Accepts ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins. In vitro catalyzes 'Lys-48'-linked polyubiquitination. Involved in endoplasmic reticulum-associated degradation (ERAD). Required for sterol-induced ubiquitination of 3-hydroxy-3-methylglutaryl coenzyme A reductase and its subsequent proteasomal degradation. 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 AUP1 (via C-terminus); the interaction recruits UBE2G2 to lipid droplets. Interacts with ubiquitin ligases AMFR/gp78 and RNF139/TRC8; recruitment to lipid droplets by AUP1 facilitates interaction of UBE2G2 with AMFR and RNF139, leading to sterol-induced ubiquitination of 3-hydroxy-3-methylglutaryl coenzyme A reductase and its subsequent proteasomal degradation. Belongs to the ubiquitin-conjugating enzyme family.