Phosphorylation of Aly3 C-terminus impedes aberrant endocytosis of S. pombe hexose transporter Ght5

In fission yeast, Schizosaccharomyces pombe, transcriptional upregulation and cell-surface localization of the hexose transporter, Ght5, are required for cell proliferation in low glucose. As the target of rapamycin complex 2 (TORC2) signaling pathway inhibits -arrestin Aly3-dependent endocytosis of Ght5, we hypothesized that this endocytosis was inhibited by phosphorylation. To identify phosphorylation sites required for cell proliferation in low glucose, serine and threonine residues of Aly3 and Ght5 reportedly phosphorylated were replaced with alanine. We found that C-terminal serine residues of Aly3, but not Ght5, are necessary for proliferation in low glucose. Expression of Aly3 protein unphosphorylated at the C-terminus led to increased ubiquitination and vacuolar accumulation of Ght5 in low glucose, but reversion of one of the alanine residues to serine reduced ubiquitination and vacuolar accumulation of Ght5. Also, Aly3 physically interacted with the HECT-type ubiquitin ligases Pub1 and Pub3, and these interactions were required for surface localization of Ght5 and proliferation in low glucose. This study reveals mechanisms by which Aly3 is regulated so that fission yeast can adapt to nutritional stress.