Identification of a nucleoside/nucleobase transporter from Plasmodium falciparum, a novel target for anti-malarial chemotherapy

Plasmodium, the aetiologic agent of malaria, cannot synthesize purines de novo, and hence depends upon salvage from the host. Here we describe the molecular cloning and functional expression in Xenopus oocytes of the first purine transporter to be identified in this parasite. This 422-residue protein, which we designate PfENT1, is predicted to contain 11 membrane-spanning segments and is a distantly related member of the widely distributed eukaryotic protein family the equilibrative nucleoside transporters (ENTs). However, it differs profoundly at the sequence and functional levels from its homologous counterparts in the human host. The parasite protein exhibits a broad substrate specificity for natural nucleosides, but transports the purine nucleoside adenosine with a considerably higher apparent affinity (Km 0.32±0.05 mM) than the pyrimidine nucleoside uridine (Km 3.5±1.1 mM). It also efficiently transports nucleobases such as adenine (Km 0.32±0.10 mM) and hypoxanthine (Km 0.41±0.1 mM), and anti-viral 3ʹ-deoxynucleoside analogues. Moreover, it is not sensitive to classical inhibitors of mammalian ENTs, including NBMPR {6-[(4-nitrobenzyl)thio]-9-β-D-ribofuranosylpurine, or nitrobenzylthioinosine} and the coronary vasoactive drugs, dipyridamole, dilazep and draflazine. These unique properties suggest that PfENT1 might be a viable target for the development of novel anti-malarial drugs.