Heat shock factor 1 (HSF1) primarily regulates various cellular stress responses. The role of alpha-helix1 (H1) in its DNA-binding domain (DBD) during HSF1 activation remains unknown. Here, HSF1 lacking H1 loses its heat-induced activity, suggesting the importance of the latter. Furthermore, the CD spectra and AMBER prediction show that this HI deficiency does not change the structure of HSF1 monomer, but does impact its heat-induced trimerization. Point Mutation showed that Phe18 in H1 interacts with Tyr60, and that Trp23 interacts with Phe 104 by an aromatic-aromatic interaction. Thus, the presence of H I stabilizes the DBD Structure, which facilitates the heat-induced trimerization and DNA-binding of HSF1. (c) 2009 Elsevier Inc. All rights reserved.