Characterization of a protein correlated with the production of knob-like protrusions on membranes of erythrocytes infected with Plasmodium falciparum.

Membranes of erythrocytes infected with Plasmodium falciparum develop protrusions called "knobs." These protrusions are not apparent on erythrocytes infected with young parasites (rings) but develop with the growth of parasites to the trophozoite and schizont stages. The nature and origin of knobs were characterized by comparing the stage-specific proteins of two culture lines of P. falciparum, K+ and K-. K+ parasites produce knobs; K- parasites do not. Erythrocytes infected with both types of parasites were labeled metabolically and samples were analyzed by electrophoresis in sodium dodecyl sulfate/polyacrylamide gels. There were no apparent differences in Coomassie blue-stained or radioactive components of rings of K+ and K- parasites. However, erythrocytes infected with K+ trophozoites or schizonts showed a major labeled protein, with an apparent molecular weight of approximately 80,000, that was not present in any developmental stage of K- parasites or in K+ rings. A fraction enriched in membranes from erythrocytes infected with K+ trophozoites showed enrichment of this protein. The results indicate that this protein, synthesized by the parasites, is correlated with the formation of knobs on the host cell membrane. Two additional labeled components were identified. These appeared with the development of schizonts in both K+ and K- parasites and are therefore stage-dependent and not correlated with knobs.