Biochemical characterization of recombinant polypeptides corresponding to the predicted βαα fold in Aux IAA proteins

The plant hormone indoleacetic acid (IAA or auxin) transcriptionally activates a select set of early genes. The Aux/IAA class of early auxin-responsive genes encodes a large family of short-lived, nuclear proteins. Aux/IAA polypeptides homo-and heterodimerize, and interact with auxin-response transcription factors (ARFs) via C-terminal regions conserved in both protein families. This shared region contains a predicted beta alpha alpha motif similar to the prokaryotic beta-ribbon DNA binding domain, which mediates both protein dimerization and DNA recognition. Here, we show by circular dichroism spectroscopy and by chemical cross-linking experiments that recombinant peptides corresponding to the predicted beta alpha alpha region of three Aux/IAA proteins from Arabidopsis thaliana contain substantial alpha-helical secondary structure and undergo homo- and heterotypic interactions in vitro. Our results indicate a similar biochemical function of the plant beta alpha alpha domain and suggest that the beta alpha alpha fold plays an important role in mediating combinatorial interactions of Aux/IAA and ARF proteins to specifically regulate secondary gene expression in response to auxin, (C) 1999 Federation of European Biochemical Societies.