Synthesis of merozoite proteins and glycoproteins during the schizogony of Plasmodium falciparum

We have investigated the protein and glycoprotein content of Plasmodium falciparum merozoites by metabolically labeling cultures of schizont-stage parasites with ³⁵S]methionine or with ³H]glucosamine followed by incubation in nonradioactive medium to allow the schizonts to mature into merozoites, infect new erythrocytes, and develop into ring-stage parasites. The ring stages were separated from schizonts by sedimentation through Percoll. Labeled proteins were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and visualized by fluorography. Using ³⁵S]methionine, four major proteins (p) with apparent relative molecular weights (M_r) = 202k, 136k, 82k, and 46k and two proteins of intermediate labeling (M_r = 185k and 142k) were observed in the schizont-labeled ring-stage parasites. Because corresponding proteins were also observed in the schizont stage, we conclude that they had been present in the invading merozoite. In contrast, prominent proteins which were generally labeled during the ring stage and some major schizont-stage proteins were virtually absent in the schizont-labeled ring-stage. By labeling the parasite proteins with ³H]glucosamine followed by separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, five major glycoproteins (gp) of apparent M_r = 185k, 88k, 56k, 46k, and 34k were identified. Their presence in both the schizont and the schizont-labeled ring stage demonstrated that the merozoite contains glycoproteins. Immune owl monkey serum recognized all five glycoproteins. A comparison of proteins by two-dimensional gel electrophoresis (isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis) suggested that p185 and gp185 were identical, as were p46 and gp46.