Has E3 ubiquitin-protein ligase activity. 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. Testis-specific. Expression begins in the testis at day 21 and increases dramatically from day 28 and thereafter. Auto-ubiquitinated.