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Zuo1 is a component of the eukaryotic Ribosome Associated Complex (RAC), a group of chaperone proteins that associate with the ribosome and aid in the cotranslational folding of nascent proteins. When cells encounter environmental stress, yeast Zuo1 dissociates from the ribosome and moves into the nucleus, where it serves as a transcription factor to activate genes which aid in the stress response. Previous work from our laboratory has demonstrated that the absence of functional RAC on ribosomes significantly increases the frequency at which prions – self-propagating, misfolded protein conformations – form in the budding yeast, Saccharomyces cerevisiae. Moreover, previous studies have shown that environmental stress conditions elevate prion formation rates, and that prion formation can enhance fitness in some stress conditions. Thus, dissociation of Zuo1 from ribosomes during stress may be a mechanism underlying stress-induced prion formation. However, the mechanism by which Zuo1 dissociation from ribosomes is triggered during stress is currently not understood. We hypothesized that post-translational modification of Zuo1 is the mechanism which allows for the conformational change required for Zuo1 to dissociate from the ribosome. We tested this hypothesis by engineering mutant versions of Zuo1 that are unable to be post-translationally modified at likely sites of modification. The normal ribosomal function of the mutant forms of Zuo1 was not compromised, as indicated by their ability to rescue the growth defect of a strain lacking Zuo1 in non-stress conditions. We also tested how wild-type Zuo1 behaves under stressful conditions such as cyclohexamide and ethanol stress. Although no significant conclusions were drawn from the experiments with the mutant Zuo1 variants, wild-type Zuo1 was found to associate more with ribosomes under stressful conditions, a novel result that was inconsistent with our hypothesis. This test also revealed that either new isoforms of Zuo1 exist under stressful conditions or that the degradation pattern of Zuo1 is altered under stressful conditions, potentially suggesting differences in modification and/or folding in these conditions.
Tessitore, Thomas, "Understanding the Function of the Chaperone Protein Zuo1 in Saccharomyces cerevisiae" (2018). Biology Honors Papers. 19.