Catalytic subunit of the KPC complex that acts as E3 ubiquitin-protein ligase. 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. 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. Functions also as an inhibitor of innate antiviral signaling mediated by RIGI and IFIH1 independently of its E3 ligase activity. Interacts with the N-terminal CARD domains of RIGI and IFIH1 and competes with the downstream adapter MAVS