The discovery of molecular chaperones
From Chaperome
The classical experiments of Anfinsen demonstrated that all the information required for the folding of polypeptide chains into functional native three-dimensional conformations, is encoded in their primary sequences (Anfinsen, 1973). However, subtle changes in amino acid sequences, or in the milieu, can affect the folding process and the stability of the native state, often leading to the formation of stable inactive aggregated states (Gregersen et al., 2000; Stevens and Argon, 1999). Protein aggregates are often toxic to cells (Kopito and Ron, 2000; Taylor et al., 2002). Cells can express molecular chaperones and proteases to prevent, revert or eliminate aggregates (Parsell and Lindquist, 1993; Sakahira et al., 2002).
The term “molecular chaperone” was initially used to describe how a nuclear protein, nucleoplasmin, mediates the assembly of nucleosomes, but does not take part in the final assembled complex (Laskey and Earnshaw, 1980). The term molecular chaperones was later extended to a group of highly conserved proteins, most of which stress-induced, involved in the folding and assembly of other protein complexes in the cell (Ellis, 1987; Hemmingsen et al., 1988; Pelham, 1986). Using mutagenesis, the Escherichia coli proteins DnaK and GroEL were shown to be chaperones that control bacterial thermo-tolerance and participate in the proper folding and assembly of large protein complexes (Kusukawa and Yura, 1988; Paek and Walker, 1987; Tilly et al., 1981). GroEL was shown to assist the assembly of numerous self and foreign recombinant proteins in E. coli cells (Goloubinoff et al., 1989b; Van Dyk et al., 1989). Similarly, a chloroplast protein highly homologous to bacterial GroEL, was shown to control the proper folding and assembly of Ribulose bisphosphate Carboxylase/Oxygenase (RubisCO) complexes in chloroplasts of higher plants (Hemmingsen et al., 1988). These in vivo findings were confirmed in vitro with purified GroEL and GroES proteins, who significantly promoted in an ATP-dependent manner the correct refolding and assembly of urea-unfolded or acid-denatured RubisCO, into an active enzyme (Goloubinoff et al., 1989a).
As demonstrated by the essential role of GroEL in the cell at all temperatures (Fayet et al., 1989), molecular chaperones have important functions at physiological temperatures. In addition, the fact that most chaperone mutants are thermo-sensitive indicates that molecular chaperones assume vital functions under stress, such as repairing stress-induced damages in proteins (Fayet et al., 1989; Mogk et al., 1999; Paek and Walker, 1987). Overproduction of chaperones increases the tolerance of organisms to various stresses (Parsell and Lindquist, 1993) and, moreover, can suppress thermo-sensitive mutant phenotypes (Van Dyk et al., 1989). Deficiency in chaperone expression may reveal mutant phenotypes in populations, which otherwise exhibit wild-type phenotypes. Molecular chaperones thus allow the accumulation of conditional “silent” or “neutral” allelic mutations in populations, which become expressed only under the selective pressure of a stress (Rutherford and Lindquist, 1998; Queitsch et al., 2002). This suggests that molecular chaperones can serve as a buffer in the process of evolution (Rutherford and Lindquist, 1998)
