ISOLATION OF A HAMSTER CDNA CLONE CODING FOR A FUNCTION INVOLVED IN METHOTREXATE UPTAKE

A clone has been isolated from a Chinese hamster ovary cell cDNA expre ssion library that complements mutant cells defective in the uptake of the folate analogue methotrexate. When transfected with this clone, t he mutant cells regain the ability to bind and transport the drug and, as a consequence, become sensitive to its cytotoxic action. The clone is 2314 base pairs long and has an open reading frame of 1557 base pa irs that codes for a putative protein of 58 kDa. This novel putative p rotein has a high content of hydrophobic residues and has a large part of its predicted secondary structure in the form of P-sheets. In the wild-type cell Line and in one of the mutant cell lines, this clone de tects an mRNA of 2.5 kilobases, while in another mutant cell line, thi s message is absent. The data are consistent with this clone encoding either the reduced folate transporter or an auxiliary function that in teracts with this transporter. This is the first report of a cDNA codi ng for a function, other than folate-binding protein, that is involved in the transport of methotrexate.