Monoclonal antibodies detect an auxin-induced conformational change in the maize auxin-binding protein

The monoclonal antibody MAC 256 precipitates specifically the auxin-binding protein (ABP) of maize membranes. Auxin-binding activity was recovered from the immunoprecipitate and MAC 256 can, therefore, bind undenatured, native ABP. A sandwich enzyme-linked immunosorbent assay was used to present native ABP to MAC 256 and under these conditions auxins inhibit antibody binding. Millimolar naphthalene-1-acetic acid completely blocks MAC 256 binding and the characteristics of monoclonal antibody MAC 259 are similar. The ability of a range of auxins and related compounds to displace MAC 256 correlates with the known structure-activity relationships of these compounds in vivo and in binding assays. The results are interpreted in terms of an auxin-induced conformational change in ABP, auxin binding leading to a change in, or concealment of, the epitope of the antibody. The epitope for MAC 256 and 259 lies close to the carboxy terminus of the protein, implying that the part of ABP containing the sequence of amino acids responsible for retention within the endoplasmic reticulum is conformationally active.