Molecular mechanism of thermosensory function of human heat shock transcription factor Hsf1

Abstract

The heat shock response is a universal homeostatic cell autonomous reaction of organisms to cope with adverse environmental conditions. In mammalian cells, this response is mediated by the heat shock transcription factor Hsf1, which is monomeric in unstressed cells and upon activation trimerizes, and binds to promoters of heat shock genes. To understand the basic principle of Hsf1 activation we analyzed temperature-induced alterations in the conformational dynamics of Hsf1 by hydrogen exchange mass spectrometry. We found a temperature-dependent unfolding of Hsf1 in the regulatory region happening concomitant to tighter packing in the trimerization region. The transition to the active DNA binding-competent state occurred highly cooperative and was concentration dependent. Surprisingly, Hsp90, known to inhibit Hsf1 activation, lowered the midpoint temperature of trimerization and reduced cooperativity of the process thus widening the response window. Based on our data we propose a kinetic model of Hsf1 trimerization.Cells cope with excessive heat, toxic compounds and other adverse environmental conditions by triggering an internal repair process called the heat shock response. In mammalian cells, a protein called Hsf1 is activated by stress and regulates the activity of a large set of target genes. These genes code for proteins that help the cell cope with the effects of stress, for example, by repairing or breaking down damaged proteins. Under normal conditions, Hsf1 exists as a single molecule, but when it is activated, three molecules come together to make a complex called a trimer that is able to bind to DNA and activate the target genes.Proteins are made of long chains that then fold into specific three-dimensional shapes. It is not known how Hsf1 is kept in an inactive state in healthy, unstressed cells. One possibility is that the protein folds into a three-dimensional shape that prevents it from being activated. Alternatively, Hsf1 may be bound to other proteins called chaperones that move away when the cell is under stress because they are needed to help the damaged proteins refold into their own three-dimensional shapes.Hentze et al. used a variety of biochemical techniques to study the human Hsf1 protein. The experiments showed that there are two regions of the Hsf1 protein that changed shape dramatically when the temperature increased. A region that regulates the activity of Hsf1 unfolded, while a region involved in making the trimer became more stable. Detailed analysis showed that once the regulatory region unfolded, the protein was able to interact with other Hsf1 units to make the trimer. Therefore, Hsf1 can directly sense and respond to changes in temperature without the aid of any chaperone proteins.Further experiments showed that the formation of Hsf1 trimers and the ability of these trimers to bind to DNA depend upon both the temperature and the amount of Hsf1 present. In addition, a chaperone protein called Hsp90 – which is known to be able to interact with Hsf1 – influenced how Hsf1 responded to changes in temperature. Hentze et al. also present a model for the activation of Hsf1 that allows for flexibility in the response of Hsf1 to changes in temperature. Previous studies have shown that Hsf1 is chemically modified during stress and also while the cell recovers from stressful conditions. Therefore, the next challenge will be to find out how these modifications influence the way in which Hsf1 responds to stress.