Acts as an E3 ubiquitin-protein ligase (By similarity). May have a role during the programmed cell death of hematopoietic cells. 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. Expression is highest in liver, with lesser amounts in the lung, spleen, brain, heart, kidney and testis. Up-regulated in response to interleukin-3 (IL-3) withdrawal-induced apoptosis of 32Dcl3 cells (derived from bone marrow).