EVIDENCE FOR A ROLE OF HSP7O IN THE REGULATION OF THE HEAT-SHOCK RESPONSE IN MAMMALIAN-CELLS

Heat and other environmental insults (stress) cause unfolding of prote ins, triggering the activation of heat shock transcription factor HSF (HSF1 in vertebrates) that, in higher eukaryotes, involves trimerizati on of the factor and acquisition of heat shock element (HSE) DNA-bindi ng ability. Interaction of activated HSF1 with HSEs in promoters of ge nes encoding heat shock proteins (Hsps) enhances their expression. It was suggested that Hsp70 may function as the negative regulator of HSF 1. In the simplest model, stress-unfolded proteins would compete with monomeric HSF1 for Hsp70 binding. This competition would result in dis sociation of an HSF1-Hsp70 complex, allowing trimerization of released HSF1 monomers. In support of this model, we present evidence herein t hat 1) non-activated HSF1 forms a 1:1 complex with Hsp70, 2) both rate s of heat-induced appearance of HSF1 oligomers and rates of disappeara nce of HSF1 heterodimers and monomers decrease when concentrations of unengaged Hsps are increased, and 3) transient overexpression of Hsp70 inhibits heat activation of HSF1.