Targeted disruption of an erythrocyte binding antigen in Plasmodium falciparum is associated with a switch toward a sialic acid-independent pathway of invasion

Erythrocyte invasion by Plasmodium requires molecules present both on the m erozoite surface and within the specialized organelles of the apical comple x. The Plasmodium erythrocyte binding protein family includes the Plasmodiu m falciparum sialic acid-binding protein, EBA-175 (erythrocyte binding anti gen-175). which binds sialic acid present on glycophorin A of human erythro cytes. We address the role of the conserved 3'-cysteine rich region, the tr ansmembrane, and cytoplasmic domains through targeted gene disruption. Trun cation of EBA-175 had no measurable effect on either the level of EBA-175 p rotein expression or its subcellular localization. Similarly, there appears to be no impairment in the ability of soluble EBA-175 to be released into the culture supernatant after schizont rupture. Additionally, the 3'-cys ri ch region, transmembrane. and cytoplasmic domains of EBA-175 are apparently non-essential for merozoite invasion. In contrast, erythrocyte invasion vi a the EBA-175/glycophorin A route appears to have been disrupted to such a degree that the mutant lines have undergone a stable switch in invasion phe notype, As such, EBA-175 appears to have been functionally inactivated with in the truncation mutants. The sialic acid-independent invasion pathway wit hin the mutant parasites accounts for approximately 85% of invasion into no rmal erythrocytes, These data demonstrate the ability of P, falciparum to u tilize alternate pathways for invasion of red blood cells, a property that most likely provides a substantial survival advantage in terms of overcomin g host receptor heterogeneity and/or immune pressure.