Multiple components of the HSP90 chaperone complex function in regulation of heat shock factor 1 in vivo

Rapid and transient activation of heat shock genes in response to stress is mediated in eukaryotes by the heat shock transcription factor HSF1. It is well established that cells maintain a dynamic equilibrium between inactive HSF1 monomers and transcriptionally active trimers, but little is known ab out the mechanism linking HSF1 to reception of various stress stimuli or th e factors controlling oligomerization. Recent reports have revealed that HS P90 regulates key steps in the HSF1 activation-deactivation process. Here, we tested the hypothesis that components of the HSP90 chaperone machine, kn own to function in the folding and maturation of steroid receptors, might a lso participate in HSF1 regulation. Mobility supershift assays using antibo dies against chaperone components demonstrate that active HSF1 trimers exis t in a heterocomplex with HSP90, p23, and FKBP52. Functional in vivo experi ments in Xenopus oocytes indicate that components of the HSF1 heterocomplex , as well as other components of the HSP90 cochaperone machine, are involve d in regulating oligomeric transitions. Elevation of the cellular levels of cochaperones affected the time of HSF1 deactivation during recovery: atten uation was delayed by immunophilins, and accelerated by HSP90, Hsp/c70, Hip , or Hop. In immunotargeting experiments with microinjected antibodies, dis ruption of HSP90, Hip, Hop, p23, FKBP51, and FKBP52 delayed attenuation. In addition, HSF1 was activated under nonstress conditions after immunotarget ing of HSP90 and p23, evidence that these proteins remain associated with H SF1 monomers and function in their repression in vivo. The remarkable simil arity of HSF1 complex chaperones identified here (HSP90, p23, and FKBP52) a nd components in mature steroid receptor complexes suggests that HSF1 oligo merization is regulated by a foldosome-type mechanism similar to steroid re ceptor pathways. The current evidence leads us to propose a model in which HSF1, HSP90 and p23 comprise a core heterocomplex required for rapid confor mational switching through interaction,vith a dynamic series of HSP90 subco mplexes.