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. The RING-CH-type zinc finger domain is required for E3 ligase activity.