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

The plant hormone indoleacetic acid (IAA or auxin) transcriptionally activa tes a select set of early genes. The Aux/IAA class of early auxin-responsiv e genes encodes a large family of short-lived, nuclear proteins. Aux/IAA po lypeptides homo-and heterodimerize, and interact with auxin-response transc ription factors (ARFs) via C-terminal regions conserved in both protein fam ilies. This shared region contains a predicted beta alpha alpha motif simil ar to the prokaryotic beta-ribbon DNA binding domain, which mediates both p rotein dimerization and DNA recognition. Here, we show by circular dichrois m spectroscopy and by chemical cross-linking experiments that recombinant p eptides corresponding to the predicted beta alpha alpha region of three Aux /IAA proteins from Arabidopsis thaliana contain substantial alpha-helical s econdary structure and undergo homo- and heterotypic interactions in vitro. Our results indicate a similar biochemical function of the plant beta alph a alpha domain and suggest that the beta alpha alpha fold plays an importan t 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.