HSP90 INTERACTS WITH AND REGULATES THE ACTIVITY OF HEAT-SHOCK FACTOR-1 IN XENOPUS OOCYTES

Transcriptional activation of heat shock genes is a reversible and mul tistep process involving conversion of inactive heat shock factor 1 (H SF1) monomers into heat shock element (HSE)-binding homotrimers, hyper phosphorylation, and further modifications that induce full transcript ional competence. HSF1 is controlled by multiple regulatory mechanisms , including suppression by additional cellular factors, physical inter actions with HSP70, and integration into different cellular signaling cascades. However, the signaling mechanisms by which cells respond to stress and control the HSF1 activation-deactivation pathway are not kn own. Here we demonstrate that HSP90, a cellular chaperone known to reg ulate several signal transduction molecules and transcription factors, functions in the regulation of HSF1. The existence of HSF1-HSP90 hete rocomplexes was shown by coimmunoprecipitation of HSP90 with HSF1 from unshocked and heat-shocked nuclear extracts, recognition of HSF1-HSE complexes in vitro by using HSP90 antibodies (Abs), and recognition of HSF1 in vivo by HSP90 Abs microinjected directly into oocyte nuclei. The functional impact of HSP90-HSF1 interactions was analyzed by using two strategies: direct nuclear injection of HSP90 Abs and treatment o f cells with geldana-mycin (GA), an agent that specifically blocks the chaperoning activity of HSP90. Both HSP90 Abs and GA delayed the disa ssembly of HSF1 trimers during recovery from heat shock and specifical ly inhibited heat-induced transcription from a chloramphenicol acetylt ransferase reporter construct under control of the hsp70 promoter. HSP 90 Abs activated HSE binding in the absence of heat shock, an effect t hat could be reversed by subsequent injection of purified HSP90, GA di d not activate HSE binding under nonshock conditions but increased the quantity of HSE binding induced by heat shock. On the basis of these findings and the known properties of HSP90, we propose a new regulator y model in which HSP90 participates in modulating HSF1 at different po ints along the activation-deactivation pathway, influencing the interc onversion between monomeric and trimeric conformations as well as tran scriptional activation. We also put forth the hypothesis that HSP90 li nks HSF1 to cellular signaling molecules coordinating the stress respo nse.