Stage-dependent processing and localization of a Plasmodium falciparum protein of 130,000 molecular weight.

A Plasmodium falciparum protein of 130,000 molecular weight (m.w.) has been identified, cloned in Escherichia coli, and completely sequenced (Kochan et al. 1986). The protein appeared to bind to soluble glycophorin, a host erythrocyte surface protein. In the present study, extracts of parasites from different intraerythrocytic stages were immunoblotted with antibodies, raised against a 30,000 m.w. fusion protein corresponding to the 3' end of the 130,000 m.w. protein. It was demonstrated that the protein is synthesized at the trophozoite stage, accumulates at the schizont stage, and is processed at the merozoite stage to a triplet of three polypeptides. The processed proteins are present in the culture supernatant at the time of merozoite burst from the red cell. Immunofluorescent staining of the parasite at different intracellular stages indicates that the protein is localized on the parasite at the trophozoite stage. At late trophozoite stage, it appears to be transported to the erythrocyte cytoplasm, where it is present in small vesicles or inclusions. In mature schizonts the protein accumulates around the plasma membrane of the erythrocyte. At the segmenter stage, just prior to merozoite release, it appears also to surround the intracellular merozoite, as well as the erythrocyte plasma membrane. The soluble 130,000 m.w. protein binds to erythrocytes but binds significantly greater to erythrocyte membranes, suggesting it binds to an internal domain of glycophorin rather than the domain exposed on the surface. The 130,000 m.w. protein is present in 11 different geographic isolates of P. falciparum from diverse geographic origins. Its molecular weight is similar in all isolates.