The Ribosome‐Associated Complex Suppresses [PSI+] Prion Formation in Yeast

Abstract

Prions are self‐propagating infectious protein conformations. In mammals prions underlie the transmissible spongiform encephalopathies, a set of invariably fatal neurodegenerative disorders associated with misfolding of the mammalian prion protein, PrP. In recent years increasing numbers of fungal proteins capable of infectiously propagating altered conformations have been discovered. To gain insight into the mechanisms by which cells regulate prion formation, propagation and toxicity, we previously examined the role of the ribosome‐associated complex (RAC) chaperones in the formation and toxicity of the [PSI+] prion in yeast. The [PSI+] prion results from a self‐propagating amyloid form of the essential translation termination factor Sup35, and is manifested phenotypically as elevated levels of nonsense suppression. The RAC consists 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. Previously we showed that yeast cells lacking Zuo1, and thus lacking RAC function on ribosomes, exhibit higher frequencies of spontaneous and induced [PSI+] formation. Cells expressing variants of Zuo1 that are unable to associate with ribosomes, or that are unable to stimulate Ssb ATPase activity, exhibit similarly high levels of prion formation. These findings are consistent with a role for the RAC in chaperoning nascent Sup35 to prevent misfolding of the N‐terminal prion domain as it emerges from the ribosome. Zuo1 can additionally function as transcriptional co‐activator for the pleiotropic drug resistance pathway, which enhances resistance to various drugs and environmental toxins. Its role in this pathway requires unfolding of the C‐terminal domain of Zuo1, which prevents its association with ribosomes. Since a variety of environmental stress conditions are known to induce prion formation, RAC dissociation from ribosomes under these conditions may facilitate prion formation. Our current work aims to understand the mechanism by which Zuo1 dissociation from ribosomes is triggered in response to environmental stress.Support or Funding InformationThis work was supported by an award from the National Institutes of Health National Institute of General Medical Sciences to DMC.