string: Targeting of proteins in the biosynthetic pathway can also be influenced by ubiquitylation, especially at the TGN. Monoubiquitylation of the yeast permease Gap1 results in transport to the plasma membrane, whereas polyubiquitylation targets it to the vacuole, a process regulated by the ubiquitin ligase Rsp5 and the ubiquitin ligase adaptors BUL1 and BUL2 (Helliwell et al., 2001).