Catalyzes the covalent attachment of ubiquitin to other proteins (By similarity). Involved in female fertility and embryo implantation (PubMed:23108111). May be involved in hormonal homeostasis in females (PubMed:23108111). Involved in regulation of B4GALT1 cell surface expression, B4GALT1-mediated cell adhesion to laminin and embryoid body formation (PubMed:18511602). 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. Monomer and homodimer. Only the homodimer is linked to ubiquitin through thiolester activation. Interacts (via N-terminus) with B4GALT1 (via N-terminal cytoplasmic domain); the interaction is direct. Expressed in liver, brain, heart, spleen, lung, kidney, muscle, ovary, epididymis, testis and placenta (PubMed:23108111). Also expressed in thymus and ES cells (PubMed:18511602). Only expressed in the uterus during pregnancy (PubMed:23108111). Expressed in oocytes and during subsequent embryonic development stages (4-cell stage, blastocyst, 8.5 dpc, 13.5 dpc, 16.5 dpc and 18.5 dpc) (PubMed:23108111). Autoubiquitinated in vitro in the presence of NEDD4L. No reproductive defects in males but females have reduced litter size due to embryo implantation failure, normal ovulation but a longer estrus cycle, reduced levels of serum prolactin at the beginning of lactation, abnormal sexual behavior and show reduced offspring care. Belongs to the ubiquitin-conjugating enzyme family. Truncated N-terminus. Truncated N-terminus.