Liberation of the intramolecular interaction as the mechanism of heat-induced activation of HSP90 molecular chaperone

The molecular chaperone function of HSP90 is activated under heat-stress co nditions. In the present study, we investigated the role of the interaction s in the heat-induced activation of HSP90 molecular chaperone. The precedin g paper demonstrated two domain-domain interactions of HtpG, an Escherichia coli homologue of mammalian HSP90, i.e. an intra-molecular interaction bet ween the N-terminal and middle domains and an intermolecular one between th e middle and C-terminal domains. A bacterial two-hybrid system revealed tha t the two interactions also existed in human HSP90 alpha. Partners of the i nteraction between the N-terminal and middle domains of human HSP90 alpha c ould, but those between the middle and C-terminal domains could not, be rep laced by the domains of HtpG. Thus, the interface between the N-terminal an d middle domains is essentially unvaried from bacterial to human members of the HSP90-family proteins. The citrate synthase-binding activity of HtpG a t an elevated temperature was solely localized in the N-terminal domain, bu t HSP90 alpha possessed two sites in the N-terminal and other domains. The citrate-synthase-binding activity of the N-terminal domain was suppressed b y the association of the middle domain. The complex between the N-terminal and middle domains is labile at elevated temperatures, but the other is sta ble even at 70 degreesC. Taken to-ether, we propose the liberation of the N -terminal client-binding domain from the middle suppressor domain is involv ed in the temperature-dependent activation mechanism of HSP90 molecular cha perone.