E3 ubiquitin-protein ligase component of the ribosome quality control complex (RQC), a ribosome-associated complex that mediates ubiquitination and extraction of incompletely synthesized nascent chains for proteasomal degradation (PubMed:23178123). Mediates ubiquitination of proteins derived from mRNAs lacking stop codons (non-stop proteins) and other translation arrest products induced by poly-lysine sequences and tandem rare codons. Ubiquitination leads to CDC48 recruitment for extraction and degradation of the incomplete translation product (PubMed:20835226, PubMed:23825054, PubMed:24261871). May indirectly play a role in chromatin function and transcription (PubMed:17283062). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Component of the ribosome quality control complex (RQC), composed of the E3 ubiquitin ligase RKR1/LTN1, RQC1 and RQC2, as well as CDC48 and its ubiquitin-binding cofactors associated with the 60S ribosomal subunits (PubMed:23178123, PubMed:23479637, PubMed:25349383). Present with 222 molecules/cell in log phase SD medium. Belongs to the LTN1 family.