TIC22

PF3D7_0529300 (TIC22) is a component of the apicoplast protein import machinery in Plasmodium falciparum, functioning as a chaperone that maintains imported cargo in an unfolded state during translocation across the apicoplast intermembrane space (PMID:23027875). It localizes to the periphery of the apicoplast as a membrane-associated (not integral) protein and is expressed predominantly during the trophozoite stage of the asexual cycle (PMID:19502580). Its crystal structure, solved to 2.15 Å, reveals a fold conserved from cyanobacteria to plants bearing a non-polar groove on each side of the molecule that mediates its cargo-binding chaperone activity (PMID:23027875, PMID:22442242). Beyond this membrane-associated intermembrane-space chaperone role, no further mechanistic detail—such as its specific import partners in P. falciparum—has been characterized in the available corpus.

1. 2009 Medium

   Establishing where TIC22 resides and how it associates with the organelle was the first step toward placing it in the apicoplast import pathway.

   Evidence Endogenous-promoter HA-tagging with immunofluorescence and fractionation in P. falciparum

   PMID:19502580

   Open questions at the time

   • Does not define which membrane face or compartment TIC22 associates with
   • No functional role in import tested at this stage
   • Import partners in P. falciparum not identified
2. 2012 Medium

   Producing recombinant TIC22 and obtaining diffraction-quality crystals demonstrated the soluble domain was tractable and set up structural determination of its fold.

   Evidence Recombinant overexpression, purification, and X-ray diffraction to 2.15 Å in P. falciparum TIC22

   PMID:22442242

   Open questions at the time

   • Structure not yet solved at this reporting stage
   • No functional assay accompanying crystallization
3. 2012 High

   Solving the crystal structure revealed a conserved fold with non-polar grooves and linked it to chaperone activity, explaining how TIC22 could keep translocating cargo unfolded across the intermembrane space.

   Evidence 2.15 Å X-ray structure (SeMet SAD) with chaperone activity assays for P. falciparum TIC22; essentiality and import role validated in the T. gondii ortholog by genetic knockdown

   PMID:23027875

   Open questions at the time

   • Essentiality and direct import role shown in the T. gondii ortholog, not directly in P. falciparum TIC22
   • Specific cargo proteins bound by P. falciparum TIC22 not identified
   • Direct interaction with translocon components not defined in the corpus

• How TIC22 engages specific cargo and integrates with the apicoplast translocon machinery in P. falciparum remains unresolved.
• No identified P. falciparum-specific cargo substrates
• No characterized physical partners within the apicoplast import complex
• Genetic essentiality demonstrated only for the ortholog, not directly for PF3D7_0529300