Catalytic subunit of the KPC complex that acts as E3 ubiquitin-protein ligase (PubMed:15531880, PubMed:16227581, PubMed:25860612). Promotes the ubiquitination and proteasome-mediated degradation of CDKN1B which is the cyclin-dependent kinase inhibitor at the G0-G1 transition of the cell cycle (PubMed:15531880, PubMed:16227581). Also acts as a key regulator of the NF-kappa-B signaling by promoting maturation of the NFKB1 component of NF-kappa-B: acts by catalyzing ubiquitination of the NFKB1 p105 precursor, leading to limited proteasomal degradation of NFKB1 p105 and generation of the active NFKB1 p50 subunit (PubMed:25860612, PubMed:33168738, PubMed:34873064). Also functions as an inhibitor of innate antiviral signaling mediated by RIGI and IFIH1 independently of its E3 ligase activity (PubMed:27312109). Interacts with the N-terminal CARD domains of RIGI and IFIH1 and competes with the downstream adapter MAVS (PubMed:27312109). {ECO:0000269|PubMed:15531880, ECO:0000269|PubMed:16227581, ECO:0000269|PubMed:25860612, ECO:0000269|PubMed:27312109, ECO:0000269|PubMed:33168738, ECO:0000269|PubMed:34873064}. This is the function of RNF123 (ring finger protein 123, ENSG00000164068).