Comparative analysis of thePlasmodium falciparumhistidine-rich proteins HRP-I, HRP-II and HRP-III in malaria parasites of diverse origin

Abstract

SUMMARYPlasmodium falciparum-infectederythrocytes (IRBC) synthesize 3 histidine-rich proteins: HRP-I or the knob-associated HRP, HRP-II and HRP-III or SHARP. In order to distinguish these proteins immunochemically we prepared monoclonal antibodies which react with HRP-I, HRP-II and HRP-III, and rabbit antisera against synthetic peptides derived from the HRP-II and HRP-III sequences. A comparative analysis of diverseP. falciparumparasites was made using these antibodies and immunoprecipitation or Western blotting. HRP-I(Mr80000–115000) was identified in all knob-positiveP. falciparumparasites including isolates examined directly from Gambian patients. However, this protein was of lower abundance in these isolates and in 6 knob-positive, culture-adapted parasites compared toAotusmonkey-adapted parasites or culture-adapted parasites studied previously. HRP-II(Mr60000–105000) was identified in allP. falciparumparasites regardless of knob-phenotype, and was recovered from culture supernatants as a secreted water-soluble protein. Within IRBC, HRP-II was found as a complex of several closely spaced bands. Cell surface radio-iodination of IRBC from several isolates and immunoprecipitation with a rabbit antiserum against the HRP-II repeat sequence identified HRP-II as a surface-exposed protein. Like HRP-I, the abundance of HRP-II was lower in the Gambian isolates than withAotusmonkey-adapted parasites studied earlier. Neither HRP-I nor HRP-II were identified in a knob-positive isolate ofP. malariaecollected from a Gambian patient. Analogues of these HRP were also absent from asexual parasites of diverse primate and murine malaria species screened with this panel of antibodies. HRP-III(Mr40000–55000) was distinguished by its lower apparent size and by specific reaction with rabbit antibody against its 5-mer repeat sequence. HRP-III was of lowest abundance compared with the other two HRP. These antibody reagents and distinguishing properties should prove useful in studies on the separate functions of the 3P. falciparumHRP.