Phosphorylation of the Aly3 C-terminus impedes aberrant endocytosis of <i>Schizosaccharomyces pombe</i> hexose transporter Ght5

In the 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 the α-arrestin Aly3-dependent endocytosis of Ght5, we hypothesized that phosphorylation inhibits this endocytosis. To identify phosphorylation sites required for proliferation in low glucose, putatively phosphorylated serine/threonine residues of Aly3 and Ght5 were replaced with alanine, showing that C-terminal serine residues of Aly3, but not Ght5, are necessary for proliferation in low glucose. Expression of Aly3 that could not be phosphorylated at the C-terminus led to increased ubiquitylation and vacuolar accumulation of Ght5 in low glucose, but reversion of one of the alanine residues to serine reversed those defects. 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 the mechanisms by which Aly3 is regulated so that fission yeast can adapt to nutritional stress.