Identification of amino acid residues responsible for the pyrimidine and purine nucleoside specificities of human concentrative Na+ nucleoside cotransporters hCNT1 and hCNT2

被引:66
作者
Loewen, SK
Ng, AML
Yao, SYM
Cass, CE
Baldwin, SA
Young, JD
机构
[1] Univ Alberta, Dept Physiol, Membrane Transport Res Grp, Edmonton, AB T6G 2H7, Canada
[2] Univ Alberta, Dept Oncol, Membrane Transport Res Grp, Edmonton, AB T6G 2H7, Canada
[3] Univ Leeds, Sch Biochem & Mol Biol, Leeds LS2 9JT, W Yorkshire, England
关键词
D O I
10.1074/jbc.274.35.24475
中图分类号
Q5 [生物化学]; Q7 [分子生物学];
学科分类号
071010 ; 081704 ;
摘要
hCNT1 and hCNT2 mediate concentrative (Na+-linked) cellular uptake of nucleosides and nucleoside drugs by human cells and tissues. The two proteins (650 and 658 residues, 71 kDa) are 72% identical in sequence and contain 13 putative transmembrane helices (TMs), When produced in Xenopus oocytes, recombinant hCNT1 is selective for pyrimidine nucleosides (system cit), whereas hCNT2 is selective for purine nucleosides (system cif), Both transport uridine, We have used (i) chimeric constructs between hCNT1 and hCNT2, (ii) sequence comparisons with a newly identified broad specificity concentrative nucleoside transporter (system cib) from Eptatretus stouti, the Pacific hagfish (hfCNT), and (iii) site-directed mutagenesis of hCNT1 to identify two sets of adjacent residues in TMs 7 and 8 of hCNT1 (Ser(319)/Gln(320) and Ser(353)/Leu(354)) that, when converted to the corresponding residues in hCNT2 (Gly(313)/Met(314) and Thr(347)/Val(348)), changed the specificity of the transporter from cit to cif. Mutation of Ser(319) in TM 7 of hCNT1 to Gly enabled transport of purine nucleosides, whereas concurrent mutation of Gln(320) to Met (which had no effect on its own) augmented this transport. The additional mutation of Ser(353) to Thr in TM 8 converted hCNT1/S319G/Q320M, from cib to cif, but with relatively low adenosine transport activity. Additional mutation of Leu(354) to Val (which had no effect on its own) increased the adenosine transport capability of hCNT1/S319G/Q320M/S353T, producing a full cif-type transporter phenotype, On its own, the S353T mutation converted hCNT1 into a transporter with novel uridine-selective transport properties. Helix modeling of hCNT1 placed Ser319 (TM 7) and Ser(353) (TM 8) within the putative substrate translocation channel, whereas Gln(320) (TM 7) and Leu(354) (TM 8) may exert their effects through altered helix packing.
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页码:24475 / 24484
页数:10
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