Monitoring chaperone aggregation to measure ribosomal stalling

Abstract

Proteostatic disequilibrium, the inability for cellular components to synchronously balance protein synthesis, folding and degradation, has been implicated in various ageing-related diseases, including Alzheimer’s and Parkinson’s disease, which often manifest through protein aggregation, toxicity and eventual cell death. The ribosome-associated quality control (RQC) complex targets incomplete polypeptides at stalled ribosomes through C-terminal addition of Alanine and Threonine residues by Rqc2p, followed by ubiquitin tagging for proteasomal degradation by Ltn1p. When they are not degraded, these stalled polypeptides have heightened propensity for aggregate formation that cytosolically sequesters molecular chaperones, prominently including Sis1p. In this study, we used the altered RQC genotype ltn1Δ with RQC2 overexpression (ltn1Δ/RQC2OE) in conjunction with mCherry-labelled molecular chaperone Sis1p, to characterise aggregate formation upon altered proteostasis. By using Sis1p aggregation, we could monitor increased ribosomal stalling in cells treated with a translation inhibitor as well as defects in mRNA decay. In ltn1Δ/RQC2OE cells, factors involved in mRNA decay, namely the Superkiller complex, Ccr4p, Caf1p and Dcp1 were found crucial to proteostasis. Furthermore, the E3 ubiquitin ligase Hel2p was shown to delimit endonucleolytic cleavage upon ribosome collision. Together, we demonstrated the feasibility of using molecular chaperones to monitor ribosomal stalling from multiple origins.