Chaperones substrate complexes

Fig. 5. Model for sHsp chaperone activity. The sHsp oligomer (T Hspl6.9 shown here) is in rapid equilibrium with a smaller species (possibly a dimer). Heat-denatured substrates bind hydrophobic sites exposed on the sHsp subunits to form soluble sHsp/substrate complexes, preventing formation of insoluble aggregates of denatured proteins. The sHsp/substrate complexes may also be in rapid equilibrium, and when dissociated, the denatured substrate can be picked up and refolded in an ATP-dependent fashion by the Hsp70 or DnaK (plus cochaperone) machinery. Note that sHsp/substrate complexes can also become larger and insoluble, and the fate of these latter complexes is unknown.

The model in Figure 5 includes formation of both soluble and insoluble complexes of sHsp and substrate. The formation of insoluble sHsp/substrate complexes is consistent with the in vivo transition of sHsps to an insoluble, structure-bound form under many stress conditions as discussed above. At present we can provide only speculative explanations for this insolubility in the context of the chaperone model of sHsp function. From in vitro studies, it is clear that the ability of sHsps to keep substrates soluble is dependent on the sHsp-to-substrate ratio, the rate of substrate denaturation, and other factors in vitro conditions can be manipulated to cause precipitation of sHsp and substrate, as well as to maintain substrate solubility. Thus, insolubilization could result from a type of overload of the soluble binding capacity of the sHsps. Since in vivo there is good evidence that the insolubilization is reversible, this leads to the intriguing question of the mechanism of resolubilization, and whether this is also a function of Hsp70 systems, or if additional components are required. Alternatively, sHsp insolubilization in vivo could result from interaction with insoluble components in the cell. 

Bennett, J.C.Q., Thomas, J., Fraser, G.M. and Hughes, C. (2001). Substrate complexes and domain organization of the Salmonella flagellar export chaperones FlgN and FliT. Mol. Microbiol. 39, 781-791. 

Figure 11 Pharmacological chaperones (grey triangles) bind to the active site of a protein, misfolded due to a mutation (black ellipse) and restore the functional protein conformation. Thus the protein is prevented from early degradation. The stable chaperone-protein complex is transferred by the endoplasmic reticulum vesicles to the Golgi and later to the lysosome, where the inhibitor is replaced by the natural substrate.

The general types of protein-protein interactions that occur in cells include receptor-ligand, enzyme-substrate, multimeric complex formations, structural scaffolds, and chaperones. However, proteins interact with more targets than just other proteins. Protein interactions can include protein-protein or protein-peptide, protein-DNA/RNA or protein-nucleic acid, protein-glycan or protein-carbohydrate, protein-lipid or protein-membrane, and protein-small molecule or protein-ligand. It is likely that every molecule within a cell has some kind of specific interaction with a protein. 

Parkin substrates. (B) A model for Parkin-dependent degradation of misfolded proteins. Parkin recruits a complex containing molecular chaperones and the unfolded substrates to the proteasome. Degradation may be facilitated by... 

Fig. 9.4. Hypothetical reaction cycle for the 26S proteasome. A polyubiquitylated substrate is delivered to a 26S hybrid proteasome in some cases by chaperones such as VCP/p97 (step 1). Substrate is bound by polyubiquitin recognition components within the regulatory complex (RC) until the polypeptide chain is engaged by the ATPases (step 2). As the...

Fig. 5. Copper homeostasis in Enterococcus hirae. Under copper-limiting conditions, copper is pumped into the cell by CopA. The CopZ copper chaperone picks up copper at this site of entry. Under physiological copper conditions, Zn(II)CopY binds to the promoter and represses transcription of the cop operon. Under conditions of copper excess, Cu-CopZ donates Cu(I) to CopY, which leads to the replacement of the Zn(II), loss of DNA-binding affinity, and ultimately synthesis of the operon products. Excess copper is secreted by the CopB efflux pump. The substrate for this pump may be a copper-glutathione (GSH) complex, rather than Cu-CopZ.

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