Critical Glycosylated Residues in Exon Three of Erythrocyte Glycophorin A Engage Plasmodium falciparum EBA-175 and Define Receptor Specificity

Erythrocyte invasion is an essential step in the pathogenesis of malaria. The erythrocyte binding-like (EBL) family of Plasmodium falciparum proteins recognizes glycophorins (Gp) on erythrocytes and plays a critical role in attachment during invasion. However, the molecular basis for specific receptor recognition by each parasite ligand has remained elusive, as is the case with the ligand/receptor pair P. falciparum EBA-175 (PfEBA-175)/GpA. This is due largely to difficulties in producing properly glycosylated and functional receptors. Here, we developed an expression system to produce recombinant glycosylated and functional GpA, as well as mutations and truncations. We identified the essential binding region and determinants for PfEBA-175 engagement, demonstrated that these determinants are required for the inhibition of parasite growth, and identified the glycans important in mediating the PfEBA-175–GpA interaction. The results suggest that PfEBA-175 engages multiple glycans of GpA encoded by exon 3 and that the presentation of glycans is likely required for high-avidity binding. The absence of exon 3 in GpB and GpE due to a splice site mutation confers specific recognition of GpA by PfEBA-175. We speculate that GpB and GpE may have arisen due to selective pressure to lose the PfEBA-175 binding site in GpA. The expression system described here has wider application for examining other EBL members important in parasite invasion, as well as additional pathogens that recognize glycophorins. The ability to define critical binding determinants in receptor-ligand interactions, as well as a system to genetically manipulate glycosylated receptors, opens new avenues for the design of interventions that disrupt parasite invasion.