Transcriptional activator that specifically binds DNA sequence 5'-AGAAnnTTCT-3' known as heat shock promoter elements (HSE). Homotrimer (PubMed:11807141). Interacts with HSP70-1 and HSP70-4 (PubMed:11807141). Binds to CRK1 (PubMed:18466301). Binds to HSBP (PubMed:20388662, PubMed:20657173). Constitutively expressed. By heat stress. The hydrophobic-rich region (HR-A/B) corresponds to the oligomerization domain. AHA motif is a transcriptional activator element. Exhibits temperature-dependent phosphorylation (By similarity). Phosphorylated by CRK1. Belongs to the HSF family. Class A subfamily.