Critical role of Erythrocyte Binding-Like protein of the rodent malaria parasite Plasmodium yoelii to establish an irreversible connection with the erythrocyte during invasion.

Plasmodium malaria parasites multiply within erythrocytes and possess a repertoire of proteins whose function is to recognize and invade these vertebrate host cells. One such protein involved in erythrocyte invasion is the micronemal protein, Erythrocyte Binding-Like (EBL), which has been studied as a potential target of vaccine development in Plasmodium vivax (PvDBP) and Plasmodium falciparum (EBA-175). In the rodent malaria parasite model Plasmodium yoelii, specific substitutions in the EBL regions responsible for intracellular trafficking (17XL parasite line) or receptor recognition (17X1.1pp. parasite line), paradoxically increase invasion ability and virulence rather than abolish EBL function. Attempts to disrupt the ebl gene locus in the 17XL and 17XNL lines were unsuccessful, suggesting EBL essentiality. To understand the mechanisms behind these potentially conflicting outcomes, we generated 17XL-based transfectants in which ebl expression is suppressed with anhydrotetracycline (ATc) and investigated merozoite behavior during erythrocyte invasion. In the absence of ATc, EBL was secreted to the merozoite surface, whereas following ATc administration parasitemia was negligible in vivo. Merozoites lacking EBL were unable to invade erythrocytes in vitro, indicating that EBL has a critical role for erythrocyte invasion. Quantitative time-lapse imaging revealed that with ATc administration a significant number of merozoites were detached from the erythrocyte after the erythrocyte deformation event and no echinocytosis was observed, indicating that EBL is required for merozoites to establish an irreversible connection with erythrocytes during invasion.