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 the phosphotyrosine interaction domain of all isoforms of NUMB (By similarity). Interacts with MAGEB18 and MAGEF1. Interacts with the Coxsackievirus and adenovirus receptor CXADR. Interacts with endogenous retrovirus K protein Np9. IGSF5/JAM4 interacts with isoform 2 through the second PDZ domain, other isoforms may also interact with IGSF5/JAM4 (By similarity). Expressed in heart, placenta, kidney, pancreas and 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 (By similarity).