The Hsp70 and Hsp60 Chaperone Machines

Abstract

An essential cellular machinery that has been identified and studied only relatively recently is a collective of specialized proteins, molecular chaperones, that bind nonnative states of other proteins and assist them to reach a functional conformation, in most cases through the expenditure of ATP. Originally identified by their increased abundance following heat shock, chaperone proteins in general recognize exposed hydrophobic surfaces of nonnative species, surfaces that will ultimately be buried in the native state, and form noncovalent interactions with them, stabilizing them against irreversible multimeric aggregation. Release of polypeptide then follows, in many cases driven by an ATP-directed conformational change of the chaperone, permitting subsequent steps of polypeptide folding or biogenesis to occur. When such steps fail to proceed productively, recognition and rebinding by the same or another chaperone can occur, allowing another opportunity for a productive conformation to be reached. Different classes of molecular chaperones appear to be directed to binding specific nonnative states, the nature of which are beginning to be understood. For example, the two best-studied families, examined in detail below, the ubiquitous Hsp70 and Hsp60 (chaperonin) chaperones, recognize hydrophobic surface in the context of, respectively, extended and collapsed (globular) conformations, which are bound correspondingly either by local enclosure of the chain or by global enclosure of the polypeptide in a central cavity. Because there is not yet the detailed level of structural and mechanistic understanding for other recognized families of chaperones, they are not considered here, but important observations concerning binding, nucleotide use, and cellular actions are summarized in Table 1.Table 1Topology of Polypeptide Binding and Action of Chaperone Families Hsp70 chaperones, with their co-chaperones, comprise a set of abundant cellular machines that assist a large variety of protein folding processes in almost all cellular compartments. Historically, they were identified by induction under conditions of stress, during which they are now known to provide an essential action of preventing aggregation and assisting refolding of misfolded proteins. But they also play an essential role under normal conditions, including (1) assisting folding of some newly translated proteins; (2) guiding translocating proteins across organellar membranes through action at both the cis and trans sides; (3) disassembling oligomeric protein structures; (4) facilitating proteolytic degradation of unstable proteins; and in selected cases, (5) controlling the biological activity of folded regulatory proteins, including transcription factors (for a discussion of these actions, see69Morimoto R.I. Tissieres A. Georgopoulos C. The Biology of Heat Shock Proteins and Molecular Chaperones. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY1994Google Scholar, 40Hartl F.U. Molecular chaperones in cellular protein folding.Nature. 1996; 381: 571-580Crossref PubMed Scopus (2944) Google Scholar). All of these activities rely on the ATP-regulated association of Hsp70 with short hydrophobic segments in substrate polypeptides (23Flynn G.C. Pohl J. Flocco M.T. Rothman J.E. Peptide-binding specificity of the molecular chaperone BiP.Nature. 1991; 353: 726-730Crossref PubMed Scopus (610) Google Scholar; 83Rüdiger S. Buchberger A. Bukau B. Interaction of Hsp70 chaperones with substrates.Nature Struct. Biol. 1997; 4 (a): 342-349Crossref PubMed Scopus (264) Google Scholar, 84Rüdiger S. Germeroth L. Schneider-Mergener J. Bukau B. Substrate specificity of the DnaK chaperone determined by screening cellulose-bound peptide libraries.EMBO J. 1997; 16 (b): 1501-1507Crossref PubMed Scopus (598) Google Scholar), which prevents further folding or aggregation by shielding these segments. In Hsp70-assisted folding reactions, substrates undergo repeated cycles of binding/release (92Szabo A. Langer T. Schröder H. Flanagan J. Bukau B. Hartl F.U. The ATP hydrolysis-dependent reaction cycle of the Escherichia coli Hsp70 system-DnaK, DnaJ and GrpE.Proc. Natl. Acad. Sci. USA. 1994; 91: 10345-10349Crossref PubMed Scopus (425) Google Scholar, 9Buchberger A. Schröder H. Hesterkamp T. Schönfeld H.-J. Bukau B. Substrate shuttling between the DnaK and GroEL systems indicates a chaperone network promoting protein folding.J. Mol. Biol. 1996; 261: 328-333Crossref PubMed Scopus (131) Google Scholar), frequently at a stoichiometry of a single Hsp70 monomer per substrate molecule. Hsp70 binding does not appear to induce global conformational changes in the substrate but, rather, appears to act locally. Substrates released from the chaperone undergo kinetic partitioning between folding to native state, aggregation, rebinding to Hsp70, and binding to other chaperones or proteases as part of a multidirectional folding network. Hsp70 proteins all consist of the same working parts: a hig