Functional analysis of an inosine-guanosine transporter from Leishmania donovani -: The role of conserved residues, aspartate 389 and arginine 393

Equilibrative nucleoside transporters encompass two conserved, charged residues that occur within predicted transmembrane domain 8. To assess the role of these "signature" residues in transporter function, the Asp(389) and Arg(393) residues within the LdNT2 nucleoside transporter from Leishmania donovani were mutated and the resultant phenotypes evaluated after transfection into Deltaldnt2 parasites. Whereas an R393K mutant retained transporter activity similar to that of wild type LdNT2, the R393L, D389E, and D389N mutations resulted in dramatic losses of transport capability. Tagging the wild type and mutant ldnt2 proteins with green fluorescent protein demonstrated that the D389N and D389E mutants targeted properly to the parasite cell surface and flagellum, whereas the expression of R393L at the cell surface was profoundly compromised. To test whether Asp(389) and Arg(393) interact, a series of mutants was generated, D389R/R393R, D389D/R393D, and D389R/ R393D, within the green fluorescent protein-tagged LdNT2 construct. Although all of these ldnt2 mutants were transport-deficient, D389R/ R393D localized properly to the plasma membrane, while neither D389R/ R393R nor D389D/R393D could be detected. Moreover, a transport-incompetent D389N/R393N double ldnt2 mutant also localized to the parasite membrane, whereas a D389L/R393L ldnt2 mutant did not, suggesting that an interaction between residues 389 and 393 may be involved in LdNT2 membrane targeting. These studies establish genetically that Asp(389) is critical for optimal transporter function and that a positively charged or polar residue at Arg(393) is essential for proper expression of LdNT2 at the plasma membrane.