title: The broadly selective human Na+/nucleoside cotransporter (hCNT3) exhibits novel cation-coupled nucleoside transport characteristics.
abstract: The concentrative nucleoside transporter (CNT) protein family in humans is represented by three members, hCNT1, hCNT2, and hCNT3. hCNT3, a Na+/nucleoside symporter, transports a broad range of physiological purine and pyrimidine nucleosides as well as anticancer and antiviral nucleoside drugs, and belongs to a different CNT subfamily than hCNT1/2. H+-dependent Escherichia coli NupC and Candida albicans CaCNT are also CNT family members. The present study utilized heterologous expression in Xenopus oocytes to investigate the specificity, mechanism, energetics, and structural basis of hCNT3 cation coupling. hCNT3 exhibited uniquely broad cation interactions with Na+, H+, and Li+ not shared by Na+-coupled hCNT1/2 or H+-coupled NupC/CaCNT. Na+ and H+ activated hCNT3 through mechanisms to increase nucleoside apparent binding affinity. Direct and indirect methods demonstrated cation/nucleoside coupling stoichiometries of 2:1 in the presence of Na+ and both Na+ plus H+, but only 1:1 in the presence of H+ alone, suggesting that hCNT3 possesses two Na+-binding sites, only one of which is shared by H+. The H+-coupled hCNT3 did not transport guanosine or 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine, demonstrating that Na+- and H+-bound versions of hCNT3 have significantly different conformations of the nucleoside binding pocket and/or translocation channel. Chimeric studies between hCNT1 and hCNT3 located hCNT3-specific cation interactions to the C-terminal half of hCNT3, setting the stage for site-directed mutagenesis experiments to identify the residues involved.
authors.affiliation: Membrane Protein Research Group, Departments of Physiology and Oncology, University of Alberta Cross Cancer Institute, Edmonton, Alberta T6G 2H7, Canada.
authors.forename: Kyla M
authors.initials: KM
authors.lastname: Smith
authors.forename: Melissa D
authors.initials: MD
authors.lastname: Slugoski
authors.forename: Shaun K
authors.initials: SK
authors.lastname: Loewen
authors.forename: Amy M L
authors.initials: AM
authors.lastname: Ng
authors.forename: Sylvia Y M
authors.initials: SY
authors.lastname: Yao
authors.forename: Xing-Zhen
authors.initials: XZ
authors.lastname: Chen
authors.forename: Edward
authors.initials: E
authors.lastname: Karpinski
authors.forename: Carol E
authors.initials: CE
authors.lastname: Cass
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Baldwin
authors.forename: James D
authors.initials: JD
authors.lastname: Young
mesh_terms: D000818:Animals; D001665:Binding Sites; D006801:Humans; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008094:Lithium / Q000378:metabolism; D008564:Membrane Potentials / Q000502:physiology; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009705:Nucleosides / Q000378:metabolism*; D009865:Oocytes / Q000502:physiology; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D014529:Uridine / Q000378:metabolism; D014981:Xenopus
chemical_list: D026901:Membrane Transport Proteins; D009705:Nucleosides; D011993:Recombinant Fusion Proteins; C065171:cif nucleoside transporter; D006859:Hydrogen; D008094:Lithium; D012964:Sodium; D014529:Uridine