ATP4

ATP4 (TgATP4) is a plasma membrane P-type ATPase of Toxoplasma gondii that functions as a Na+ efflux pump essential for cytosolic Na+ homeostasis (PMID:30723156). Genetic knockdown abolishes the parasite's ability to regulate cytosolic Na+ without affecting resting pH or Ca2+, defining a selective role in sodium regulation (PMID:30723156). TgATP4 is the molecular target of PfATP4-class antimalarial compounds, including cipargamin, which elevate cytosolic Na+ and alkalinize the cytosol in wild-type but not TgATP4-knockdown parasites (PMID:30723156). Loss of TgATP4 reduces the viability of extracellular parasites and attenuates virulence in mice (PMID:30723156). Phylogenetically, ATP4 proteins form an apicomplexan-restricted clade representing a conserved Na+ regulatory mechanism across these parasites (PMID:30723156). Beyond these findings, no further mechanistic or structural detail has been characterized in the available corpus.

1. 2019 Medium

   Establishing where TgATP4 acts was prerequisite to assigning its transport function; localization placed it at the plasma membrane, the interface for ion exchange with the extracellular environment.

   Evidence Subcellular fractionation and localization experiments in T. gondii

   PMID:30723156

   Open questions at the time

   • Topology and membrane insertion determinants not resolved
   • No structural model of the transporter
2. 2019 High

   Whether TgATP4 regulates a specific ion was unknown; knockdown showed it is required for cytosolic Na+ regulation but dispensable for pH and Ca2+ homeostasis, defining it as a selective Na+ efflux pump.

   Evidence Genetic knockdown with cytosolic Na+, pH, and Ca2+ physiological measurements in T. gondii

   PMID:30723156

   Open questions at the time

   • Direct demonstration of Na+/H+ exchange stoichiometry not shown
   • Transport activity not reconstituted in vitro
3. 2019 High

   The molecular target of PfATP4-class antimalarials in Toxoplasma was untested; epistatic pharmacology-genetics established TgATP4 as the target, since the compounds' ionic effects require the protein.

   Evidence Pharmacological inhibition (cipargamin) combined with knockdown and ion/pH measurements in T. gondii

   PMID:30723156

   Open questions at the time

   • Drug-binding site on TgATP4 not mapped
   • Resistance mutations not characterized
4. 2019 Medium

   The physiological importance of Na+ regulation was addressed by loss-of-function; disruption reduced extracellular parasite viability and virulence in mice, linking Na+ homeostasis to fitness and pathogenesis.

   Evidence Genetic knockdown/disruption with growth assays and in vivo mouse virulence assays

   PMID:30723156

   Open questions at the time

   • Mechanism linking extracellular Na+ dysregulation to loss of viability unresolved
   • Stage-specific requirements not defined
5. 2019 Medium

   Whether ATP4 function is conserved beyond Toxoplasma was addressed phylogenetically; ATP4 proteins form an apicomplexan-restricted P-type ATPase clade, indicating a conserved parasite Na+ regulatory mechanism.

   Evidence Phylogenetic analysis alongside functional genetic and physiological characterization

   PMID:30723156

   Open questions at the time

   • Functional conservation across other apicomplexans not directly tested here
   • Evolutionary origin of the clade not resolved

• The catalytic and structural basis of Na+/H+ exchange by TgATP4 and the precise drug-binding determinants remain unresolved.
• No in vitro reconstitution of transport activity
• No structure or drug-binding site mapping
• Ion exchange stoichiometry undetermined