Role of the tryptophan residues in proton-coupled folate transporter (PCFT-SLC46A1) function

Am J Physiol Cell Physiol. 2016 Jul 1;311(1):C150-7. doi: 10.1152/ajpcell.00084.2016. Epub 2016 Jun 1.

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.

Keywords: HFM; PCFT; folate; hereditary folate malabsorption; methotrexate; pemetrexed; proton-coupled folate transporter; solute transporter; substituted cysteine accessibility assay.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Binding Sites
  • Biotinylation
  • Cell Membrane / drug effects
  • Cell Membrane / metabolism*
  • Cysteine
  • Folic Acid / metabolism*
  • Folic Acid Antagonists / pharmacology
  • Genotype
  • HeLa Cells
  • Humans
  • Hydrogen-Ion Concentration
  • Hydrophobic and Hydrophilic Interactions
  • Kinetics
  • Mutagenesis, Site-Directed
  • Mutation
  • Pemetrexed / pharmacology
  • Phenotype
  • Protein Binding
  • Protein Conformation, alpha-Helical
  • Protein Domains
  • Protein Stability
  • Proton-Coupled Folate Transporter / chemistry
  • Proton-Coupled Folate Transporter / drug effects
  • Proton-Coupled Folate Transporter / genetics
  • Proton-Coupled Folate Transporter / metabolism*
  • Structure-Activity Relationship
  • Transfection
  • Tryptophan

Substances

  • Folic Acid Antagonists
  • Proton-Coupled Folate Transporter
  • SLC46A1 protein, human
  • Pemetrexed
  • Tryptophan
  • Folic Acid
  • Cysteine