Small heat shock protein Hsp16.3 modulates its chaperone activity by adjusting the rate of oligomeric dissociation.

Small heat shock proteins usually exist as oligomers and appear to undergo dynamic dissociation/reassociation, with oligomeric dissociation being a prerequisite for their chaperone activities. However, contradictory cases were also reported that chaperone activities could be enhanced with no change or even increase in oligomeric sizes. Using Hsp16.3 as a model system, our studies show the following: (1) Although a preheat (over 60 degrees C) treatment or the presence of low concentrations of urea (around 0.8M) hardly caused any change in the oligomeric size of Hsp16.3 proteins when examined by size exclusion chromatography, its chaperone activities were increased significantly. (2) Further analysis using the unique pore-gradient polyacrylamide gel electrophoresis revealed a dramatic increase in the tendency of oligomeric dissociation for both the preheated and urea-containing Hsp16.3. (3) Meanwhile, for both cases, an apparent increase in the rate constants of oligomeric dissociation was also observed, as determined by utilizing conjugated fluorescence probes whose quantum yield increases accompanying oligomeric dissociation. (4) Moreover, the fluorescence anisotropy analysis also demonstrated that the oligomeric structures for the preheated or urea-containing Hsp16.3 proteins seem to be more dynamic and variable. In light of these observations, we propose that the small heat shock proteins like Hsp16.3 can modulate their chaperone activities by adjusting the rate of oligomeric dissociation in responding to environmental changes. Results obtained here also suggest that small heat shock proteins might be able to "remember" their stress experiences via certain structural alterations which will allow them to act as better chaperones when the stress conditions reappear.