MODELING THE AUXIN-BINDING SITE OF AUXIN-BINDING PROTEIN-1 OF MAIZE

We have modelled the auxin-binding site of maize auxin-binding protein 1 (ABP1) using published Site I binding data for 45 different auxin analogues. We propose an auxin phytophore model based on the analysis of structure-activity relationships for these compounds. The phytophore incorporates the key structural features of the three compounds which bind with the highest affinity, naphthalene-1-acetic acid, naphthalene-2-acetic acid and indole-3-acetic acid. We propose that their phytophoric conformation is one in which the planes described by their carboxylic acid groups are perpendicular to the plane of their aromatic ring systems and that their carboxylic acid oxygens are positioned directly astride the ring system. Our model of the auxin-binding site describes three regions of the protein: a planar aromatic ring binding platform, a carboxylic acid binding site and a hydrophobic transition region which separates the two binding sites. The exact orientation and position of the ABP1 groups involved in carboxylic acid binding is somewhat flexible. This is the first model proposed which describes the auxin-binding site of a single auxin-binding protein. We have used this model to propose a number of compounds which should bind ABP1 well.