Nucleoside transporter proteins of Saccharomyces cerevisiae - Demonstration of a transporter (FUI1) with high uridine selectivity in plasma membranesand a transporter (FUN26) with broad nucleoside selectivity in intracellular membranes

FUI1 and function unknown now 26 (FUN26) are proteins of uncertain function with sequence similarities to members of the uracil/allantoin permease and equilibrative nucleoside transporter families of transporter proteins, res pectively, [H-3]Uridine influx was eliminated by disruption of the gene enc oding FUI1 (fui1) and restored by expression of FUI1 cDNA, whereas influx i n transport-competent and full-negative yeast were unaffected, respectively , by disruption of the FUN26 gene or overexpression of FUN26 cDNA, FUI1 tra nsported uridine with high affinity (K-m, 22 +/- 3 mu M) and was unaffected or inhibited only partially by high concentrations (1 mM) Of a variety of ribo- and deoxyribonucleosides or nucleobases. When FUN26 cDNA was expresse d in oocytes of Xenopus laevis, inward fluxes of [H-3]uridine, [H-3]adenosi ne, and [H-3]cytidine were stimulated, and uridine influx was independent o f pH and not inhibited by dilazep, dipyridamole, or nitrobenzylmercaptopuri ne ribonucleoside. Fractionation of yeast membranes containing immunotagged recombinant FUN26 (shown to be functional in oocytes) demonstrated that th e protein was primarily in intracellular membranes. These results indicated that FUI1 has high selectivity for uracil-containing ribonucleosides and i mports uridine across cell-surface membranes, whereas FUN26 has broad nucle oside selectivity and most likely functions to transport nucleosides across intracellular membranes.