Investigating the Roles of the NAC and RAC in Stress Induced Prion Formation in Yeast
Authors: Kelly Christina , Cameron Dale M. 
Published: June 2021

Many mammalian neurodegenerative diseases and the prion‐transmitted spongiform encephalopathies are associated with the misfolding of proteins into amyloid conformations. In Saccharomyces cerevisiae, there exist a number of proteins capable of folding into prion forms, yet the acquisition of prion phenotypes can occasionally result in increased fitness, possibly by serving as epigenetic switches that can rapidly confer beneficial heritable phenotypes. Thus, the possible control mechanisms yeast use to modulate the acquisition of prion conformations makes yeast an important model system for understanding amyloid formation. Eukaryotes have two conserved ribosome‐associated chaperone complexes that interact with nascent polypeptide chains to promote correct folding of newly synthesized proteins: the nascent polypeptide‐associated complex (NAC), a heterodimeric complex consisting of an α subunit, paired with one of two possible β subunit paralogs; and the ribosome‐associated complex (RAC), consisting of the Hsp70 chaperone Ssz1 and the Hsp40 chaperone Zuo1, which anchors the complex onto ribosomes and stimulates the ATPase activity of the Hsp70 chaperone Ssb. Both of these complexes associate with ribosomes near the polypeptide exit tunnel and both have roles in co‐translational protein homeostasis and ribosome biogenesis. Previous work in our lab has shown that yeast cells lacking Zuo1, and thus lacking RAC chaperone function on ribosomes, exhibit higher frequencies of spontaneous and induced formation of the [PSI+] prion, the self‐propagating amyloid form of Sup35, a translation termination factor. These findings are consistent with a role for the RAC in chaperoning nascent Sup35 to prevent misfolding of its N‐terminal prion domain as it emerges from the ribosome. An alternative role for Zuo1 is as a transcriptional co‐activator for genes in the pleiotropic drug response pathway (PDR), which enhances resistance to various drugs and environmental toxins. Zuo1's role in this pathway requires unfolding of its C‐terminal domain, which prevents Zuo1's association with ribosomes. Since a variety of environmental stressors are known to induce prion formation, interactions that promote RAC's dissociation from ribosomes under those conditions, may result in increased prion formation. We conducted a screen for Zuo1 dependent enhancers of PDR that revealed a possible role for the NAC minor β subunit Btt1. The contributions of ribosome‐associated processes in prion formation is a critical step in understanding the regulation and physiological impacts of amyloid formation. Our current work examines the role of RAC and NAC as stress‐responsive modulators of protein folding and seeks to understand the role of the NAC in facilitating a RAC dependent stress‐response.Support or Funding InformationNational Institutes of Health, NIGMS Award Number R15GM119081 to DMC and by Ursinus CollegeThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.