Figure 1. Models representing the basic mechanisms of the Hsp70 and chaperonin systems. (a) Mechanism of the Escherichia coli Hsp70 system DnaK (Hsp70), DnaJ (Hsp40), and GrpE. Unfolded protein (U) is presented to ATP-bound DnaK by the Hsp40 DnaJ. Interaction of DnaJ with DnaK leads to rapid ATP hydrolysis on DnaK and to tight binding of U to DnaK. U remains tightly associated with the ADP-bound form of DnaK. The nucleotide exchange factor, GrpE, causes dissociation of ADP, which allows DnaK to rebind ATP. This then shifts DnaK to a low substrate affinity mode, resulting in release of U. Whenever U is released, folding to the native state (N) may occur, provided that all structural elements necessary for folding are available (e.g. upon release of a nascent chain from the ribosome). Otherwise, U will undergo another cycle of interaction with the Hsp70 system. (b) Mechanism of E. coli GroEL–GroES. A vertical section of the GroEL double ring is shown. Binding of U occurs by interaction with hydrophobic surfaces lining the GroEL cavity. Binding of ATP and GroES encapsulates the substrate, which is released into the GroEL cavity. Successive binding of ATP and GroES leads to the release of U into an enclosed cage for folding. Following ATP hydrolysis ATP binding in the opposite GroEL ring then causes the release of GroES, opening the cage. Folded polypeptide escapes into the solution while incompletely folded intermediates can rebind for another folding trial.

Figure 2. Possible protein folding pathways in the eukaryotic (a) and bacterial cytosol (b). Alternative pathways for folding are possible, as well as folding without the participation of chaperones. The two models demonstrate pathways likely to be used by a subset of aggregation-sensitive polypeptides. (a) Folding pathway for TRiC-dependent substrates, such as actin. Nascent chain-associated complex (NAC) interacts with nascent chains early during translation; Hsp70 also interacts with nascent chains. The action of Hsp70 and TRiC may be functionally coupled. Prefoldin (GimC) is known to interact with TRiC during actin folding and may also be involved in targeting actin to TRiC, either co- or posttranslationally. (b) Trigger factor (TF) and DnaK (E. coli Hsp70) associate with nascent chains in a successive fashion as indicated. DnaK associates with ribosome-released proteins as well. A subset of proteins is transferred to GroEL for successive folding trials.

Figure 3. Protein folding pathway for a eukaryotic multidomain protein accomplished by sequential domain folding, assisted by the Hsp70 system. Folding is assisted by the Hsp70 system. Folding of a completed domain occurs as Hsp70 dissociates from the nascent chain in an ATP-dependent manner. For some proteins cotranslational folding of a completed domain may be mediated by the cytosolic chaperonin TRiC (not shown).