Control of Ste6 recycling by ubiquitination in the early endocytic pathway in yeast

We present evidence that ubiquitination controls sorting of the ABC-transporter Ste6 in the early endocytic pathway. The intracellular distribution of Ste6 variants with reduced ubiquitination was examined. In contrast to wild-type Ste6, which was mainly localized to internal structures, these variants accumulated at the cell surface in a polar manner. When endocytic recycling was blocked by Ypt6 inactivation, the ubiquitination deficient variants were trapped inside the cell. This indicates that the polar distribution is maintained dynamically through endocytic recycling and localized exocytosis ("kinetic polarization"). Ste6 does not appear to recycle through late endosomes, because recycling was not blocked in class E vps (vacuolar protein sorting) mutants (Deltavps4, Deltavps27), which are affected in late endosome function and in the retromer mutant Deltavps35. Instead, recycling was partially affected in the sorting nexin mutant Deltasnx4, which serves as an indication that Ste6 recycles through early endosomes. Enhanced recycling of wild-type Ste6 was observed in class D vps mutants (Deltapep12, Deltavps8, and Deltavps21). The identification of putative recycling signals in Ste6 suggests that recycling is a signal-mediated process. Endocytic recycling and localized exocytosis could be important for Ste6 polarization during the mating process.