Mutational analysis of Hsp110 suggests an integral role for the chaperone in yeast prion propagation.
Moran, Ciara (2010) Mutational analysis of Hsp110 suggests an integral role for the chaperone in yeast prion propagation. PhD thesis, National University of Ireland Maynooth.
Prions are highly aggregated infectious proteins that have been identified in mammals and in some fungal species. The discovery that prions exist in the yeast species S. cerevisiae has enabled researchers to explore the characteristics of these unusual proteins and how they propagate within the cell. Heat shock proteins are implicated as having a major role in the propagation of prions. The work presented in this thesis focuses on the heat shock protein Hsp110. Recent reports have demonstrated how the Hsp110 proteins Sse1 and Sse2 are necessary for promoting the propagation of the yeast prions [PSI+] and [URE3]. A group of Sse1 mutants with impaired ability to propagate [PSI+] were created. These mutants were characterised according to the degree in which they affect [PSI+]. Molecular modeling of the mutants onto the Sse1 structure allowed us to assess the potential structural affects the mutants are having on Sse1. These changes could affect normal Sse1 functions necessary for [PSI+] propagation such as nucleotide exchange factor function and interaction with the molecular chaperone Hsp70. SSE1 and SSE2 are an essential gene pair and have been implicated in yeast cellular homeostasis. The Sse1 mutants disrupted cellular thermostability at 37oC and 39oC. Mutant G616D displayed osmoremedial temperature sensitivity at 37oC. Disruption to cell wall integrity signaling was explored as a possible reason for this phenotype but evidence from this study suggests that G616D may disrupt PKA signaling. Comparative analysis of Sse1 and Sse2 reveals that Sse1 is more thermostable than Sse2 and Sse1 propagates [PSI+] efficiently whereas Sse2 fails to promote [PSI+] propagation. Small structural differences between the proteins may be responsible for this. However, differentially phosphorylating Sse2 may impact its ability to act as a [PSI+] promoting factor. Overall, this work further highlights the important role Sse1 plays in yeast prion propagation and in cellular homeostasis.
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