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Updated: Aug 15, 2025
High-throughput Screening for Protein-based Inheritance in S. cerevisiae
Published on: August 8, 2017
The human ribosome-associated complex suppresses prion formation in yeast
Christina Kelly1, Yusef Ahmed1,2, Omar Elghawy1,3
1Biology Department, Ursinus College, Collegeville, Pennsylvania, USA.
Abstract:
Many human diseases are associated with the misfolding of amyloidogenic proteins. Understanding the mechanisms cells employ to ensure the integrity of the proteome is therefore a crucial step in the development of potential therapeutic interventions. Yeast cells possess numerous prion-forming proteins capable of adopting amyloid conformations, possibly as an epigenetic mechanism to cope with changing environmental conditions. The ribosome-associated complex (RAC), which docks near the ribosomal polypeptide exit tunnel and recruits the Hsp70 Ssb to chaperone nascent chains, can moderate the acquisition of these amyloid conformations in yeast. Here we examine the ability of the human RAC chaperone proteins Mpp11 and Hsp70L1 to function in place of their yeast RAC orthologues Zuo1 and Ssz1 in yeast lacking endogenous RAC and investigate the extent to which the human orthologues can perform RAC chaperone activities in yeast. We found that the Mpp11/Hsp70L1 complex can partially correct the growth defect seen in RAC-deficient yeast cells, although yeast/human hetero species complexes were variable in this ability. The proportion of cells in which the Sup35 protein undergoes spontaneous conversion to a [PSI+ ] prion conformation, which is increased in the absence of RAC, was reduced by the presence of the human RAC complex. However, the toxicity in yeast from expression of a pathogenically expanded polyQ protein was unable to be countered by the human RAC chaperones. This yeast system can serve as a facile model for studying the extent to which the human RAC chaperones contribute to combating cotranslational misfolding of other mammalian disease-associated proteins.
Insights
Human ribosome-associated complex (RAC) proteins Mpp11 and Hsp70L1 partially restored growth and reduced prion formation in yeast lacking their own RAC components. However, they did not prevent toxicity from expanded polyQ proteins.
Area of Science:
- Molecular biology
- Cell biology
- Protein folding
Background:
- Amyloidogenic protein misfolding is linked to human diseases.
- Cells utilize proteome integrity mechanisms, including chaperones like the ribosome-associated complex (RAC).
- Yeast prions, like [PSI+], are amyloid conformations of proteins such as Sup35, potentially serving as an epigenetic response.
Purpose of the Study:
- To investigate if human RAC proteins (Mpp11 and Hsp70L1) can functionally replace yeast RAC orthologues (Zuo1 and Ssz1).
- To assess the extent to which human RAC chaperones can perform RAC chaperone activities in yeast.
- To evaluate the efficacy of human RAC in combating protein misfolding and toxicity in a yeast model.
Main Methods:
- Yeast strains deficient in endogenous RAC components were utilized.
- Human RAC proteins Mpp11 and Hsp70L1 were expressed in yeast to substitute for Zuo1 and Ssz1.
- Cell growth, Sup35 prion conversion to [PSI+], and toxicity from expanded polyQ protein expression were assessed.
Main Results:
- The human Mpp11/Hsp70L1 complex partially rescued the growth defects of RAC-deficient yeast.
- Yeast/human hetero-species RAC complexes showed variable functional abilities.
- The human RAC complex reduced the spontaneous conversion of Sup35 to the [PSI+] prion conformation.
- Human RAC chaperones did not mitigate the toxicity caused by expressing a pathogenic expanded polyQ protein.
Conclusions:
- Human RAC chaperones exhibit partial functional conservation in yeast, capable of moderating prion formation.
- The yeast system provides a valuable model for studying human RAC chaperone function in combating cotranslational misfolding.
- Further research is needed to understand the limitations of human RAC in addressing specific proteinopathies, such as those involving expanded polyQ proteins.
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