Structural characterization of a minimal functional transactivation domain from the human glucocorticoid receptor

A 58-amino acid polypeptide containing the functional core region, the τ1 core, of the major transactivation domain of the human glucocorticoid receptor has been expressed in Escherichia coli and purified to homogeneity. The polypeptide retains 60-70% of the activity of the intact domain when assayed in vivo or in vitro. This report describes a structural characterization of the τ1 core peptide fragment. Circular dichroism spectroscopy shows that the τ1 core and a larger fragment encompassing the intact τ1 domain are largely unstructured in water solution under a variety of pH conditions. The τ1 core, however, acquires a significant α-helical structure when analyzed in the presence of trifluoroethanol, an agent that favors secondary structure formation in regions that have propensity for α- helical conformation. Two- and three-dimensional NMR spectroscopy of ¹⁵N- labeled τ1 core, in the presence of trifluoroethanol, has allowed sequential assignment of ¹H and ¹⁵N resonances and identification of three protein segments with α-helical character. Potentially helix-breaking proline substitutions, in proposed α-helical regions, lead to reduced activity, suggesting that α-helices are important for transactivation in vivo.