E3 ubiquitin-protein ligase that mediates ubiquitination of target proteins, including p53/TP53, TP73, HDAC1 and CDKN1B (PubMed:16914734, PubMed:17721809, PubMed:18006823, PubMed:19043414, PubMed:19483087, PubMed:21994467). Mediates ubiquitination and degradation of p53/TP53; preferentially acts on tetrameric p53/TP53 (PubMed:19043414, PubMed:19483087). Catalyzes monoubiquitinates the translesion DNA polymerase POLH (PubMed:21791603). Involved in the ribosome-associated quality control (RQC) pathway, which mediates the extraction of incompletely synthesized nascent chains from stalled ribosomes: RCHY1 acts downstream of NEMF and recognizes CAT tails associated with stalled nascent chains, leading to their ubiquitination and degradation (PubMed:33909987). {ECO:0000269|PubMed:16914734, ECO:0000269|PubMed:17721809, ECO:0000269|PubMed:18006823, ECO:0000269|PubMed:19043414, ECO:0000269|PubMed:19483087, ECO:0000269|PubMed:21791603, ECO:0000269|PubMed:21994467, ECO:0000269|PubMed:33909987}. [Isoform 4]: Has no E3 ubiquitin-protein ligase activity. . This is the function of RCHY1 (ring finger and CHY zinc finger domain containing 1, Ensembl gene identifier ENSG00000163743).