Genetic Factors Involved In Yeast Prion Propagation

Diabetes, Digestive, Kidney Diseases

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Abstract

Prion disease is a type of amyloidosis, like Alzheimer's disease and type 2 diabetes, but with the important distinction of being infectious. In these diseases abnormal folding of a cellular protein leads to its aggregation into fibrillar structures (amyloid), whose accumulation is associated with tissue degeneration. Amyloid formation appears to require a nucleation event, or seeding, but the precise nature of the seeds and what triggers their formation is unknown. What allows prions to be transmissible is also unknown. Our research, using yeast as a genetic system, is aimed at better understanding amyloid formation and replication, and in particular, what roles cellular factors play in prion propagation and transmissibility. Hsp104 is a protein chaperone whose role is to disaggregate stress-damaged proteins. Yeast prion propagation is universally dependent upon Hsp104 and is arrested by the presence of 5 millimolar guanidine HCl in growth media. We showed that this level of guanidine completely inhibits Hsp104 activity in vivo, providing an explanation for its effect in curing yeast prions. We then isolated Hsp104 mutations that conferred resistance to this guanidine curing, which demonstrated that effects of guanidine on yeast prions are specific to Hsp104 and revealed that Hsp104 interacts differently with amorphous protein aggregates generated by thermal denaturation and the structured aggregates of yeast prions. In attempting to isolate strains with different combinations of yeast prions we discovered that different prions interfered with each other's de novo appearance and propagation. We found that the prions were differentially sensitive to elevations in various protein chaperones, and that each stimulated increased expression of different chaperones. A current hypothesis suggests that positive interactions between different prions can be mediated through physical associations. Our data are consistent with the idea that there can be physical associations between different prions but show that such interactions can also be antagonistic, and indicate that distinct interactions between different prions and protein chaperones contributes to their mutual antagonism.