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. May be mitochondrial in the R2C Leydig cell line. In testis sections, expressed in interstitial tissue and seminiferous tubules. In tubules, expression is mainly in postmeiotic germ cells and to a much lesser extent in Sertoli cells (at protein level). Expressed at high levels in liver, lung, stomach, heart and thymus. Regulated by lutenising hormone (LH) in Leydig cells but not in germ cells.