title: A P425R mutation of the proton-coupled folate transporter causing hereditary folate malabsorption produces a highly selective alteration in folate binding.
abstract: Proton-coupled folate transporter (PCFT) mediates folate intestinal absorption and transport across the choroid plexus, processes defective in subjects with hereditary folate malabsorption (HFM). PCFT is also widely expressed in human solid tumors where it contributes to the transport of pemetrexed and other antifolates. This study defines the basis for the functional changes due to a P425R mutation detected in a subject with HFM. Among various substitutions, only positively charged mutants (P425R and P425K) lost function but in a highly selective manner. Transport of reduced folates mediated by P425R-PCFT was virtually abolished; the methotrexate influx K(t) was increased fivefold (from 2 to 10 μM). In contrast, the pemetrexed influx K(t) mediated by P425R-PCFT was decreased 30% compared with wild-type (WT)-PCFT. Methotrexate inhibition of pemetrexed influx was competitive with a K(i) for WT-PCFT comparable to its influx K(t). However, the methotrexate influx K(i) for P425R-PCFT was ∼15-fold higher than the WT-PCFT influx K(t) and threefold higher than the methotrexate influx K(t) for the P425R-PCFT mutant. The confirmed secondary structure and homology modeling place the P425 residue at the junction of the 6th external loop and 12th transmembrane domain, remote from the aqueous translocation pathway, a prediction confirmed by the failure to label P425C-PCFT with N-biotinylaminoethyl methanethiosulfonate-biotin and the absence of inhibition of P425C-PCFT function by water-soluble sulfhydryl reagents. Hence, despite its location, the P425R-PCFT mutation produces a conformational change that fully preserves pemetrexed binding but markedly impairs binding of methotrexate and other folates to the carrier.
authors.affiliation: Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
authors.forename: Daniel Sanghoon
authors.initials: DS
authors.lastname: Shin
authors.forename: Rongbao
authors.initials: R
authors.lastname: Zhao
authors.forename: Enghui H
authors.initials: EH
authors.lastname: Yap
authors.forename: Andras
authors.initials: A
authors.lastname: Fiser
authors.forename: I David
authors.initials: ID
authors.lastname: Goldman
mesh_terms: D000970:Antineoplastic Agents / Q000737:chemistry / Q000493:pharmacokinetics; D015153:Blotting, Western; D002478:Cells, Cultured; D005492:Folic Acid / Q000378:metabolism*; D005971:Glutamates / Q000737:chemistry / Q000493:pharmacokinetics; D006147:Guanine / Q000031:analogs & derivatives / Q000737:chemistry / Q000493:pharmacokinetics; D006801:Humans; D047468:Immunoprecipitation; D008286:Malabsorption Syndromes / Q000235:genetics* / Q000378:metabolism; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D000068437:Pemetrexed; D011485:Protein Binding; D017433:Protein Structure, Secondary; D058979:Proton-Coupled Folate Transporter / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D014162:Transfection
chemical_list: D000970:Antineoplastic Agents; D005971:Glutamates; D058979:Proton-Coupled Folate Transporter; C505058:SLC46A1 protein, human; D000068437:Pemetrexed; D006147:Guanine; D005492:Folic Acid