Mimicry of Canonical Translation Elongation Underlies Alanine Tail Synthesis in RQC

Aborted translation produces large ribosomal subunits obstructed with tRNA-linked nascent chains, which are substrates of ribosome-associated quality control (RQC). Bacterial RqcH, a widely conserved RQC factor, senses the obstruction and recruits tRNA(Ala(UGC)) to modify nascent-chain C termini with a polyalanine degron. However, how RqcH and its eukaryotic homologs (Rqc2 and NEMF), despite their relatively simple architecture, synthesize such C-terminal tails in the absence of a small ribosomal subunit and mRNA has remained unknown. Here, we present cryoelectron microscopy (cryo-EM) structures of Bacillus subtilis RQC complexes representing different Ala tail synthesis steps. The structures explain how tRNA(Ala) is selected via anticodon reading during recruitment to the A-site and uncover striking hinge-like movements in RqcH leading tRNA(Ala) into a hybrid A/P-state associated with peptidyl-transfer. Finally, we provide structural, biochemical, and molecular genetic evidence identifying the Hsp15 homolog (encoded by rqcP) as a novel RQC component that completes the cycle by stabilizing the P-site tRNA conformation. Ala tailing thus follows mechanistic principles surprisingly similar to canonical translation elongation.

Automated summary

The study found that aborted translation leads to large ribosomal subunits obstructed with tRNA-linked nascent chains, which are substrates of ribosome-associated quality control. Bacterial RqcH, a widely conserved RQC factor, can modify the C-termini of nascent chains to form a polyalanine degron. The research revealed the mechanism of tRNA selection and the role of RqcH in the synthesis of nascent chains.