RBCK1/HBV associated factor 4

Acts as an E3 ubiquitin-protein ligase, or as part of an E3 complex, which accepts ubiquitin from specific E2 ubiquitin-conjugating enzymes, such as UBE2L3/UBCM4, and then transfers it to substrates. Functions as an E3 ligase for oxidized IREB2 and both heme and oxygen are necessary for IREB2 ubiquitination. Promotes ubiquitination of TAB2 and IRF3 and their degradation by the proteoasome. Component of the LUBAC complex which conjugates linear polyubiquitin chains in a head-to-tail manner to substrates. LUBAC conjugates linear polyubiquitin to IKBKG at 'Lys-285' and 'Lys-309' and is involved in activation of the canonical NF-kappa-B and the JNK signaling pathways. LUBAC is proposed to be recruited to the TNF-R1 signaling complex (TNF-RSC) following polyubiquitination of TNF-RSC components by BIRC2 and/or BIRC3 and to conjugate linear polyubiquitin to IKBKG and possibly other components contributing to the stability of the complex. Binds polyubiquitin of different linkage types.