CTP synthetase and its role in phospholipid synthesis in the yeast Saccharomyces cerevisiae.

CTP synthetase is a cytosolic-associated glutamine amidotransferase enzyme that catalyzes the ATP-dependent transfer of the amide nitrogen from glutamine to the C-4 position of UTP to form CTP. In the yeast Saccharomyces cerevisiae, the reaction product CTP is an essential precursor of all membrane phospholipids that are synthesized via the Kennedy (CDP-choline and CDP-ethanolamine branches) and CDP-diacylglycerol pathways. The URA7 and URA8 genes encode CTP synthetase in S. cerevisiae, and the URA7 gene is responsible for the majority of CTP synthesized in vivo. The CTP synthetase enzymes are allosterically regulated by CTP product inhibition. Mutations that alleviate this regulation result in an elevated cellular level of CTP and an increase in phospholipid synthesis via the Kennedy pathway. The URA7-encoded enzyme is phosphorylated by protein kinases A and C, and these phosphorylations stimulate CTP synthetase activity and increase cellular CTP levels and the utilization of the Kennedy pathway. The CTPS1 and CTPS2 genes that encode human CTP synthetase enzymes are functionally expressed in S. cerevisiae, and rescue the lethal phenotype of the ura7Deltaura8Delta double mutant that lacks CTP synthetase activity. The expression in yeast has revealed that the human CTPS1-encoded enzyme is also phosphorylated and regulated by protein kinases A and C.