Characterization of a cysteine-less human reduced folate carrier: localization of a substrate-binding domain by cysteine-scanning mutagenesis and cysteine accessibility methods

The human reduced folate carrier (hRFC) mediates the transport of reduced folates and classical anti-folates into mammalian cells. Whereas the functionally important domains in hRFC are poorly characterized. previous studies with anti-folate-resistant cells suggest critical roles for transmembrane domain (TMD) 1 and residues (Gly(44), GIu(45), Ser(46) and Ile(48)) in or flanking this region. An hRFC mutant devoid of cysteine residues was prepared by deleting the C-terminal 56 amino acids, including four cysteine residues, and mutagenizing the remaining cysteine residues to serine residues. A fully functional cysteine-less hRFC protein was expressed in transport-unpaired MtxRIIOua(R)2-4 Chinese-hamster ovary cells. To explore the role of residues in or flanking TMD1 in transport, all 24 amino acids from Trp(25) to Ile(48) of hRFC were mutated individually to cysteine residues, and the mutant hRFCs were transfected into MtxRIIOua(R)2-4 cells. All of the 24 cysteine mutants were expressed and, with the exception of R42C (Arg(42) --> Cys), were capable of mediating methotrexate uptake above the low level in MtxRIIOua(R)2-4 cells. We found that by treating the transfected cells with the small, water-soluble, thiol-reactive anionic reagent, sodium (2-sulphonatoethyl) methanethiosulphonate, methotrexate transport by several of the cysteine-substituted hRFC mutants was significantly inhibited, including Q40C, G44C, E45C and I48C. Sodium (2-sulphonatoethyl) methanethiosulphonate transport inhibition of the Q40C, G44C and I48C mutants was protected by leucovorin [(6R,S)-5-formyltetrahydrofolate], indicating that these residues lie at or near a substrate-binding site. Using surface-labelling reagents [N-biotinylaminoethyl methanethiosulphonate and 3-(N-maleimidyl-propionyl)biocytin, combined with 4-acetamido-4'-male-imidylstilbene-2,2'-disulphonic acid] with cysteine mutants from positions 37-48, the extracellular TMD1 boundary was found to lie between residues 39 and 40, and amino acids 44-46 and 48 were localized to the TMD1 exofacial loop. Collectively, our results imply that amino acids 40, 44, 48 and, possibly, 42 serve important roles in hRFC transport, albeit not as structural components of the putative transmembrane channel for folate substrates.