Crystal Structure of the DNA Binding Domain of the Heat Shock Transcription Factor

The structure of the DNA binding domain, determined at 1.8 angstrom resolution, contains a three-helix bundle that is capped by a four-stranded antiparallel β sheet. This structure is a variant of the helix-turn-helix motif, typified by catabolite activator protein. In the heat shock transcription factor, the first helix of the motif (α2) has an α-helical bulge and a proline-induced kink. The angle between the two helices of the motif (α2 and α3) is about 20° smaller than the average for canonical helix-turn-helix proteins. Nevertheless, the relative positions of the first and third helices of the bundle (α1 and α3) are conserved. It is proposed here that the first helix of the three-helix bundle be considered a component of the helix-turn-helix motif.