ROP8

ROP8 is a rhoptry-secreted pseudokinase of Toxoplasma gondii that acts as a scaffolding component of the parasite's effector machinery targeting host immunity-related GTPases (IRGs) (PMID:24390541). Crystal structures show ROP8 adopts a bilobate protein-kinase fold stabilized by a distinctive N-terminal extension, but it lacks catalytic activity and is instead a substrate phosphorylated in vitro by the active rhoptry kinase ROP18 (PMID:19197235). Within IFN-γ-activated macrophages, ROP8 assembles in a complex with ROP18 and the dense-granule effector GRA7 that acts on IRGs to promote parasite survival (PMID:24390541), although deletion of the ROP2/ROP8 locus does not impair recruitment of host mitochondria to the parasitophorous vacuole membrane (PMID:20637758). ROP8 localizes to the rhoptry organelles and is processed to a mature ~52 kDa protein (PMID:9364966). It has been identified as a directly druggable target: thiazolidin-4-one derivatives bind recombinant ROP8 and inhibit parasitophorous vacuole formation (PMID:41727434).

1. 1997 Medium

   Establishing where the ROP8 gene product resides was the first step in assigning it to the parasite's invasion/secretion apparatus, showing it is a rhoptry-targeted protein.

   Evidence Heterologous expression in Neospora caninum transfectants and immunoblot with rhoptry-specific antibodies revealing a processed ~52 kDa product

   PMID:9364966

   Open questions at the time

   • Function within the rhoptry unresolved at this stage
   • No enzymatic or interaction data
2. 2009 High

   Solving the crystal structure resolved whether ROP8 is an active kinase, establishing it as a catalytically inactive pseudokinase that itself serves as a phosphorylation substrate.

   Evidence X-ray crystallography, in vitro kinase assay with mass spectrometry, and mutagenesis showing ROP18 phosphorylates ROP8 and defining the stabilizing N-terminal extension

   PMID:19197235

   Open questions at the time

   • Functional consequence of ROP18-mediated phosphorylation of ROP8 in vivo not defined
   • No cellular role assigned
3. 2010 Medium

   Genetic ablation tested whether the ROP2/ROP8 locus drives host mitochondrial recruitment to the PV, ruling out this proposed role.

   Evidence Knockout of the ROP2a/ROP2b/ROP8 locus with fluorescence microscopy of host mitochondrial association, indistinguishable from parental strain

   PMID:20637758

   Open questions at the time

   • Does not isolate ROP8-specific contribution from ROP2a/ROP2b
   • Negative result from a single lab
4. 2013 Medium

   Comparative structural analysis defined the family features distinguishing active from degenerate rhoptry kinases, placing ROP8 among the pseudokinase members.

   Evidence Comparative analysis of solved ROP2/ROP5/ROP8 structures with sequence-conservation and evolutionary mapping, identifying a hinge insert, substrate-lobe disulfide, and shared N-terminal extension

   PMID:23742205

   Open questions at the time

   • No functional mutagenesis of the identified conserved features
   • Purely computational/structural inference
5. 2014 High

   Co-IP and genetic epistasis assigned ROP8 a role in a multiprotein effector complex, framing it as a pseudokinase scaffold acting with ROP18 and GRA7 against host IRGs.

   Evidence Reciprocal co-immunoprecipitation, gra7/rop18 double-mutant epistasis, IFN-γ-activated macrophage survival assay, and in vitro GTPase binding

   PMID:24390541

   Open questions at the time

   • ROP8-specific contribution to the complex not separated from ROP18/GRA7 by targeted deletion
   • Mechanism by which ROP8 scaffolds the complex not defined
6. 2021 Low

   Drug-resistance profiling probed whether ROP8 influences chemotherapeutic sensitivity, implicating it as a minor determinant of resistance to the compound KG8.

   Evidence Chemical mutagenesis, in vitro selection, and whole-genome sequencing of resistant clones identifying a ROP8 mutation alongside the primary ROP1 determinant

   PMID:33361291

   Open questions at the time

   • ROP8 mutation not functionally validated for the resistance phenotype
   • ROP8 was a minor contributor relative to ROP1
7. 2025 Medium

   Long-read genomics revealed intra-strain genomic plasticity at the ROP8 locus, showing copy-number expansion at this virulence locus.

   Evidence Oxford Nanopore long-read sequencing and de novo assembly of ME49 isolates detecting a 13 kb expansion increasing ROP2A-ROP8 copies from three to six

   PMID:41339799

   Open questions at the time

   • Functional consequences of copy-number variation not tested
   • Impact on ROP8 expression or virulence unknown
8. 2026 Medium

   Target-engagement assays established ROP8 as a directly druggable protein whose engagement blocks parasitophorous vacuole formation.

   Evidence DARTS assay showing direct binding of thiazolidin-4-one derivatives to recombinant ROP8, with confocal microscopy of impaired PV formation

   PMID:41727434

   Open questions at the time

   • Whether the PV-formation defect is caused specifically by ROP8 engagement rather than off-target effects not established
   • Mechanistic link between ROP8 and PV formation not defined

• How ROP8's pseudokinase scaffolding activity and its ROP18-mediated phosphorylation mechanistically contribute to IRG targeting and PV formation remains unresolved.
• No ROP8-specific knockout phenotype for IRG targeting
• Functional role of ROP18 phosphorylation of ROP8 in vivo undefined
• Molecular basis of ROP8's contribution to PV formation unknown