The 70-kDa Heat Shock Protein (Hsp70) Chaperone System: Linking ATP Turnover and Complex Formation To Protein Homeostasis.

Abstract

The 70-kDa heat shock protein (Hsp70) is a molecular chaperone that binds unfolded proteins and directs them towards a number of divergent pathways, including folding, trafficking, and degradation. This delicate balance is critical for normal protein homeostasis and becomes disrupted in a variety of diseases, including neurodegenerative disorders. However, it isn’t yet clear what factors direct Hsp70-bound substrates to adopt a given fate.

Hsp70 is a two-domain protein capable of ATP hydrolysis, substrate binding, and the formation of discrete multi-protein complexes. In this thesis, we aimed to characterize the effect of these different variables on substrate fate. First, using a series of point mutants in the prokaryotic Hsp70, DnaK, we discovered that the ATPase rate is not directly related to molecular chaperone activities, such as protection from heat stress or refolding of denatured model substrates. To further probe how ATP hydrolysis may influence chaperone structure and function, we explored how nucleotide state regulates the oligomerization of DnaK. This work showed that ADP-bound DnaK formed small oligomers that retained some chaperone functions, such as substrate binding. However, these oligomers had reduced refolding activity and they were poorly stimulated by the co-chaperone, DnaJ. These studies suggest that oligomerization might be an important step in Hsp70 chaperone cycling. Finally, we explored how chaperones, including Hsp70, change in their association with the pathologically-relevant substrate, tau (MAPT), during an acute, small molecule-induced switch to a degradation fate. We hypothesized that this system would, for the first time, provide insight into the early events associated with a change in protein fate. These studies suggested a rapid switch from an Hsp70-bound complex to an Hsp90-bound complex during tau degradation. Further, Hsp70 and Hsp90 seemed to compete for binding to similar regions of tau, suggesting that competition between these chaperones might control tau fate. Taken together these studies improve our understanding of the factors that link Hsp70 to the various fates of its substrates. These studies also suggest previously unanticipated therapeutic strategies to rebalance protein homeostasis.