The reactions of folate metabolism are distributed between the cytosolic and mitochondrial compartments of mammalian cells and both components are essential for mammalian cell survival. The enzyme folylpoly-γ-glutamate synthetase (FPGS) has been shown to be essential for the intracellular retention of folates as polyglutamate derivatives. The fpgs gene produces two transcripts, one for an isoform with a leader peptide targeting it to the mitochondrial matrix, and the other is transcribed without this sequence, so that the encoded isoform remains in the cytosol. Transfer of folates into the mitochondrial matrix is mediated by an inner mitochondrial membrane transport protein, the mitochondrial folate transporter (MFT). Either deletion of the MFT or of mitochondrial FPGS is cytotoxic and renders the cell auxotrophic for glycine; the mechanism of this effect is far from apparent. To better understand this compartmentalization, we generated cell lines that express only the cytosolic or mitochondrial isoform of FPGS under tetracycline-inducible control in FPGS-null AuxB1 cells. Western blot analysis of the FPGS protein in these cell lines indicate that both FPGS isoforms are expressed near endogenous levels in the presence of doxycycline and that each cell line solely expresses the FPGS isoform in the respective compartment. Clonogenic survival of each doxycycline-induced cell line demonstrates a cellular requirement for each FPGS isoform. Additional studies showed that cells complemented with cytosolic FPGS only accumulated and retained folates in the cytosol but not mitochondria, leading us to conclude that folate polyglutamates are not substrates for the MFT. In addition, expression of only the mitochondrial form of FPGS resulted in cells that progressively trapped folates in the mitochondria, leading us to the concept that folate polyglutamates do not escape the mitochondria. However, cytosolic folates were observed in cells expressing only mitochondrial FPGS. We are currently using these cell lines to study the transfer of folates across the mitochondrial membrane and polyglutamation in each compartment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 79.