Non-native conformations of proteins have the tendency to misfold and aggregate. Cells have developed a quality control system that ensures that the cellular proteome folds correctly, keeps its native conformation and that unproductive side reactions are prevented. This is especially important under stress conditions when massive protein unfolding occurs or in the context of diseases when the cellular protein homeostasis is out of control.
The key elements of the cellular stress defense system are molecular chaperones. These molecular machines of protein folding share the remarkable ability of specifically recognizing non-native proteins and assisting their folding to the native state. There are several classes of molecular chaperones that evolved independently and are both structurally and mechanistically not related, such as the small heat shock proteins, Hsp70 and Hsp90. Progress in recent years allows now to define mechanistic principles and chaperone networks in health and disease. For the most complex chaperone machinery of the eukaryotic cell, the Hsp90 system, large conformational changes of the dimeric protein which are connected to its ATPase cycle are rate-limiting. A cohort of cochaperones binds to Hsp90 in a conformation- specific manner and modulates its properties. With a view to define the key traits of this chaperone machine, we set out to reconstitute the interaction of Hsp90 with cochaperones and client proteins using purified components. Our biophysical analysis reveals an intriguing interplay between Hsp90, its cochaperones and clients.