Catalyzes the covalent attachment of ubiquitin to other proteins. Regulates pronephros development, possibly by promoting ubiquitination and thus inactivation or degradation of map3k10/mlk2. 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 map3k10/mlk2. At embryonic stages 28 to 35, expressed in the somites, eye primordia, otic vesicle and branchial arches. By stage 35, also weakly expressed in the pronephros. Expressed maternally. Present at near constant levels throughout early development. Belongs to the ubiquitin-conjugating enzyme family.