Unusual Kinetic and Structural Properties Control Rapid Assembly and Turnover of Actin in the ParasiteToxoplasma gondii

Abstract

Toxoplasma is a protozoan parasite in the phylum Apicomplexa, which contains a number of medically important parasites that rely on a highly unusual form of motility termed gliding to actively penetrate their host cells. Parasite actin filaments regulate gliding motility, yet paradoxically filamentous actin is rarely detected in these parasites. To investigate the kinetics of this unusual parasite actin, we expressed TgACT1 in baculovirus and purified it to homogeneity. Biochemical analysis showed that Toxoplasma actin (TgACT1) rapidly polymerized into filaments at a critical concentration that was 3-4-fold lower than conventional actins, yet it failed to copolymerize with mammalian actin. Electron microscopic analysis revealed that TgACT1 filaments were 10 times shorter and less stable than rabbit actin. Phylogenetic comparison of actins revealed a limited number of apicomplexan-specific residues that likely govern the unusual behavior of parasite actin. Molecular modeling identified several key alterations that affect interactions between monomers and that are predicted to destabilize filaments. Our findings suggest that conserved molecular differences in parasite actin favor rapid cycles of assembly and disassembly that govern the unusual form of gliding motility utilized by apicomplexans.

The other side of the coin: Functional and structural versatility of ADF/cofilins

“…It is established that actins from lower eukaryotes, such as the Apicomplexan parasites, have very low inherent stability, which is required for the effective adaptation for gliding motility (Dobrowolski et al, 1997;Hirono et al, 1989;Sahoo et al, 2006;Schüler et al, 2005b;Skillman et al, 2011;Skillman et al, 2013). Also these actins can have very different characteristics compared to eukaryotic and bacterial actins.…”

A broad spectrum of actin paralogs inParamecium tetraureliacells display differential localization and function

“…However, in lower eukaryotes the situation is rather complex. For instance, Apicomplexan parasites (Plasmodium, Toxoplasma), close relatives of our non-parasitic ciliates, contain abundant actin, mainly in monomeric or short polymeric forms (Dobrowolski et al, 1997;Poupel et al, 2000;Schmitz et al, 2005;Sahoo et al, 2006). Also surprising is the inability of any previous work to visualize in Paramecium a cleavage furrow by fluorescent phalloidin (Kersken et al, 1986a) whereas phagocytosis could easily be inhibited by cytochalasins (Cohen et al, 1984;Allen and Fok, 1985;Fok et al, 1985;Allen et al, 1995).…”

Importance of a Lys113–Glu195 Intermonomer Ionic Bond in F-actin Stabilization and Regulation by Yeast Formins Bni1p and Bnr1p

“…Such a result emanating from a predicted interaction cannot be assumed to be true. In a similar situation, a simulation predicted that a 39 -275 bond was necessary for stabilization as well (29). However, in that case, experimental results demonstrated that the predicted interaction was not important (30).…”