Amino terminus of Plasmodium falciparum acidic basic repeat antigen interacts with the erythrocyte membrane through band 3 protein.

The acidic basic repeat antigen (ABRA) of Plasmodium falciparum is localised in the parasitophorous vacuole, and associates with the merozoite surface at the time of schizont rupture. By virtue of its protease-like activity, it is implicated in the process of merozoite invasion and schizont rupture, and therefore, possibly interacts with erythrocyte membrane proteins to execute its function during these events. In this study, using Escherichia coli expressed recombinant fragments of ABRA, we have demonstrated that ABRA interacts with red blood cells through its N-terminus. Out of the four human erythrocyte proteins tested, namely, band 3, glycophorin A and B and spectrin, ABRA showed dose-dependent and saturable binding with the band 3 protein. This binding was lost on chymotrypsin treatment of erythrocytes or their membrane extract. Studies with the deletion constructs of the N-terminus revealed that the binding domain lies in the cysteine-rich N-proximal region of ABRA. In addition to the recombinant fragments, native ABRA derived from the P. falciparum-infected erythrocytes also showed binding to band 3 protein. Sequencing of the cysteine-rich 528 bp region, amplified from fifteen field isolates of P. falciparum, showed that not only the five cysteines of mature ABRA but also the whole sequence is fully conserved, even at the nucleotide level. This sequence conservation of the N-terminus and its role in RBC binding suggests that this region may be crucial for any putative function of ABRA, therefore emphasising its importance as a vaccine/drug target.