Jlp2 is an RQC complex-independent release factor acting on aberrant peptidyl-tRNA, protecting cells against translation elongation stress

Summary Ribosome stalling results in aberrant nascent peptides attached to large ribosomal subunits after ribosome splitting. Proteasomal peptide degradation is canonically initiated by the Ribosome-associated Quality Control (RQC) complex, including K48-linked polyubiquitination by Ltn1 and Rqc1, and addition of non-templated ‘CAT tails’ by Rqc2 facilitates ubiquitination and promotes CAT tail-mediated degradation. Ubiquitinated peptides are released from tRNA by Vms1. Ubiquitination-independent processing of aberrant nascent peptides remains poorly understood. Here, we identify budding yeast Jlp2 as a factor in an RQC complex-independent mechanism that counteracts excessive CAT tailing and facilitates peptide release from the tRNA in the absence of Ltn1-mediated K48-linked ubiquitination. We characterize its ribosomal binding behavior and substrate preferences and identify residues in Jlp2 that are crucial for its function. We propose that Jlp2-mediated peptide release acts as a fail-safe for situations of RQC failure, or insufficiency, and for substrates without accessible ubiquitination-target lysines, to protect cells during ribosome stalling. Competing Interest Statement The authors have declared no competing interest.