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 conditions. In the present study, we investigated the role of the interactions in the heat‐induced activation of HSP90 molecular chaperone. The preceding paper demonstrated two domain–domain interactions of HtpG, an Escherichia coli homologue of mammalian HSP90, i.e. an intra‐molecular interaction between the N‐terminal and middle domains and an intermolecular one between the middle and C‐terminal domains. A bacterial two‐hybrid system revealed that the two interactions also existed in human HSP90α. Partners of the interaction between the N‐terminal and middle domains of human HSP90α could, but those between the middle and C‐terminal domains could not, be replaced by the domains of HtpG. Thus, the interface between the N‐terminal and middle domains is essentially unvaried from bacterial to human members of the HSP90‐family proteins. The citrate synthase‐binding activity of HtpG at an elevated temperature was solely localized in the N‐terminal domain, but HSP90α possessed two sites in the N‐terminal and other domains. The citrate‐synthase‐binding activity of the N‐terminal domain was suppressed by the association of the middle domain. The complex between the N‐terminal and middle domains is labile at elevated temperatures, but the other is stable even at 70 °C. Taken together, we propose the liberation of the N‐terminal client‐binding domain from the middle suppressor domain is involved in the temperature‐dependent activation mechanism of HSP90 molecular chaperone.