Accepts ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins. In vitro catalyzes 'Lys-48'-linked polyubiquitination. S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Detected at embryo implantation sites in the luminal epithelium of pregnant endometrium. Detected at low levels in ovary and liver. Up-regulated in pregnant endometrium during implantation. Auto-ubiquitinated in vitro. Belongs to the ubiquitin-conjugating enzyme family.