Novel strategies to an auxin-evoked transport control.

The auxin, indole acetic acid (IAA), was found to bring about a bimodal response in the activity of the inward K+ rectifier in intact Vicia faba guard cells: 10 microM IAA led to an increase in current through the K+ rectifier, whilst 100 microM IAA inhibited the current flow. Application of a synthetic oligopeptide (A6.1), corresponding in sequence to the C-terminal 12 residues of the major Zea mays auxin binding protein (ZmABP1), led to a reversible closure of the inward K+ channel with Ki/0.5 for peptide A6.1 of around 5 microM. In addition, peptide A6.1 and homologues corresponding to the C-terminal sequence of ZmABP1, were effective at 1 microM in stimulating binding of GTP gamma S to microsomal fraction membranes. In contrast, oligopeptides corresponding to other solvent-exposed domains of the auxin binding protein were ineffective in these assays.