Characterization of native PfABCG protein in Plasmodium falciparum

The Plasmodium falciparum genome encodes 16 members of ABC proteins, with one member of the ABCG subfamily (PfABCG). Analysis of PfABCG amino acid sequence shows equal sequence identity to hsABCG1 and G2. Using N-terminal directed antibody against a recombinant fragment of PfABCG, we show that PfABCG migrates with an apparent molecular mass of 65KDa polypeptide on SDS-PAGE. PfABCG is expressed in all four stages of the parasite erythrocytic life cycle, with lower and higher expression in ring and late trophozoite stages, respectively. The protein localizes to the plasma membrane and a novel spherical structure beneath the cell membrane. Similar localization is also observed in gametocytes where PfABCG is highly expressed. Analysis of PfABCG genomic sequences for polymorphisms and changes in protein expression between different strains of P. falciparum revealed identical nucleotide sequence among the different strains, but variable protein expression. PfABCG expression is least in HB3 chloroquine sensitive strain, while higher expression levels are seen in other chloroquine-sensitive and -resistant strains, with highest levels of expression in 7G8. The differential expression of PfABCG in three chloroquine-sensitive strains (e.g., 3D7, HB3 and D10) predicts the sensitivity of the different strains to ketotifen, an anti-histaminic drug, whereby low expression is associated with decreased sensitivity to ketotifen. Taken together, the results in this report provide the first description of native PfABCG expression and subcellular localization in asexual stages of the parasite and its localization in gametocytes. It remains to be determined if PfABCG is functionally equivalent to mammalian ABCG1, ABCG2 or both.