title: Identification and functional impact of homo-oligomers of the human proton-coupled folate transporter.
abstract: The proton-coupled folate transporter (PCFT; SLC46A1) is a proton-folate symporter that is abundantly expressed in solid tumors and normal tissues, such as duodenum. The acidic pH optimum for PCFT is relevant to intestinal absorption of folates and could afford a means of selectively targeting tumors with novel cytotoxic antifolates. PCFT is a member of the major facilitator superfamily of transporters. Because major facilitator superfamily members exist as homo-oligomers, we tested this for PCFT because such structures could be significant to PCFT mechanism and regulation. By transiently expressing PCFT in reduced folate carrier- and PCFT-null HeLa (R1-11) cells and chemical cross-linking with 1,1-methanediyl bismethanethiosulfonate and Western blotting, PCFT species with molecular masses approximating those of the PCFT dimer and higher order oligomers were detected. Blue native polyacrylamide gel electrophoresis identified PCFT dimer, trimer, and tetramer forms. PCFT monomers with hemagglutinin and His(10) epitope tags were co-expressed in R1-11 cells, solubilized, and bound to nickel affinity columns, establishing their physical associations. Co-expressing YPet and ECFP*-tagged PCFT monomers enabled transport and fluorescence resonance energy transfer in plasma membranes of R1-11 cells. Combined wild-type (WT) and inactive mutant P425R PCFTs were targeted to the cell surface by surface biotinylation/Western blots and confocal microscopy and functionally exhibited a "dominant-positive" phenotype, implying positive cooperativity between PCFT monomers and functional rescue of mutant by WT PCFT. Our results demonstrate the existence of PCFT homo-oligomers and imply their functional and regulatory impact. Better understanding of these higher order PCFT structures may lead to therapeutic applications related to folate uptake in hereditary folate malabsorption, and delivery of PCFT-targeted chemotherapy drugs for cancer.
authors.affiliation: Developmental Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
authors.forename: Zhanjun
authors.initials: Z
authors.lastname: Hou
authors.forename: Sita
authors.initials: S
authors.lastname: Kugel Desmoulin
authors.forename: Erika
authors.initials: E
authors.lastname: Etnyre
authors.forename: Mary
authors.initials: M
authors.lastname: Olive
authors.forename: Benjamin
authors.initials: B
authors.lastname: Hsiung
authors.forename: Christina
authors.initials: C
authors.lastname: Cherian
authors.forename: Patrick A
authors.initials: PA
authors.lastname: Wloszczynski
authors.forename: Kamiar
authors.initials: K
authors.lastname: Moin
authors.forename: Larry H
authors.initials: LH
authors.lastname: Matherly
mesh_terms: D001692:Biological Transport; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism*; D005492:Folic Acid / Q000235:genetics / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D006863:Hydrogen-Ion Concentration; D055503:Protein Multimerization / Q000502:physiology*; D058979:Proton-Coupled Folate Transporter / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D058979:Proton-Coupled Folate Transporter; C505058:SLC46A1 protein, human; D005492:Folic Acid