The heat shock transcription factor in liver exists in a form that has DNA binding activity but no transcriptional activity.

The binding activity of the heat shock transcription factor (HSF) to the heat shock element (HSE) is observed in non-stressed liver and freshly isolated hepatocytes when the expression of hsp70 is undetectable. HSF binding activity in non-stressed liver/hepatocytes is specific for HSE and similar to the HSF binding activity observed in heat shocked hepatocytes that is associated with hsp70 transcription. However, the HSF binding activity in non-stressed and heat shock cells can be distinguished on the basis of the thermal stability in vitro. The HSE binding activity of cell extracts isolated from non-stressed liver/hepatocytes was lost rapidly when the extracts were incubated at 37 degrees C. In contrast, the HSF binding activity of cell extracts isolated from heat shocked hepatocytes was relatively stable at 37 degrees C. Based on our observations, we propose that the activation of HSF is a multistep process that involves a change in conformation after oligomerization and the acquisition of DNA binding to a form that is more thermostable and is associated with increased hsp70 transcription.