Hsp70 and Hsp90 chaperones

National Institute Of Diabetes And Digestive And Kidney Diseases

Investigators

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Abstract

Organisms encode multiple Hsp70s to regulate abundance of Hsp70 in accordance with need in response to changing environments and to provide a range of distinct Hsp70 functions for carrying out many different tasks within cells and across cell types. Little is know of the mechanisms that underlie how the highly homologous Hsp70s function differently. We constructed a yeast genetic system to evaluate Hsp70s from any source and have found that each of the four functionally redundant essential yeast Hsp70s possess distinct activities. We also developed our system to investigate differences in functions of the two Hsp90 paralogs. We are continuing to use this system to investigate how Hsp70s and Hsp90s within and across species act in cellular PQC processes or influence propagation of amyloid in vivo. The large number of co-chaperones that act on Hsp70 and Hsp90 can cooperate to provide both a broad range of function and fine tuning of Hsp70 or Hsp90 activity to specify proper functions for defined roles in cells. The many ways that phenotypes change when activities or abundance of Hsp70s or their many co-chaperones are altered provides a sensitive way to investigate even subtle functional distinctions among highly redundant Hsp70s, and a useful approach to uncover the underlying mechanisms. Hsp70 cooperates with Hsp90 to promote the proper folding and functions of many "client" proteins that regulate various fundamental cellular processes. Altering abundance or function of Hsp70 and Hsp90 can lessen pathology in models of protein folding disorders, while in the same models reducing activity of either chaperone can cause or exacerbate pathology. These protein chaperones therefore are promising therapeutic candidates for amyloid and other protein folding disorders and they are being evaluated intensively as drug targets. To provide insight into development of such approaches we are using yeast, human and disease organism paralogs of Hsp70 and Hsp90 to study specific and conserved functional interactions of these chaperones. Using human Hsp90s alpha and beta (hHsp90), which function in place of the yeast Hsp90s, we have identified specific sites on these paralogs that are important for regulating functional output of interactions with Hsp70 and other co-chaperones. We also discovered that a mutation in Hsp90 that destroys its ability to hydrolyze ATP does not prevent hHsp90 from supporting growth of evolutionarily distant cells. Similarly mutated versions of Hsp90s from three disease organisms also retained ability to support cell growth, but to different degrees, which reveal functional differences. Using biophysical and biochemical assays we showed how the mutation affects conformational dynamics of Hsp90 upon binding of ATP or ADP. Together our findings connect how specific changes in the Hsp90 reaction cycle relate to its functions in vivo, and provide a framework for understanding the molecular mechanics needed to specify action of these Hsp90s under different physiological and environmental conditions. Showing ATP hydrolysis is dispensable for essential and non-essential Hsp90 functions is obligating the field to reconsider the role of ATP in Hsp90 activity. More recently we showed that rather than providing a source of energy, ATP plays a structural role in Hsp90 function. Hsp90 undergoes large and complex conformational rearrangements of its structural domains and ATP acts as one element of a tether via ionic interaction of its gamma phosphate with an arginine in an adjacent domain. This arrangement strongly resembles so-called arginine fingers that stabilize interactions between protomers of oligomeric complexes in protein families such as Snares, Septins and AAA+ ATPases. Insights gained from our studies therefore have much broader implication regarding the role of ATP in these other systems. Our work toward understanding what underlies specificity in activities of functionally redundant Hsp70s and Hsp90s provide insight for new approaches to modify Hsp90 activity in specific ways. This understanding can help guide decisions about which Hsp70 or Hsp90-family members would be most useful for such applications, or point to potential problems that could arise due to differences in ways that different Hsp70 or Hsp90 paralogs respond to specific compounds. Overall our work provides insight into functions of protein quality control factors that can help guide strategies for using chaperones as targets for therapy in amyloid diseases and other protein folding disorders.