An auxin binding protein (ABP) was isolated from the shoots of rice seedlings and characterized. The ABP was found to be a monomer with a molecular mass of 57 kDa and play a crucial role via auxin binding in regulating H+ translocation activity of the plasma membrane in a typical biphasic manner. The results of binding equilibrium experiments indicate that the ABP binds indole 3-acetic acid with a high affinity (Kd = 1.9 x 10(-8) M), having four primary binding sites for auxin and some secondary sites with low auxin affinities. The ABP appears to have an unambiguous auxin receptor function.
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