Chaperones and protein folding

The folding and translocation of many newly synthesized proteins in the cell is kinetically assisted by ubiquitous, abundant, specialized proteins known as molecular chaperones. These components generally recognize hydrophobic surfaces, exposed specifically by non-native conformations, through their own solvent-exposed hydrophobic surfaces, with different classes of chaperone recognizing such surfaces in the context of extended (Hsp70) vs. collapsed (Hsp60/chaperonin) topology of substrate protein. Such binding prevents substrate proteins from misfolding and from forming multimolecular aggregates. Chaperone-bound proteins are then released from Hsp70 and Hsp60 machines via the binding of ATP to chaperone domains physically separated from the substrate protein binding domains, via allosterically directed conformational changes. Molecular chaperones also act under stress conditions, where polypeptide chains are subject to misfolding, preventing aggregation and restoring the native state. The small heat shock proteins (sHsps) are oligomeric assemblies that participate with the other chaperones in binding non-native states under such conditions. Finally, Hsp90 is an abundant clamp-shaped chaperone that participates in binding and maturation of a variety of substrate proteins via an ATP-directed cycle. This chapter reviews the structure and mechanism of action of these chaperones, with special attention directed to the variety of biophysical methods employed to reaching our current understanding.