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A more recent version of this article appeared on June 1, 2005
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Submitted on October 28, 2004
Revised on March 21, 2005
Accepted on March 23, 2005
Institut für Mikrobiologie, Heinrich-Heine-Universität Düsseldorf, D-40225 Düsseldorf, Germany
Monitoring Editor: Sandra Schmid
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 wildtype 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, since recycling was not blocked in class E vps (vacuolar protein sorting) mutants (
vps4,
vps27), which are affected in late endosome function and in the retromer mutant
vps35. Instead, recycling was partially affected in the sorting nexin mutant
snx4, which serves as an indication that Ste6 recycles through early endosomes. Enhanced recycling of wildtype Ste6 was observed in class D vps mutants (
pep12,
vps8 and
vps21). 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.
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