Plasmodium falciparum: two antigens of similar size are located in different compartments of the rhoptry.

Two previously described antigens, AMA-1 and QF3, which are located in the rhoptries of Plasmodium falciparum merozoites have polypeptides of similar relative molecular masses. On immunoblots, antibodies to both antigens recognized polypeptides of relative molecular mass 80,000 and 62,000 in all isolates tested. Two-dimensional electrophoresis showed that the isoelectric points of the two antigens were different. QF3 being more basic than AMA-1. AMA-1 was soluble in Triton X-114 whereas QF3 partitioned into the aqueous phase after temperature-dependent phase separation. In immunoelectron microscopic studies. QF3 was found in the body of the rhoptry whereas AMA-1 was consistently found in the neck of the rhoptry. Both antigens gave a punctate double-dot pattern in mature schizonts and merozoites when visualized by fluorescence microscopy, but AMA-1 antibodies also appeared to label the merozoite surface. QF3 was also detected in ring-infected erythrocytes whereas AMA-1 was not. Synthesis of both antigens was first observed in mature trophozoites and immature schizonts. Pulse-chase experiments showed that the Mr 80,000 polypeptide of the AMA-1 gene was subject to immediate processing to the Mr 62,000 product. This cleavage pattern was not stage specific. The Mr 80,000 polypeptide of QF3 was derived from a short-lived Mr 84,000 precursor polypeptide. Processing of the Mr 80,000 polypeptide to an Mr 62,000 polypeptide was restricted to the period of merozoite maturation and reinvasion. Hence AMA-1 and QF3 are different antigens with polypeptides of similar size but located in different compartments of the merozoite rhoptries.