A novel immunological approach establishes that the auxin-binding protein,Nt-abp1, is an element involved in auxin signaling at the plasma membrane

Interactions of a collection of monoclonal antibodies (mAbs) to the recombi nant. Nicotiana tabacum auxin-binding protein 1 (Nt-abp1) were extensively characterized using surface plasmon resonance. Dynamic interaction studies using combinations of Nt-abp1, synthetic peptides corresponding to conserve d sequences within auxin-binding proteins, and the mAbs have shown that a n umber of the mAbs recognized discontinuous epitopes revealing the junction of distinct domains in the folded protein. In particular, the two putative auxin binding domains and the C terminus of the protein were shown to inter act with each other in the folded protein. Using the auxin-induced electric al response of tobacco protoplasts as a functional assay, all the mAbs exhi bited either auxin antagonist or hormonomimetic properties. These effects, measured for the first time in homologous conditions, confirm that Nt-abp1 is present at the plasma membrane and is involved in the activation of the auxin-dependent electrical response of tobacco protoplasts, Based on our su rface plasmon resonance data, we propose that the key event leading to the activation of this auxin electrical response consists of a conformational c hange in Nt-abp1.