ADF

TgADF is a small ADF/cofilin-family protein that governs the actin monomer/filament equilibrium in Toxoplasma gondii, maintaining rapid filament turnover required for gliding motility, host cell invasion, and egress (PMID:20042603, PMID:21346192). It acts predominantly as a potent G-actin sequestering protein that inhibits TgACT polymerization at very low concentrations and is only a weak filament-severing protein, a mechanistic bias that arises from the absence of key F-actin binding sites otherwise conserved in canonical ADF/cofilins; experimentally restoring the C-terminal F-actin binding site stabilizes filament interaction but reduces net disassembly (PMID:20042603). Its NMR solution structure confirms a conserved ADF/cofilin fold lacking the C-terminal α4 helix implicated in F-actin binding, with conformationally flexible G-actin binding surfaces underlying high G-actin affinity (Kd ~24 nM) (PMID:21820516). In vivo, conditional depletion causes accumulation of actin-rich filaments and severe motility, invasion, and egress defects (PMID:21346192), and the filament-disassembly activity mediated by Lys-68 is specifically required for organelle segregation during parasite development while being dispensable for motility (PMID:26157165). TgADF activity is enhanced by the bradyzoite-associated binding partner TgDPA, which lowers actin polymerization and promotes filament disassembly (PMID:20433874).

1. 1997 Medium

   Establishing that T. gondii encodes a functional actin-regulatory protein answered whether the parasite possesses cofilin-like machinery to control its actin pool.

   Evidence In vitro actin binding/depolymerization assays with recombinant protein plus immunofluorescence and immunoelectron microscopy

   PMID:9274866

   Open questions at the time

   • Relative contribution of monomer sequestering versus severing not resolved
   • No structural basis for activity
   • No in vivo functional test
2. 2009 High

   Quantitative biochemistry resolved the mechanistic bias of TgADF, showing it is primarily a monomer sequestering protein rather than a severing enzyme, and tied this to the loss of conserved F-actin binding sites.

   Evidence Kinetic/steady-state polymerization and severing assays with recombinant TgADF and TgACT, and mutagenesis restoring the C-terminal F-actin binding site

   PMID:20042603

   Open questions at the time

   • In vivo relevance of sequestering-dominant activity untested here
   • Structural model of missing F-actin sites not yet determined
3. 2011 High

   Conditional knockout established that TgADF is physiologically required for rapid actin filament turnover and the resulting gliding motility, invasion, and egress.

   Evidence Conditional knockout with immunofluorescence, electron microscopy, and motility/invasion/egress assays

   PMID:21346192

   Open questions at the time

   • Does not separate which biochemical activity (sequestering vs disassembly) drives each phenotype
   • No structural correlate
4. 2011 High

   The NMR structure explained the biochemical bias structurally, showing an ADF/cofilin fold lacking the F-actin-binding α4 helix and flexible surfaces conferring high G-actin affinity.

   Evidence NMR spectroscopy with 15N-relaxation dynamics and ITC affinity measurement

   PMID:21820516

   Open questions at the time

   • No structure of the TgADF–G-actin complex
   • PIP2 shown not to affect binding but regulatory inputs otherwise unexplored
5. 2010 Medium

   Identifying TgDPA as a physical partner that enhances TgADF activity revealed a mechanism for stage-specific modulation of actin dynamics in bradyzoites.

   Evidence Yeast two-hybrid, Co-IP, GST pull-down, and actin polymerization/disassembly kinetics

   PMID:20433874

   Open questions at the time

   • In vivo consequence of the TgADF–TgDPA interaction not tested
   • Structural basis and stoichiometry of the interaction unknown
6. 2015 High

   Structure-guided complementation dissociated the two activities functionally, showing that Lys-68-dependent filament disassembly is required for organelle segregation but dispensable for motility.

   Evidence Conditional TgADF knockout complemented with defined point mutants (TgADF Lys-68, PfADF1, PfADF1 Lys-72) plus growth, motility, organelle segregation, and biochemical depolymerization assays

   PMID:26157165

   Open questions at the time

   • Molecular link between disassembly activity and organelle segregation unresolved
   • Divergent phenotype of the equivalent PfADF1 mutation not mechanistically explained

• How TgADF sequestering versus disassembly activities are spatiotemporally coordinated with the actomyosin glideosome, and how TgDPA modulation is integrated in vivo, remain unresolved.
• No structure of TgADF bound to G-actin or F-actin
• In vivo role of TgDPA-mediated enhancement untested
• Upstream regulation of TgADF activity unknown