Isolation of a human cDNA that complements a mutant hamster cell defective in methotrexate uptake

A clone has been isolated from a human lymphoblastic cDNA expression library that complements a mutant Chinese hamster cell defective in the uptake of the folate analogue methotrexate. When transfected with this clone the mutant cells regain the ability to transport the drug and, as a consequence, become sensitive to its cytotoxic action. The clone is 2863 base pairs long and has an open reading frame of 1770 base pairs that codes for a putative protein of 64 kDa. The putative protein has 51 and 50% identity at the amino acid level with the mouse and hamster functions, respectively, involved in the transport of reduced folates. Together these three proteins share 47% identity and have similar predicted structural features. The data are consistent with this human clone encoding either the reduced folate transporter or an auxiliary function that interacts with this transporter.