The Effects of Molecular Chaperone Modulation on Protein Folding, Prion Formation, and Prion Propagation in Saccharomyces cerevisiae

Proper and efficient protein folding is vital for cell survival. Many factors affect protein folding fidelity and prion formation, including molecular chaperone availability and activity. Research has shown that modulating chaperone availability and function can affect protein misfolding and aggregation, as well as de novo prion formation and propagation. However, the factors involved and underlying mechanisms influencing prion formation and protein folding are largely unknown. The following work aims to elucidate these areas. The Nascent Polypeptide-Associated Complex (NAC) is the first point of chaperone contact for nascent polypeptides. Previous work has shown that disruption of the NAC leads to improved viability in cells experiencing protein misfolding stress. This counterintuitive result led us to investigate the ability of NAC deletion to improve survivability of cells expressing misfolding human proteins. This work resulted in the identification of multiple NAC deletion strains that improve viability in cells expressing disease-causing alpha-synuclein and expanded polyglutamine proteins. Also, this work identified changes in de novo induction of a yeast prion and morphological changes in expanded polyglutamine aggregates as a result of NAC disruption. Overall, this work reveals the potential of NAC disruption as a therapeutic target for neurodegenerative diseases and sets the stage for investigating the mechanism by which NAC disruption improves viability in cells expressing disease-causing, aggregating proteins. Mutations in another chaperone, DNAJB6, have been shown to cause Limb-Girdle Muscular Dystrophy Type 1D (LGMDD1). While we know that these mutations are associated with LGMDD1, the mechanism by which they induce disease remains unknown. Because substrates of DNAJB6 have not been identified, we have turned to a homologous protein in yeast, Sis1, with known client proteins to better understand the effect of these mutations. We have also developed a Sis1-DNAJB6 chimeric protein (SDSS) to evaluate these mutations. This chimeric protein includes the J, G/M, and C-terminal domains of Sis1, and the G/F domain, in which many LGMDD1-associated mutations are found, of DNAJB6. Previous work has shown that when LGMDD1-associated mutations are introduced in Sis1 or SDSS there is disruption of client processing by Sis1. This body of work identifies multiple second-site suppressors that, when introduced in combination with LGMDD1-associated mutations, are capable of recovering client processing by Sis1 and SDSS. Overall, this work shows that second-site suppressors may be capable of recovering DNAJB6 activity when introduced in combination with LGMDD1-associated mutations. Moreover, it provides an experimental model for the continued investigation of these second-site suppressors and identification of similar therapeutic avenues for potentially treating patients with other LGMDD1-associated mutations in the future