A Yeast Model System to Study the Human Orthologs of the Ribosome‐Associated Complex

Abstract

Prions are a class of misfolded proteins that form protein aggregates that are infectious and can self‐propagate, causing human diseases like Creutzfeldt‐Jakob. Additionally, many other diseases are known to involve protein misfolding and formation of amyloid aggregates as a characteristic of their pathology. To suppress the formation of misfolded proteins, cells have evolved various quality control systems, including chaperone proteins that can help protect nascent polypeptide chains from misfolding. Saccharomyces cerevisiae, also known as brewer’s yeast, has been used as a model organism to study protein misfolding and prion formation, including the yeast [PSI+] prion formed by the misfolding of the translation termination factor Sup35. The ribosome‐associated complex (RAC) is a chaperone complex composed of two subcomponents, in yeast called Zuo1 and Ssz1. Previous work from our lab and others has shown that the presence of the RAC on ribosomes decreases the frequency of the formation of the [PSI+] prion and reduces the toxicity of aggregation‐prone polypeptides. The human ortholog of yeast RAC is composed of Mpp11 and Hsp70L1. Previous studies have shown that human RAC can rescue the growth defect associated with deletion of the yeast RAC components. We therefore hypothesized that the function of the RAC complex, including its ability to suppress prion formation and toxicity of aggregation‐prone proteins, may be conserved from yeast to humans. We transformed yeast lacking the RAC components Zuo1 and Ssz1 with plasmids expressing the human orthologs of these chaperones. Growth assays confirmed previous studies showing that the human RAC components partially restore growth to yeast lacking endogenous RAC. In addition, like the yeast counterparts, human RAC reduces the formation of the [PSI+] prion. This result highlights the potential of the yeast model system to investigate the roles of human chaperones in modulating protein folding and misfolding. To further investigate the potential of this yeast model system, we have studied the ability of the human RAC components to counter the toxicity of several human disease‐associated proteins expressed in yeast, including those with polyQ repeats, the amyloid‐β peptide, and □‐synuclein, which are associated with Huntington’s, Alzheimer’s, and Parkinson’s Diseases in humans, respectively. The results provide insights on the use of S. cerevisiaein studying human diseases involving protein misfolding.