Evolutionarily Divergent, Unstable Filamentous Actin Is Essential for Gliding Motility in Apicomplexan Parasites

Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, USA.
PLoS Pathogens (Impact Factor: 7.56). 10/2011; 7(10):e1002280. DOI: 10.1371/journal.ppat.1002280
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Apicomplexan parasites rely on a novel form of actin-based motility called gliding, which depends on parasite actin polymerization, to migrate through their hosts and invade cells. However, parasite actins are divergent both in sequence and function and only form short, unstable filaments in contrast to the stability of conventional actin filaments. The molecular basis for parasite actin filament instability and its relationship to gliding motility remain unresolved. We demonstrate that recombinant Toxoplasma (TgACTI) and Plasmodium (PfACTI and PfACTII) actins polymerized into very short filaments in vitro but were induced to form long, stable filaments by addition of equimolar levels of phalloidin. Parasite actins contain a conserved phalloidin-binding site as determined by molecular modeling and computational docking, yet vary in several residues that are predicted to impact filament stability. In particular, two residues were identified that form intermolecular contacts between different protomers in conventional actin filaments and these residues showed non-conservative differences in apicomplexan parasites. Substitution of divergent residues found in TgACTI with those from mammalian actin resulted in formation of longer, more stable filaments in vitro. Expression of these stabilized actins in T. gondii increased sensitivity to the actin-stabilizing compound jasplakinolide and disrupted normal gliding motility in the absence of treatment. These results identify the molecular basis for short, dynamic filaments in apicomplexan parasites and demonstrate that inherent instability of parasite actin filaments is a critical adaptation for gliding motility.

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    • "These remnant structures may help provide the cellular deformability properties that are needed for survival in the host circulation (Dixon et al., 2012). Although asexual P. falciparum expresses only actin-I, gametocytes also express actin-II (Wesseling et al., 1989; Skillman et al., 2011), and this isoform plays a critical role in male gametogenesis in P. berghei (Deligianni et al., 2011). A previous study reported that anti-ACTII has a diffuse (cytoplasmic) labelling pattern in gametocytes (Rupp et al., 2011). "
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    • "In the linear motor model that has dominated the field for the last decade, TgMyoA walks along short actin filaments that polymerize transiently between the parasite IMC and plasma membrane (Heintzelman and Schwartzman, 1997; Herm-Gotz et al., 2002; Wetzel et al., 2003; Sahoo et al., 2006; Skillman et al., 2011). The actin filaments are bound to the cytosolic tails of transmembrane surface adhesins through a bridging protein (Jewett and Sibley, 2003; Shen and Sibley, 2014; Sheiner et al., 2010). "
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    • "An intriguing open question is whether actin2 forms filaments in these cells. Actin2 can form filaments in vitro (Skillman et al., 2011) but we have no evidence that actin filaments are required at the stages studied here. This issue may be investigated by introducing specific mutations in the actin2 gene which will interfere with filament formation. "
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