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Published on July 9, 2008; 10.1104/pp.108.122044

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Received April 29, 2008
Accepted June 24, 2008

The binding of auxin to the Arabidopsis auxin influxtransporter, AUX1

David J. Carrier , Norliza Tendot Abu Bakar , Ranjan Swarup , Richard Callaghan , Richard M. Napier , Malcolm J. Bennett , and Ian D. Kerr *

School of Biomedical Sciences, University of Nottingham, Queen's Medical Centre, Nottingham, NG7 2UH; School of Biosciences, University of Nottingham, Sutton Bonington Campus, LE12 5RD; Nuffield Department of Clinical Laboratory Sciences, University of Oxford, John Radcliffe Hospital, Oxford, OX3 9DS; Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF

* Corresponding author; email: ian.kerr{at}nottingham.ac.uk.

The cellular import of the hormone auxin is a fundamental requirement for the generation of auxin gradients which control a multitude of plant developmental processes. The AUX/LAX family of auxin importers, exemplified by AUX1 from Arabidopsis thaliana, has been shown to mediate auxin import when expressed heterologously. The quantitative nature of the interaction between AUX1 and its transport substrate indole-3-acetic acid (IAA) is incompletely understood and we sought to address this in the present investigation. We expressed AUX1 to high levels in a baculovirus expression system and prepared membrane fragments from baculovirus-infected insect cells. These membranes proved suitable for determination of the binding of IAA to AUX1 and enabled us to determine a Kd of 2.6 µM, comparable with estimates for the Km for IAA transport. The efficacy of a number of auxin analogues and auxin transport inhibitors to displace IAA binding from AUX1 has also been determined and can be rationalized in terms of their physiological effects. Determination of the parameters describing the initial interaction between a plant transporter and its hormone ligand provides novel quantitative data for modelling auxin fluxes.






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