HSF1 transcriptional activity is modulated by IER5 and PP2A/B55

•IER5, a target gene of HSF1, interacts with PP2A and its B55 regulatory subunits.•IER5 and B55 enhance HSF1 dephosphorylation and expression of HSF1 target genes.•IER5 functions as a positive feedback regulator of HSF1.•This process involves PP2A/B55 and HSF1 dephosphorylation. Heat shock factor 1 (HSF1) is the master transcriptional regulator of chaperone genes. HSF1 regulates the expression of the immediate-early response gene IER5, which encodes a protein that has roles in the stress response and cell proliferation. Here, we have shown that IER5 interacts with protein phosphatase 2A (PP2A) and its B55 regulatory subunits. Expression of IER5 and B55 in cells leads to HSF1 dephosphorylation and activation of HSF1 target genes. The B55 subunits directly bind to HSF1. These results suggest that IER5 functions as a positive feedback regulator of HSF1 and that this process involves PP2A/B55 and HSF1 dephosphorylation.