Investigation of the yeast prion factor (PSI+)

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DESCRIPTION (provided by applicant): Certain proteins can exist in distinct heritable conformations. In their "prion" conformation, they form amyloid-like aggregates and seed the joining of non-prion molecules of the same protein to the aggregates. Since similar alterations in protein conformations are associated with neurodegenerative diseases such as Alzheimer's, Huntington's, Parkinson's and Creutzfeldt-Jacob disease, exploring the underlying mechanisms that dictate prion biology will provide a better understanding of the etiology of these diseases. This novel prion mechanism of epigenetic variation exists in yeast. Investigations of new yeast prions will clarify the different roles of heritable protein conformations and the relationships between prions. Prion candidates have been, and will be, retrieved from genetic and proteomic screens. Prionization of two candidates, Cyc8 and Swi1, members of transcriptional regulation and chromatin remodeling complexes, would have global effects on the cell. A prion-protein candidate that may be involved with memory storage (CPEB) and mammalian homologs of the yeast [PSI+] prion protein will also be studied in the yeast system. Curiously, different heritable "strains" or variants of individual prions (with identical primary sequences) exist in both mammals and yeast. The causes of variant specific phenotypic differences will be investigated by studying the physical properties and structures of variant-specific yeast prion fibers made in vitro and by identifying the proteins associated with prion aggregates in cells. The efficiencies with which variants of one prion can "cross-seed" the de novo formation of another prion will be tested in vitro using variant specific fibers. Subparticles of prion aggregates extracted from cells will be tested for infectivity and variant specificity to learn if these subspecies are infectious. Paradoxically, certain prion variants that enhance the de novo appearance of the heterologous [PSI+] prion, also eliminate already existing [PSI+]. To investigate this we will examine the segregation of [PSI+] "seeds" into daughter cells during the curing process. Similar experiments will examine the mechanism by which heterologous QN-rich aggregates destabilize [PSI+]. To investigate how prion seeds first arise in the absence of infection, genes that enhance or inhibit de novo appearance or propagation of a prion will be identified and their roles in prion biogenesis will be defined. These are likely to include proteins involved in protein folding and degradation, and others. Finally, the mechanisms of prion propagation and de novo prion seed formation will be compared by in vitro analysis.