A specialized cellular protein that binds nonnative forms of other proteins and assists them to reach a functional conformation, in most cases through the expenditure of adenosine triphosphate (ATP). Originally identified by their increased abundance after heat shock, chaperone proteins in general bind to exposed hydrophobic surfaces of nonnative proteins, preventing them from forming intermolecular interactions that lead to irreversible multimolecular aggregation. Thus, the role of chaperone proteins under conditions of stress, such as heat shock, is to protect proteins by binding to incipiently misfolded conformations, preventing aggregation; then, following return of normal conditions, they allow refolding to occur, associated with protein release.