O. Poupel et al., Toxofilin, a novel actin-binding protein from Toxoplasma gondii, sequesters actin monomers and caps actin filaments, MOL BIOL CE, 11(1), 2000, pp. 355-368
Toxoplasma gondii relies on its actin cytoskeleton to glide and enter its h
ost cell. However, T. gondii tachyzoites are known to display a strikingly
low amount of actin filaments, which suggests that sequestration of actin m
onomers could play a key role in parasite actin dynamics. We isolated a 27-
kDa tachyzoite protein on the basis of its ability to bind muscle G-actin a
nd demonstrated that it interacts with parasite G-actin. Cloning and sequen
ce analysis of the gene coding for this protein, which we named Toxofilin,
showed that it is a novel actin-binding protein. In in vitro assays, Toxofi
lin not only bound to G-actin and inhibited actin polymerization as an acti
n-sequestering protein but also slowed down F-actin disassembly through a f
ilament end capping activity. In addition, when green fluorescent protein-t
agged Toxofilin was overexpressed in mammalian nonmuscle cells, the dynamic
s of actin stress fibers was drastically impaired, whereas green fluorescen
t protein-Toxofilin copurified with G-actin. Finally, in motile parasites,
during gliding or host cell entry, Toxofilin was localized in the entire cy
toplasm, including the rear end of the parasite, whereas in intracellular t
achyzoites, especially before they exit from the parasitophorous vacuole of
their host cell, Toxofilin was found to be restricted to the apical end.