GRA72

GRA72 is a Toxoplasma gondii dense granule protein that governs small-molecule permeability across the parasitophorous vacuole membrane (PVM), where it localizes together with the PV (PMID:38380952, PMID:38936501). Structural prediction combined with histidine-residue mutagenesis and electrophysiology indicates that GRA72 assembles into a hexameric pore whose conserved pore-lining histidines are essential for function and channel conductance (PMID:38380952). Loss of GRA72 produces an aberrant 'bubble vacuole' morphology with reduced PVM permeability, phenocopying loss of GRA17 (PMID:38380952, PMID:38936501), and GRA72 acts in a partially non-redundant pathway alongside GRA17/GRA23 to maintain PV integrity, with the Plasmodium falciparum ortholog PfExp2 fully cross-complementing the single knockouts (PMID:38936501). GRA72 physically interacts with GRA47 at the PVM (PMID:38380952, PMID:38936501) and is synthetically lethal with GRA17 (PMID:37269286). In the Pru strain, GRA72 deletion attenuates virulence in mice, reducing organ parasite load and brain cyst burden, establishing it as an in vivo virulence factor (PMID:39237959).

1. 2023 Medium

   Whether GRA72 functionally intersects with the known PVM permeability machinery was unknown; an in vivo genetic-interaction screen placed GRA72 in the GRA17 pathway by showing it is essential when GRA17 is lost.

   Evidence In vivo CRISPR screen in C57BL/6 and Ifngr1-/- mice identifying synthetic lethality with GRA17 and a role in GRA17/GRA23 localization

   PMID:37269286

   Open questions at the time

   • Role in GRA17/GRA23 localization is a screen-level inference without direct localization experiments for GRA72
   • Molecular basis of the synthetic lethality not defined
2. 2024 High

   It was unknown what GRA72 does at the vacuole; knockout established it as a PVM/PV dense granule protein whose loss disrupts vacuole morphology and small-molecule flux, phenocopying GRA17 loss.

   Evidence Delta-gra72 knockout with vacuole morphology imaging and PVM small-molecule permeability assays, replicated across two labs

   PMID:38380952 PMID:38936501

   Open questions at the time

   • Does not resolve whether the permeability defect is direct (pore) or indirect (mislocalization of partners)
3. 2024 High

   The physical basis of GRA72 action was unknown; structural and biophysical work showed it forms a hexameric pore with functionally essential histidines lining the channel.

   Evidence Computational structural prediction, site-directed mutagenesis of pore histidines, and electrophysiology of membrane conductance (single lab)

   PMID:38380952

   Open questions at the time

   • Predicted hexamer lacks an experimental high-resolution structure
   • Ion/substrate selectivity of the pore not defined
   • Whether GRA72 forms a homo-hexamer or heteromeric assembly with GRA47/GRA17 unresolved
4. 2024 High

   The interaction network of GRA72 was undefined; reciprocal Co-IP identified GRA47 as a direct PVM binding partner.

   Evidence Immunoprecipitation/pulldown confirming GRA72-GRA47 interaction in two independent studies

   PMID:38380952 PMID:38936501

   Open questions at the time

   • Stoichiometry and structural interface with GRA47 unknown
   • Whether GRA47 co-assembles into the pore is not established
5. 2024 Medium

   Whether GRA72 is functionally redundant with related GRAs was unknown; double-knockout and cross-species rescue showed GRA72, GRA17, GRA23 and GRA47 act in partially non-redundant but convergent pathways for PV morphology.

   Evidence Delta-gra23/Delta-gra72 double deletion, individual overexpression rescue, and PfExp2 cross-complementation with morphology quantification (single lab)

   PMID:38936501

   Open questions at the time

   • Mechanistic reason each GRA only partially rescues while PfExp2 fully rescues not explained
   • Direct interplay between the GRA72 and GRA23 pathways undefined
6. 2024 Medium

   The in vivo importance of GRA72 was untested; Pru-strain knockout established it as a virulence factor required for normal parasite growth and cyst formation.

   Evidence Pru-Delta-gra72 knockout with mouse infection measuring survival, organ parasite load, and brain cyst burden

   PMID:39237959

   Open questions at the time

   • Whether attenuation stems specifically from the permeability defect vs other functions not dissected
   • Strain-generality of the virulence phenotype beyond Pru untested
7. 2026 Low

   Downstream consequences of GRA72 loss on parasite gene expression were unknown; transcriptomics linked GRA72 deletion to bradyzoite-associated and metabolic gene reprogramming.

   Evidence RNA-seq of Pru-Delta-gra72 vs wild-type with RT-qPCR validation, showing 858 DEGs enriched in membrane and metabolic pathways

   PMID:42068418

   Open questions at the time

   • Expression-level correlation only; no direct mechanistic link between GRA72 protein and the affected metabolic or membrane pathways
   • Cannot distinguish primary transcriptional effect from secondary consequence of stage conversion
   • Single lab, single strain

• How the GRA72 pore mediates selective small-molecule transport and how it assembles with GRA47/GRA17 into a functional PVM channel complex remain unresolved.
• No experimental atomic structure of the pore or its complexes
• Transported substrate spectrum and selectivity undefined
• Regulation of pore assembly and gating unknown