Crithidia luciliae: functional expression of nucleoside and nucleobase transporters in Xenopus laevis oocytes.

The expression of purine-specific nucleoside and base transporters of Crithidia luciliae has been demonstrated in Xenopus laevis oocytes. Poly(A)+-mRNA from C. luciliae, cultured in either purine-replete or purine-starved conditions, was microinjected into X. laevis oocytes. For "purine-replete" mRNA, expression of adenosine and hypoxanthine uptake in microinjected X. laevis oocytes was increased on average 9- and 3-fold above water-injected controls, respectively. Expression of adenosine and hypoxanthine uptake in oocytes microinjected with "purine-starved" mRNA was 8 and 3-fold above water-injected controls, respectively. Substrate competition indicated an adenosine/deoxyadenosine transporter and a separate base transporter specific for hypoxanthine. In contrast to C. luciliae in vivo, where the level of activity of adenosine and hypoxanthine transport was regulated by the level of purines in the medium, the heterologous expression of these transporters (from both purine replete and deplete cultures) in X. laevis oocytes was independent of the extracellular purine concentration. These results may suggest that the presence of specific transporter message is independent of the extracellular purine content, indicating that the regulation of activation and expression of these transporters in C. luciliae may not be under transcriptional control.