Perturbation of translation elongation induces both No-Go Decay (NGD), in which the mRNA in the vicinity of stalled ribosomes undergoes endonucleolytic cleavage, and Ribosome Quality Control (RQC), in which the arrested protein product is co-translationally degraded by the proteasome. Ribosome stalled by translationally defective 18S rRNA is eliminated by 18S nonfunctional rRNA decay (18S NRD). However, the mechanisms by which stalled ribosomes induce these quality control systems are poorly understood.

We have identified the factors that are involved in the induction of translation arrest by a polybasic sequence (R12). Receptor for activated C-kinase 1 (RACK1) is a 40S ribosomal subunit-associated factor that is involved in translation arrest and is required for RQC. In addition, an E3 ubiquitin ligase Hel2 is also involved in translation arrest by polybasic sequences. We found that Hel2 and its E2 ubiquitin-conjugating enzyme Ubc4 play fundamental roles in the quality control systems induced by stalled ribosome on an aberrant mRNAs (NGD), by defective nascent peptides (RQC) and with deleterious mutations of decoding center (18S NRD). The results demonstrate that Hel2 and Ubc4 are required for three quality controls. We attempted to determine the substrates of Hel2-dependent ubiquitylation by using ribosome affinity purification system, and identified ribosomal proteins as ubiquitylation substrates of Hel2. We propose that Hel2 recognizes and ubiquitylates stalled ribosomes to induce quality control systems that degrade aberrant mRNAs and nascent peptides, as well as nonfunctional 18S rRNAs.