Binding of non-native protein to Hsp25 during heat shock creates a reservoir of folding intermediates for reactivation

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Abstract
Small heat shock proteins (sHsps) are a conserved and ubiquitous protein family. Their ability to convey thermoresistance suggests their participation in protecting the native conformation of proteins. However, the underlying functional principles of their protective properties and their role in concert with other chaperone families remain enigmatic. Here, we analysed the influence of Hsp25 on the inactivation and subsequent aggregation of a model protein, citrate synthase (CS), under heat shock conditions in vitro. We show that stable binding of several non‐native CS molecules to one Hsp25 oligomer leads to an accumulation of CS unfolding intermediates, which are protected from irreversible aggregation. Furthermore, a number of different proteins which bind to Hsp25 can be isolated from heat‐shocked extracts of cells. Under permissive folding conditions, CS can be released from Hsp25 and, in cooperation with Hsp70, an ATP‐dependent chaperone, the native state can be restored. Taken together, our findings allow us to integrate sHsps functionally in the cellular chaperone system operating under heat shock conditions. The task of sHsps in this context is to efficiently trap a large number of unfolding proteins in a folding‐competent state and thus create a reservoir of non‐native proteins for an extended period of time, allowing refolding after restoration of physiological conditions in cooperation with other chaperones.