Models sHsp chaperone

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. 

The chaperone model for sHsp function provides a basic framework to explain the many proposed sHsp/protein interactions and potential functions. The diversity of the sHsp family, however, indicates that care must be taken in generalizing biochemical properties and activities across different family members. Nonetheless, we now have a firmer structural foundation on which to design future experiments to build a biochemical mechanism of action. 

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