Accepts ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins. In vitro, in the presence or in the absence of BRCA1-BARD1 E3 ubiquitin-protein ligase complex, catalyzes the synthesis of 'Lys-48'-linked polyubiquitin chains. Does not transfer ubiquitin directly to but elongates monoubiquitinated substrate protein. Mediates the selective degradation of short-lived and abnormal proteins, such as the endoplasmic reticulum-associated degradation (ERAD) of misfolded lumenal proteins. Ubiquitinates huntingtin. May mediate foam cell formation by the suppression of apoptosis of lipid-bearing macrophages through ubiquitination and subsequence degradation of p53/TP53. Proposed to be involved in ubiquitination and proteolytic processing of NF-kappa-B; in vitro supports ubiquitination of NFKB1. Involved in stabilization of CASP12 during ER stress-mediated amyloid-beta neurotoxicity probably by inhibiting proteasome activity; in vitro ubiquitinates CASP12. 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. Interacts with RNF138/NARF. Interacts with BRCA1. Expressed in the brain, with highest levels found in the mitral cells of the olfactory bulb, the pyramidal cell layer of the hippocampus and the Purkinje cells of the cerebellar cortex. Expressed at all stages of brain development and increases significantly between postnatal days 7 and 14. Sumoylation at Lys-14 impairs catalytic activity. Neurons are resistant to amyloid-beta neurotoxicity. Significantly lower CASP12 expression in brain. Belongs to the ubiquitin-conjugating enzyme family.