SCP2

Toxoplasma gondii SCP2 is a lipid-transfer and acyl-CoA-binding protein that supports the parasite's acquisition and inter-organelle trafficking of lipids required for intracellular replication (PMID:19005217, PMID:30362657). It originates as the two tandem C-terminal SCP-2 domains of an unusual d-bifunctional protein (TgHAD-2SCP-2), whose proteolytic processing releases free SCP-2; the most C-terminal SCP-2 domain carries a PTS1 signal that directs the protein to vesicles, and mutation of this signal causes cytoplasmic accumulation (PMID:19005217). Biochemically, the protein binds cholesterol in vitro—though with lower affinity than phosphatidylcholine—and functions as a cytosolic acyl-CoA transporter confirmed by fluorometric binding and yeast complementation (PMID:19005217, PMID:30362657). Overexpression of TgHAD-2SCP-2 enhances circulation of lipids between organelles, boosts cholesterol and oleate uptake, and accelerates replication (PMID:19005217). Genetically, SCP2 is dispensable alone, but its combined disruption with TgACBP1 impairs intracellular growth, reduces virulence in mice, and causes defects in fatty acid elongation, glyceride, and phospholipid synthesis, establishing a synergistic role with TgACBP1 in parasite lipid metabolism (PMID:30362657).

1. 2008 Medium

   Established the origin, processing, and targeting logic of parasite SCP-2, showing it is liberated from a fused d-bifunctional protein and routed by a PTS1 signal — defining where the transfer protein acts in the cell.

   Evidence Molecular characterization with PTS1-abrogated transfection constructs and subcellular imaging in T. gondii

   PMID:19005217

   Open questions at the time

   • Exact protease(s) and processing sequence releasing free SCP-2 not defined
   • Vesicular identity of the PTS1-targeted compartment not fully resolved
2. 2008 Medium

   Determined the lipid-binding selectivity of the protein, showing it binds cholesterol but favors phosphatidylcholine ~10-fold, indicating it is a selective lipid-transfer protein.

   Evidence In vitro lipid-binding assay

   PMID:19005217

   Open questions at the time

   • Binding measured in vitro without structural basis for selectivity
   • Full spectrum of physiological lipid ligands not defined
3. 2008 Medium

   Linked SCP-2 function to a cellular phenotype by showing overexpression promotes inter-organelle lipid circulation, enhances cholesterol/oleate uptake, and accelerates replication — connecting lipid transfer to parasite growth.

   Evidence Overexpression with fluorescent lipid trafficking and replication assays in T. gondii

   PMID:19005217

   Open questions at the time

   • Gain-of-function readout; loss-of-function contribution of native SCP-2 not addressed here
   • Mechanistic step at which SCP-2 acts in trafficking not resolved
4. 2018 High

   Defined SCP2 as a cytosolic acyl-CoA transporter and placed it in a lipid-metabolic pathway acting synergistically with TgACBP1, explaining why the single knockout is silent while the double knockout cripples fatty acid elongation, phospholipid synthesis, growth, and virulence.

   Evidence Fluorometric binding, yeast complementation, single and double gene disruption, GC-MS/13C lipidomics, HPLC-HRMS, and mouse virulence assay

   PMID:30362657

   Open questions at the time

   • Molecular basis of the functional redundancy with TgACBP1 not resolved
   • Directionality and organellar endpoints of acyl-CoA transport in vivo not established
5. 2018 Low

   Provided the molecular basis for SCP2 targeting by showing its PTS1 sequence binds the PTS1-binding domain of Pex5, identifying SCP2 as cargo of the peroxisomal import receptor.

   Evidence In vitro protein–protein interaction assay between Pex5 PTS1-binding domain and SCP2 PTS1 ligand

   PMID:30158461

   Open questions at the time

   • Single in vitro interaction assay without functional follow-up for SCP2
   • Whether SCP2 is genuinely imported into a peroxisome-like compartment in vivo not demonstrated

• How SCP2-mediated lipid and acyl-CoA transfer is spatially organized between organelles and coordinated with TgACBP1 at the biochemical level remains unresolved.
• No structural model of SCP2 lipid/acyl-CoA binding
• In vivo trafficking route and organellar acceptor sites undefined
• Mechanism of functional redundancy with TgACBP1 unknown