Catalyzes the covalent attachment of ubiquitin to other proteins. 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. Mutants in this gene exhibit several, largely neuronal defects including lesions affecting the neuronal connectivity of the giant fiber with the 'jumping muscle', and the axons of photoreceptor cells R7 and R8 fail to make the proper right-angle turn into the medulla (hence the term 'bendless'). Belongs to the ubiquitin-conjugating enzyme family.