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

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.