Catalyzes the covalent attachment of ubiquitin to other proteins so as to signal them for selective protein degradation. Involved in the formation of multiubiquitin chains. 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. Expressed in late sporogonial stages. Belongs to the ubiquitin-conjugating enzyme family.