title: Residues 334 and 338 in transmembrane segment 8 of human equilibrative nucleoside transporter 1 are important determinants of inhibitor sensitivity, protein folding, and catalytic turnover.
abstract: Equilibrative nucleoside transporters (ENTs) are important for the metabolic salvage of nucleosides and the cellular uptake of antineoplastic and antiviral nucleoside analogs. Human equilibrative nucleoside transporter 1 (hENT1) is inhibited by nanomolar concentrations of structurally diverse compounds, including dipyridamole, dilazep, nitrobenzylmercaptopurine ribonucleoside (NBMPR), draflazine, and soluflazine. Random mutagenesis and screening by functional complementation for inhibitor-resistant mutants in yeast revealed mutations at Phe-334 and Asn-338. Both residues are predicted to lie in transmembrane segment 8 (TM 8), which contains residues that are highly conserved in the ENT family. F334Y displayed increased V(max) values that were attributed to increased rates of catalytic turnover, and N338Q and N338C displayed altered membrane distributions that appeared to be because of protein folding defects. Mutations of Phe-334 or Asn-338 impaired interactions with dilazep and dipyridamole, whereas mutations of Asn-338 impaired interactions with draflazine and soluflazine. A helical wheel projection of TM 8 predicted that Phe-334 and Asn-338 lie in close proximity to other highly conserved and/or hydrophilic residues, suggesting that they form part of a structurally important region that influences interactions with inhibitors, protein folding, and rates of conformational change during the transport cycle.
authors: Membrane Protein Research Group, Departments of Oncology and Physiology, University of Alberta, Alberta, Canada.
authors: Frank
authors: F
authors: Visser
authors: Lijie
authors: L
authors: Sun
authors: Vijaya
authors: V
authors: Damaraju
authors: Tracey
authors: T
authors: Tackaberry
authors: Yunshan
authors: Y
authors: Peng
authors: Morris J
authors: MJ
authors: Robins
authors: Stephen A
authors: SA
authors: Baldwin
authors: James D
authors: JD
authors: Young
authors: Carol E
authors: CE
authors: Cass
mesh_terms: D000241:Adenosine / Q000378:metabolism; D000595:Amino Acid Sequence; D001692:Biological Transport; D004109:Dilazep / Q000494:pharmacology; D033721:Equilibrative Nucleoside Transporter 1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005816:Genetic Complementation Test; D006571:Heterocyclic Compounds / Q000494:pharmacology*; D006801:Humans; D008565:Membrane Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009154:Mutation; D009705:Nucleosides / Q000494:pharmacology; D011487:Protein Conformation; D017510:Protein Folding; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000254:growth & development; D017386:Sequence Homology, Amino Acid; D013868:Thioinosine / Q000494:pharmacology
chemical_list: D033721:Equilibrative Nucleoside Transporter 1; D006571:Heterocyclic Compounds; D008565:Membrane Proteins; D009705:Nucleosides; C438649:SLC29A1 protein, human; D013868:Thioinosine; D004109:Dilazep; D000241:Adenosine