The wing in yeast heat shock transcription factor (HSF) DNA-binding domain is required for full activity

The yeast heat shock transcription factor (HSF) belongs to the winged helix family of proteins, HSF binds DNA as a trimer, and additional trimers can bind DNA co-operatively, Unlike other winged helix-turn-helix proteins, HSF's wing does not appear to contact DNA, as based on a previously solved crystal structure. Instead, the structure implies that the wing is involved in protein-protein interactions, possibly within a trimer or between adjacent trimers, To understand the function of the wing in the HSF DNA-binding domain, a Saccharomyces cerevisiae strain was created that expresses a wingless HSF protein. This strain grows normally at 30 degreesC, but shows a decrease in reporter gene expression during constitutive and heat-shocked conditions. Removal of the wing does not affect the stability or trimeric nature of a protein fragment containing the DNA-binding and trimerization domains. Removal of the wing does result in a decrease in DNA-binding affinity. This defect was mainly observed in the ability to form the first trimer-bound complex, as the formation of larger complexes is unaffected by the deletion, Our results suggest that the wing is not involved in the highly cooperative nature of HSF binding, but may be important in stabilizing the first trimer bound to DNA.