title: Role of the tryptophan residues in proton-coupled folate transporter (PCFT-SLC46A1) function.
abstract: The proton-coupled folate transporter (PCFT) mediates folate absorption across the brush-border membrane of the proximal small intestine and is required for folate transport across the choroid plexus into the cerebrospinal fluid. In this study, the functional role and accessibility of the seven PCFT Trp residues were assessed by the substituted-cysteine accessibility method. Six Trp residues at a lipid-aqueous interface tolerated Cys substitution in terms of protein stability and function. W85C, W202C, and W213C were accessible to N-biotinyl aminoethylmethanethiosulfonate; W48C and W299C were accessible only after treatment with dithiotreitol (DTT), consistent with modification of these residues by an endogenous thiol-reacting molecule and their extracellular location. Neither W107C nor W333C was accessible (even after DTT) consistent with their cytoplasmic orientation. Biotinylation was blocked by pemetrexed only for the W48C (after DTT), W85C, W202C residues. Function was impaired only for the W299C PCFT mutant located in the 4th external loop between the 7th and 8th transmembrane helices. Despite its aqueous location, function could only be fully preserved with Phe and, to a lesser extent, Ala substitutions. There was a 6.5-fold decrease in the pemetrexed influx Vmax and a 3.5- and 6-fold decrease in the influx Kt and Ki, respectively, for the W299S PCFT. The data indicate that the hydrophobicity of the W299 residue is important for function suggesting that during the transport cycle this residue interacts with the lipid membrane thereby impacting on the oscillation of the carrier and, indirectly, on the folate binding pocket.
authors.affiliation: Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York;
authors.forename: Mitra
authors.initials: M
authors.lastname: Najmi
authors.affiliation: Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York;
authors.forename: Rongbao
authors.initials: R
authors.lastname: Zhao
authors.affiliation: Department of Systems and Computational Biology; Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York.
authors.forename: Andras
authors.initials: A
authors.lastname: Fiser
authors.affiliation: Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York; Department of Medicine, Albert Einstein College of Medicine, Bronx, New York; i.david.goldman@einstein.yu.edu.
authors.forename: I David
authors.initials: ID
authors.lastname: Goldman
mesh_terms: D001665:Binding Sites; D019921:Biotinylation; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D003545:Cysteine; D005492:Folic Acid / Q000378:metabolism*; D005493:Folic Acid Antagonists / Q000494:pharmacology; D005838:Genotype; D006367:HeLa Cells; D006801:Humans; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D000068437:Pemetrexed / Q000494:pharmacology; D010641:Phenotype; D011485:Protein Binding; D000072756:Protein Conformation, alpha-Helical; D000072417:Protein Domains; D055550:Protein Stability; D058979:Proton-Coupled Folate Transporter / Q000737:chemistry / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014162:Transfection; D014364:Tryptophan
chemical_list: D005493:Folic Acid Antagonists; D058979:Proton-Coupled Folate Transporter; C505058:SLC46A1 protein, human; D000068437:Pemetrexed; D014364:Tryptophan; D005492:Folic Acid; D003545:Cysteine
keywords: HFM; PCFT; folate; hereditary folate malabsorption; methotrexate; pemetrexed; proton-coupled folate transporter; solute transporter; substituted cysteine accessibility assay