Regulation of the proteasome Regulatory Particle Assembly Chaperones upon stress

A balanced proteome is crucial for cell fitness throughout its life but also its adaptation to environmental challenges. To maintain protein homeostasis, cells can modulate protein synthesis, folding, disaggregation and degradation. The Ubiquitin Proteasome System (UPS) selectively tags substrates with ubiquitin for them to be processed by the proteasome, a multicatalytic protease complex. It is composed of a core particle (CP) capped by one or two regulatory particles (RP). Correct assembly of such multi-subunit complex is crucial for its function and can happen both co-translationally and with the help of assembly chaperones. The yeast proteasome has 5 core particle assembly chaperones (CPAC) and 5 regulatory particle assembly chaperones (RPAC). Upon stress, proteasome abundance and activity are increased, which requires increased translation of its assembly chaperones such as Adc17, one of the regulatory particle assembly chaperones. The induction of Adc17 requires Ede1-mediated relocalisation of its mRNA to cortical actin patches and is abolished upon deletion of the 70 nucleotides upstream of its ATG, in the 5’ untranslated region (UTR). Here, I show that the mRNAs of all 5 RPACs mainly colocalise with actin structures and that their localisation upon stress changes to different extent. I also confirmed that the induction of all RPACs was abolished upon deletion of the 70 nucleotides in their 5’UTR. Focusing on Adc17 5’UTR, I found that although it is crucial, no specific motif seems necessary, which indicates that nucleotides content, mRNA secondary structure or multiple redundant motifs may be necessary for induction of RPACs upon stress. While this will require further analysis, we may gain new knowledge by identifying a trans-acting regulatory element. After using the 70 nucleotides of the 5’UTR of two RPACs, Adc17 and Nas6, for RNA pull down and mass spectrometry analysis, I identified multiple genes which, when knocked out, changed the sensitivity of the yeast to stresses. This is promising and characterisation of these new candidate trans-regulatory factors may help define how RPACs mRNA translation is regulated by stress.