Degradation of the hexose transporter Hxt5p in Saccharomyces cerevisiae.
BACKGROUND
Hxt5p is a member of a multigene family of hexose transporter proteins which translocate glucose across the plasma membrane of the yeast Saccharomyces cerevisiae. In contrast with other major hexose transporters of this family, Hxt5p expression is regulated by the growth rate of the cells and not by the external glucose concentration. Furthermore, Hxt5p is the only glucose transporter expressed during stationary phase. These observations suggest a different role for Hxt5p in S. cerevisiae. Therefore we studied the metabolism and localization of Hxt5p in more detail.


CONCLUSIONS
Inhibition of HXT5 expression in stationary-phase cells by the addition of glucose, which increases the growth rate, led to a decrease in the amount of Hxt5 protein within a few hours. Addition of glucose to stationary-phase cells resulted in a transient phosphorylation of Hxt5p on serine residues, but no ubiquitination was detected. The decrease in Hxt5p levels is caused by internalization of the protein, as observed by immunofluorescence microscopy. In stationary-phase cells, Hxt5p was localized predominantly at the cell periphery and upon addition of glucose to the cells the protein translocated to the cell interior. Electron microscopy demonstrated that the internalized Hxt5p-HA (haemagglutinin) protein was localized to small vesicles, multivesicular bodies and the vacuole. These results suggest that internalization and degradation of Hxt5p in the vacuole occur in an ubiquitination-independent manner via the endocytic pathway.
van Suylekom
Denise
D
Cellular Architecture and Dynamics, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
van Donselaar
Elly
E
Blanchetot
Christophe
C
Do Ngoc
Lan Nguyen
LN
Humbel
Bruno M
BM
Boonstra
Johannes
J
D009004:Monosaccharide Transport Proteins / Q000378:metabolism* / Q000648:ultrastructure; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D011993:Recombinant Fusion Proteins / Q000378:metabolism / Q000648:ultrastructure; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000254:growth & development / Q000378:metabolism* / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism* / Q000648:ultrastructure; D025801:Ubiquitin / Q000378:metabolism
C466910:HXT5 protein, S cerevisiae; D009004:Monosaccharide Transport Proteins; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin