Non-catalytic subunit of the KPC complex that acts as E3 ubiquitin-protein ligase. Required for poly-ubiquitination and proteasome-mediated degradation of CDKN1B during G1 phase of the cell cycle (By similarity). Protein modification; protein ubiquitination. Component of the KPC complex composed of RNF123/KPC1 and UBAC1/KPC2. Interacts with RNF123 via its N-terminal domain. Interacts with the proteasome via its N-terminal domain (By similarity).