We use genetics, molecular biology, and biochemistry to identify and characterize cellular factors that influence propagation of prions in yeast. The prions we study are proteins that have a propensity to misfold and form highly structured fibrous aggregates called amyloid. Tissue pathalogy in many human disorders, including type 2 diabetes and Alzheimer’s disease, is associated with accumulation of amyloid forms of proteins. Prions are transmissible between strains and they propagate by recruiting the normal form of the protein and converting it into the same misshapen form as it joins the growing fiber. Prions also must replicate to be maintained in a growing population of yeast.
Hsp70 is a universally conserved and essential protein chaperone that helps proteins adopt and maintain their native structural conformations. Hsp70 activity is regulated by co-chaperones, such as Hsp40. Another protein chaperone, Hsp104, acts with Hsp70 and Hsp40 to break up and reactivate proteins that have become aggregated. The normal function of this chaperone machine appears to promote amyloid propagation by breaking fibers into more numerous pieces, each of which can continue to propagate the amyloid structure. We have identified several mutations in these chaperones that variously affect the prion seeding process. Determining how these mutations affect the chaperones’ activities and ability to interact with each other will help us understand the molecular mechanisms underlying the functions of this machinery.
We have also found that TPR co-chaperones, which are components of the Hsp90 machinery, can regulate Hsp70 function in prion propagation, independently of Hsp90.We are using our system to dissect the mechanisms of how these co-chaperones regulate Hsp70 and Hsp90 activity, and for assessing in vivo functions of Hsp90.
We have identified many mutants of various protein chaperones, in particular Hsp70 and its co-chaperones, that are revealing chaperone and co-chaperone interactions that are important for prion replication. We are continuing genetic characterization of the modified chaperones and working to define how the mutations affect their physical interactions and enzymatic activities. We are particularly interested in understanding how the various chaperones interact with each other to regulate each other’s functions and to cooperate as components of different protein remodeling machines. We also are working to understand how specific interactions among the many different components act to localize, direct, and fine-tune the chaperone machinery in its diverse cellular processes. In addition to providing a better understanding of how protein chaperones function in amyloid propagation, our continued studies will provide insight into how they act in their many important roles within the cell.