoplast swelling in the absence of auxin. This response is inhibited by pre-incubation with 2-NAA. The eVect of 2-NAA on swelling induced by agonistic antibodies appears to depend on the binding characteristics of the antibody. ScFv12, an antibody directed against box a, box c and the C-terminal domain of ABP1 also exhibits auxin-agonist activity which is, however, not abolished by 2-NAA. Neither does 2-NAA aVect the activity of the C-terminal peptide of ABP1, which is predicted to interact with putative binding proteins of ABP1. These results support the view that box a and box c of ABP1 are auxin-binding domains.

Keywords Auxin-binding protein 1 (ABP1) Auxin receptor Protoplast swelling Zea mays coleoptile

Abbreviations

ABP1 Auxin-binding protein 1AFB Auxin-signaling F boxER Endoplasmic reticulumIAA Indole-3-acetic acidNAA Naphthaleneacetic acidTIR1 Transport inhibitor response 1

Introduction

Since the Wrst description of auxin activity by Went during the 1920s (Went 1928; Went and Thimann 1937) numerous auxin-regulated processes including tropisms, lateral root development and elongation growth have been identiWed. In contrast, our understanding of auxin signal transduction is still patchy.

The recently discovered auxin-binding F-box proteins TIR1 (transport inhibitor response 1), AFB1, AFB2 and AFB3 (auxin-signaling F box) constitute intracellular receptors (Dharmasiri et al. 2005a, b; Kepinski and Leyser 2005). These F-box proteins are part of the ubiquitin ligase complex SCFTIR1 and play a role in degradation of repressors of auxin-regulated transcription. Our understanding of auxin signaling is further complicated by a second type of auxin receptor candidate, namely, auxin-binding protein 1 (ABP1). Although auxin binding to membrane fractions containing ABP1 was Wrst demonstrated in the 1970s (Hertel et al. 1972),...