E3 ubiquitin-protein ligase which accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates. Component of a RSP5 ubiquitin ligase complex which specifies polyubiquitination and intracellular trafficking of the general amino acid permease GAP1 as well as other cell surface proteins like GAP1, FUR4, MAL61, PMA1 and STE2. The RSP5-BUL1/2 complex is also necessary for the heat-shock element (HSE)-mediated gene expression, nitrogen starvation GLN3-dependent transcription, pressure-induced differential regulation of the two tryptophan permeases TAT1 and TAT2 and sorting efficiency into multivesicular bodies. Also acts on RBP1. Plays a role in tolerance to o-dinitrobenzene. Involved in actin cytoskeleton organization and dynamics. Ubiquitinates the LAS17-binding proteins LSB1 and PIN3/LSB2 without directing them for degradation and affects LAS17 levels in a SLA1-dependent and LSB1/2-independent manner. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Component of the RSP5-BUL1/2 ubiquitin ligase complex composed of at least RSP5 and BUL1 or BUL2. Forms also a ternary complex with RUP1 and UBP2. Interacts (via WW domains) with LSB1, PIN3/LSB2 and RCR1 (via PY motifs). Interacts with HSE1, LAS17, ROG3, ROD1 and RVS167. The ubiquitination appears to be the result of an intramolecular transfer of ubiquitin. A cysteine residue is required for ubiquitin-thioester formation. Belongs to the RSP5/NEDD4 family.