Protein kinase A controls the hexosamine pathway by tuning the feedback inhibition of GFAT-1
The hexosamine pathway (HP) is a key anabolic pathway whose product uridine 5-diphospho-N-acetyl-D-glucosamine (UDP-GlcNAc) is an essential precursor for all glycosylation processes in mammals. It modulates the ER stress response, is implicated in cancer and diabetes, and HP activation extends lifespan in Caenorhabditis elegans. The highly conserved glutamine fructose-6-phosphate amidotransferase 1 (GFAT-1) is the first and rate-limiting HP enzyme. GFAT-1 activity is modulated through UDP-GlcNAc feedback inhibition and by kinase signaling, including Ser205 phosphorylation by protein kinase A (PKA). The consequence and molecular mechanism of GFAT-1 phosphorylation, however, remains poorly understood. Here, we identify the GFAT-1 R203H substitution that elevates UDP-GlcNAc levels in C. elegans, leading to ER stress resistance. In human GFAT-1, the R203H substitution interfered with both UDP-GlcNAc inhibition and PKA-mediated Ser205 phosphorylation. Of note, Ser205 phosphorylation had two discernible effects: It lowered baseline GFAT-1 activity and abolished UDP-GlcNAc feedback inhibition. Thus, GFAT-1 phosphorylation by PKA uncoupled the feedback loop of the HP and, depending on UDP-GlcNAc availability, phosphorylation by PKA results in lower or enhanced GFAT-1 activity in vivo. Mechanistically, our data indicate that the relative positioning of the two GFAT-1 domains might be affected by phosphorylation and we propose a model how Ser205 phosphorylation modulates the activity and feedback inhibition of GFAT-1.