Actin in the merozoite of the malaria parasite, Plasmodium falciparum.

Merozoites of the human malaria parasite, Plasmodium falciparum, when treated with cytochalasin B, will attach irreversibly to red cells with formation of a vestigial internal (parasitophorous) vacuole, but they are inhibited from moving into the cell. The existence of an actin-based motile mechanism is implied. Immunoblotting, peptide mapping and the DNase inhibition assay have been used to show that the merozoite contains actin. It makes up an estimated 0.3% of the total parasite protein and is partitioned in the ratio of about 1:2 between the cytosolic and particulate protein fractions. In the former it is unpolymerised and in the latter filamentous. Most of the anti-actin-reactive protein in the soluble fraction and about 20% of that in the pellet has an apparent molecular weight of 55,000 and reacts with an anti-ubiquitin antibody; it is thus evidently ubiquitinyl actin, or arthrin, which has so far been detected only in insect flight muscle.