Convergence of mammalian RQC and C-end rule proteolytic pathways via alanine tailing

Incompletely-synthesized nascent-chains obstructing large ribosomal subunits are targeted for degradation by Ribosome-associated Quality Control (RQC). In bacterial RQC, RqcH marks the nascent-chains with C-terminal alanine (Ala) tails that are directly recognized by proteasome-like proteases, whereas in eukaryotes, RqcH orthologs (Rqc2/NEMF) assist the Ltn1/Listerin E3 ligase in nascent-chain ubiquitylation. Here we study RQC-mediated proteolytic targeting of ribosome stalling products in mammalian cells. We show that mammalian NEMF has an additional, Listerin-independent proteolytic role which, as in bacteria, is mediated by tRNA-Ala binding and Ala tailing. However, in mammalian cells Ala tails signal proteolysis indirectly, through a pathway that recognizes C-terminal degrons. We identify the CRL2KLHDC10 E3 ligase complex and the novel C-end rule E3 ligase, Pirh2/Rchy1, as bona-fide RQC pathway components that directly bind to Ala-tailed ribosome stalling products and target them for degradation. As Listerin mutation causes neurodegeneration in mice, functionally-redundantly E3s may likewise be implicated in molecular mechanisms of neurodegeneration.