E3 ubiquitin-protein ligase that mediates ubiquitination and subsequent proteasomal degradation of NUMB. E3 ubiquitin ligases accept ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates. Mediates ubiquitination of isoform p66 and isoform p72 of NUMB, but not that of isoform p71 or isoform p65. Isoform 2 provides an endocytic scaffold for IGSF5/JAM4. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Interacts with CXADR. Interacts with MAGEB18 and MAGEF1 (By similarity). Interacts with the phosphotyrosine interaction domain of all isoforms of NUMB. IGSF5/JAM4 interacts with isoform 2 through the second PDZ domain, other isoforms may also interact with IGSF5/JAM4. Isoform 1 and isoform 2 are expressed in the heart. Isoform 1 is also expressed in kidney, lung and skeletal muscle while isoform 2 is also expressed in brain. The NPXY motif is required for the interaction with the PID domain of NUMB. It is however not sufficient. The PDZ 1 domain participates in the interaction with the PID domain of NUMB, and participates in the isoform-specific ubiquitination of NUMB. The PDZ 2 domain of isoform 2 participates in the interaction with IGSF5/JAM4, other isoforms containing this domain may also interact with IGSF5/JAM4.