Modulating cellular balance of Rps3 mono-ubiquitination by both Hel2 E3 ligase and Ubp3 deubiquitinase regulates protein quality control

When a ribosome complex is stalled during the translation elongation process in eukaryotes, the mono-ubiquitination of Rps3 has recently been shown to be critical to ribosome quality control. We have discovered that the regulatory role of Rps3 mono-ubiquitination is controlled by a deubiquitinase. We also showed that an autophagic signal appears to be coupled to the mono-ubiquitination of Rps3p through the entrance of Ubp3p into the autophagosome in yeasts. The mono-ubiquitination of the Rps3 protein is tightly modulated by reciprocal action between the Hel2p E3 ligase and the Ubp3p deubiquitinase in yeasts and the reciprocal action between the RNF123 E3 ligase and the USP10 deubiquitinase in mammalian cells. We also found that the Ubp3p/USP10 deubiquitinases critically modulate Hel2p/RNF123-mediated Rps3p mono-ubiquitination. In addition, we found that Hel2p/RNF123 and Ubp3p/USP10 appeared to be differently localized in the ribosome complex after ultraviolet irradiation. Together, our results support a model in which coordinated ubiquitination and deubiquitination activities can finely balance the level of regulatory Rps3p mono-ubiquitination in ribosome-associated quality control and autophagy processes. Protein synthesis: Enzymes control protein quality through ribosome tagging Ribosomes that fail to make a full-length protein get tagged by specific enzymes with a label that signals to destroy defective proteins they made. Working in yeast and mammalian cells, a South Korean team led by Joon Kim from Korea University in Seoul first confirmed previous findings that whenever protein synthesis is interrupted and halted, a ubiquitin protein is added to a ribosomal subunit called Rps3p. This signals to the cell's clean-up system that aberrent nascent polypeptides should be eliminated. The researchers then identified the enzymes involved in adding and taking away the ubiquitin tag, both in yeast and in human cells. When ubiquitin is added to Rps3p in the presence of rapamycin in yeast, the enzyme that normally removes ubiquitin is destroyed in the autophage. The findings highlight how enzymes control protein quality through ubiquitin tagging and untagging of the ribosome.