E3 ubiquitin-protein ligase that mediates polyubiquitination of CD4. E3 ubiquitin ligases accept ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfer the ubiquitin to targeted substrates. May play a role in ubuquitin-dependent protein sorting in developmenting spermatids. 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 (YXXL motif) with AP1M1. Interacts (via PDZ-binding motif) with LIN7A. Interacts with unidentified fucose glycoproteins. Predominantly expressed in testis. Present in early developing spermatids. Not present in spermatogonia, spermatocytes or somatic cells (i.e. peritubular, Leydig, and Sertoli cells). Present in early round spermatids at step 4, remains until step 11, then it decreases at steps 12-15, and diminishes after step 16 (at protein level). Also expressed at lower level in brain. The RING-CH-type zinc finger domain is required for E3 ligase activity.