Component of a RSP5 ubiquitin ligase complex which specifies polyubiquitination and intracellular trafficking of the general amino acid permease GAP1 as well as other permeases such as PMA1. The RSP5-BUL1/2 complex is also necessary for the heat-shock element (HSE)-mediated gene expression, nitrogen starvation GLN3-dependent transcription and pressure-induced differential regulation of the 2 tryptophan permeases TAT1 and TAT2. Protein modification; protein ubiquitination. Component of the RSP5-BUL1/2 ubiquitin ligase complex composed of at least RSP5 and BUL1 or BUL2. The PY-motif is required for the interaction with RSP5 ubiquitin-ligase and the HSE-mediated gene expression. Present with 339 molecules/cell in log phase SD medium. Belongs to the BUL1 family.