PABPC

TgPABPC is a cytoplasmic poly(A) binding protein in Toxoplasma gondii that serves as a marker of mRNP (mRNA) granules, associating with these structures in the parasite cytoplasm (PMID:33383066). Live imaging of TgPABPC-marked granules shows that granule assembly is a dynamic, two-phase process during egress: granules form rapidly in the cytosol prior to host-cell lysis and are then cleared within 10 minutes post-egress, with granule formation occurring independently of polymerized microtubules and distributed stochastically (PMID:33383066). Granule assembly is stage-specific and coupled to translational state rather than being a universal stress response: the eIF2α phosphatase inhibitor salubrinal induces TgPABPC-positive granules in tachyzoites but not bradyzoites, while nutrient starvation and pH stress fail to trigger formation (PMID:33383066). Beyond these localization and granule-dynamics observations, no biochemical characterization of TgPABPC's RNA-binding activity or interaction partners has been established in the available corpus.

1. 2020 Medium

   Establishing where TgPABPC resides addressed whether the protein could serve as a tractable marker for mRNP granules in Toxoplasma; it localized to the cytoplasm and colocalized with mRNP granules, defining it as a cellular marker for these structures.

   Evidence Subcellular localization by imaging/fractionation with colocalization to mRNP granule markers in Toxoplasma gondii

   PMID:33383066

   Open questions at the time

   • No biochemical demonstration of poly(A)/RNA binding by TgPABPC
   • Granule composition and other constituents not defined
   • No interaction partners identified
2. 2020 Medium

   It was unknown when and how mRNP granules assemble during the parasite lytic cycle; time-course live imaging revealed rapid pre-lysis cytosolic granule formation followed by clearance within 10 min post-egress, and depolymerization assays showed assembly does not require microtubules.

   Evidence Live imaging of TgPABPC-marked granules during Ca2+-ionophore-induced egress plus microtubule depolymerization assay

   PMID:33383066

   Open questions at the time

   • Molecular trigger linking egress to granule assembly not identified
   • Functional consequence of granule formation for translation or egress not tested
   • Mechanism of granule clearance post-egress unknown
3. 2020 Medium

   Whether granule formation is a generic stress response was unresolved; pharmacological probing showed salubrinal-driven translational repression induces TgPABPC granules in tachyzoites but not bradyzoites, and neither starvation nor pH stress induced them, establishing granule assembly as stage-specific and tied to translational state.

   Evidence Pharmacological treatment (salubrinal, nutrient deprivation, pH stress) with TgPABPC granule imaging across tachyzoites and bradyzoites

   PMID:33383066

   Open questions at the time

   • Molecular basis of the tachyzoite-vs-bradyzoite difference not defined
   • Direct link between eIF2α phosphorylation status and granule assembly not biochemically confirmed
   • Whether granules represent stress granules, P-bodies, or a distinct compartment not resolved

• The molecular function of TgPABPC — its RNA-binding activity, protein partners, and the mechanistic role of the granules it marks in mRNA fate during egress and stage differentiation — remains uncharacterized.
• No demonstrated biochemical activity or substrate
• No interaction partners or complex composition
• No genetic loss-of-function phenotype for TgPABPC