Identification, expression and characterisation of a Babesia bovis hexose transporter

Babesia are tick-transmitted haemoprotozoan parasites that infect cattle, with an estimated 500 million at risk worldwide. Here, two predicted hexose transporters (BboHT1 and 2) have been identified within the Babesia bovis genome. BboHT1, having 40% and 47% amino acid sequence similarity compared with the human (GLUT1) and Plasmodium falciparum (PfHT) hexose transporters, respectively, is the only one that could be characterised functionally after expression in Xenopus laevis oocytes. Radiotracer studies on BboHT1 showed that it is a saturable, Na+-independent, stereo-specific hexose transporter, with a Km value for glucose of 0.84 ± 0.54 mM (mean ± SEM). Using d-glucose analogues, hydroxyl positions at O-4 and O-6 have been identified as important for ligand binding to BboHT1. d-Glucose transport was inhibited maximally by cytochalasin B (50 μM). A long-chain O-3 hexose derivative (compound 3361) that selectively inhibits PfHT also inhibited relatively potently BboHT1, with an apparent Ki value of 4.1 ± 0.9 μM (mean ± SEM). Compound 3361 did not inhibit B. bovis proliferation in in vitro growth assays but inhibited invasion of glucose-depleted bovine erythrocytes. Taken together with results of inhibition studies with cytochalasin B and β-glucogallin, these data provide new insights into glucose metabolism of erythrocytic-stage Babesia infections.