Heat-inducible DNA binding of purified heat shock transcription factor 1.

The heat-induced expression of heat shock proteins, called the cellular stress response, is mediated by heat shock transcription factor 1 (HSF1). HSF1 exists in unstressed cells in an inactive form, which is converted to the DNA binding from upon exposure of cells to elevated temperature. We have developed a protocol for isolation of the non-DNA binding form of recombinant mouse HSF1, involving expression and affinity purification of HSF1 as a fusion with the glutathione S-transferase protein in Escherichia coli, followed by specific protease cleavage to release pure HSF1 protein. We report here that the purified inactive HSF1 can be converted to the DNA binding form by heat treatment in vitro. Chemical cross-linking analysis demonstrates that this conversion is accompanied by oligomerization of HSF1 from a monomeric to a trimeric native structure, similar to that observed for HSF1 in heat-shocked cells. These results indicate that elements residing in the HSF1 polypeptide are sufficient both for maintenance of this factor in the non-DNA binding from and for its heat-induced conversion to the DNA binding form and support a role for HSF1 as the "molecular thermostat" in eukaryotic cells, which senses adverse environmental conditions and activates the cellular stress response.