Folylpoly-γ-glutamate synthetase: A key determinant of folate homeostasis and antifolate resistance in cancer.
Wild-type Chinese hamster ovary (CHO) cells and CHO cell transfectants expressing human folylpoly-gamma-glutamate synthetase (FPGS) activity contain mitochondrial FPGS activity of higher specific activity than the cytosolic isozyme. Expression of mitochondrial FPGS activity is required for folate accumulation by mitochondria. The mitochondrial folate pool in CHO cells is not in equilibrium with the cytosolic pool and contains folylpolyglutamates of longer glutamate chain length than cytosolic folates. The inability of AUX-coli, a CHO cell expressing high levels of Escherichia coli FPGS activity and containing pteroyltriglutamate, to support glycine synthesis is due to a lack of mitochondrial FPGS activity. AUX-coli cells lack mitochondrial folate despite containing high levels of cytosolic folate.