[Frontiers in Bioscience 1, d309-317, October 1, 1996]


Ursula Jakob

Department of Biology, University of Michigan, Ann Arbor, Michigan, USA

Received 9/9/96; Accepted 9/10/96; On-line 10/1/96


It was more than 30 years ago, when Ritossa and coworkers found that an increase in incubation temperature of Drosophila melanogaster larva results in the development of a defined set of new transcription loci on the polytene chromosome (1). Not many people took notice of this important discovery, and it was not until more than 12 years later that the first gene products were identified and the term "heat shock protein" (Hsp) was coined (2). In the following years the heat shock proteins were discovered to be present in all species tested and to be extremely well conserved (reviewed in ref. 3). Even thermophilic organisms, which optimal growth temperature lies between 50° and 90°C have been found to respond sudden temperature upshifts with the overexpression of heat shock proteins. However, while the regulation of heat shock proteins was studied extensively, relatively little was known about their physiological significance (3). It needed the combinational work of many labs to put the pieces together and to revolutionize the protein folding field. The first hint came well before the characterization of heat shock proteins in E. coli with the work of Georgopoulos and collaborators, who discovered that successful lambda phage head assembly depends on the presence of a host-produced protein, subsequently called GroE (4). A few years later, the same group of scientists found another host gene, dnaK, which was shown to be involved in lambda replication (5). That GroE mediated protein folding and assembly is not restricted to phage assembly and DnaK mediated disruption of certain protein-protein interactions is not limited to phage replication (6) became evident with the identification and functional characterization of their eukaryotic homologues. Rubisco binding protein, a chloroplast protein implicated in the assembly of the ribulose 1,5 bisphosphate carboxylase (rubisco) multimeric complex (7) was shown to be closely related to GroEL (8) and the mammalian clathrin uncoating ATPase was identified as Hsc70, the eukaryotic DnaK homologue (9). This was the beginning of the chaperone era and the end of the idea that all proteins reach their specific three dimensional structure spontaneously in the cell (10).

The discovery of folding helper proteins (chaperones) did not contradict Anfinsen's classic theory about protein folding which states that the aquisitation of the specific three dimensional protein structure depends exclusively on the amino acid sequence of the individual protein (11). Chaperones do not change the path of protein folding or the final fold. However, they do keep proteins on the correct folding pathway and prevent unspecific interactions (12), therefore regulating the kinetic competition between folding, specific association and non-specific, irreversible aggregation (13). Aggregation represents a major problem for a folding protein. Since it is a second or higher order reaction it can often be circumvented in vitro by decreasing the protein concentration (14). Cells, however, have a very high concentration of aggregation sensitive folding intermediates at any specific time in a predefined "reaction volume" (15), and therefore need the help of chaperones. Chaperones bind transiently to folding intermediates, keep their free concentration low and suppress otherwise fatal interactions (16).

Aggregation-prone folding intermediates are not unique to newly synthesized polypeptides, but are formed in even higher concentration as a result to dramatic environmental changes such as heat shock and viral infections (17, 18). This explains the high constitutive expression level of heat shock proteins under normal temperature conditions as well as their several-fold overexpression under heat shock conditions (reviewed by 19, 20).

The most prominent heat shock proteins which take care of otherwise irreversibly damaged proteins in eukaryotes are members of the Hsp90 and Hsp70 families. While Hsp70 was one of the first proteins known to function as a molecular chaperone, the function of Hsp90 both under normal and stress conditions has remained elusive. Only very recently, a substantial increase in the understanding has been reported (reviewed in ref. 21).

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