Investigating the Roles of NAC and RAC in Suppressing Prion Formation in Saccharomyces cerevisiae

Abstract

The correct three‐dimensional folding of a polypeptide is essential for its correct structure and function. The appearance of misfolded proteins can be found in relation to many disease processes. Prions are a class of infectious misfolded proteins, which are characterized by their ability to self‐propagate. In order to help ensure correct protein folding for the survival of the cell, several chaperone quality control complexes interact with nascent polypeptides as they are being synthesized by the ribosome. In yeast, two important ribosome‐tethered chaperone complexes exist, the Ribosome‐Associated Complex (RAC), composed of two subunits, Hsp70 chaperone Ssz1 and Hsp40 chaperone Zuo1, and the Nascent Polypeptide Associated Complex (NAC), a heterodimeric complex composed of an α subunit, Egd2, and one of the two β subunits, Btt1 or Egd1. Previous research by our lab and others has shown that cells lacking Zuo1 and thus, lacking functional RAC, show an increase in prion formation of the [PSI+] prion, a misfolded form of the Sup35 translation termination factor protein. In addition to its involvement in protein folding in the RAC, Zuo1 serves as a transcriptional co‐activator for genes in the pleiotropic drug resistance (PDR) response, which aids yeast survival in response to environmental stressors such as drugs or toxins. Environmental stress can also trigger prion formation in yeast. An overexpression screen for factors that increase yeast survival in the presence of the antibiotic cycloheximide, a compound known to result in PDR activation, revealed Btt1 to increase cell survival in a Zuo1 dependent manner. To investigate the interactions between RAC and NAC, as well as the role these complexes play in modulating prion formation during periods of cellular stress, we conducted prion formation assays and growth assays to assess how the complexes work together and separately in relation to the formation of the [PSI+] prion.