Figure 2. A hypothetical model of the BOR1 trafficking pathway. Under low B conditions, the polar localization of BOR1 to the inner PM domain is maintained by polar recycling from the trans­Golgi network/early endosome (TGN/EE) and the pathway is dependent on the tyrosine-based signals. Relatively slow diffusion in the PM⁴ also seems to be important for the polar localization. Under high B conditions, BOR1 is transported to a sub-compartment of the TGN/EE, which becomes a multivesicular body/late endosome (MVB/LE). This pathway also requires the tyrosine-based signals. BOR1 is ubiquitinated under high B conditions and the ubiquitination accelerates BOR1 sorting into the intraluminal vesicles of an MVB. The MVB fuses with vacuoles and the intraluminal vesicles containing BOR1 are released into the vacuolar lumen.