TGRH88_020200

None of the findings on this card come from studying RH88: they are ROP4 results from papers that do not identify this RH88 gene (TGRH88_020200), which is probably the RH88 copy of ROP4 (card TG_295125). ROP4 is a type I transmembrane rhoptry protein of Toxoplasma gondii that is secreted during host cell invasion and associates with the parasitophorous vacuole membrane (PVM), where it becomes phosphorylated at multiple sites by kinases including host cell protein kinase(s) or parasite kinase(s) activated by host cell factors (PMID:15470260). Its rhoptry targeting is directed by a signal encoded within the ROP4 coding sequence (PMID:15279957). On the PVM, ROP4 adopts a topology with its N-terminus facing the host cytoplasm, opposite to that of the ROP2-family member ROP5 (PMID:16879455). Within infected cells ROP4 is found in association with ROP2 and with the dense granule protein GRA7, the latter representing the first reported interaction between rhoptry- and dense-granule-secreted proteins (PMID:18809661). Through its host-exposed domains ROP4 binds host ligands, including human lactoferrin (PMID:17069806) and sulfated heparin/heparan sulfate glycosaminoglycans (PMID:22997241), implicating it in parasite-host interactions at the vacuole interface. Beyond these binding and localization properties, the precise enzymatic or effector function of ROP4 has not been characterized in the available corpus.

1. 2002 Low

   Whether ROP4 expression is constant or stage-regulated was unknown; profiling across differentiation showed its transcript declines during tachyzoite-to-bradyzoite conversion, placing ROP4 among developmentally regulated secretory organelle genes.

   Evidence cDNA microarray across differentiation time points

   PMID:12455982

   Open questions at the time

   • Transcript-level only; no protein-level confirmation of stage regulation
   • Functional consequence of downregulation not tested
2. 2004 High

   The localization and fate of ROP4 during invasion were undefined; ROP4 was established as a type I transmembrane rhoptry protein that is secreted, associates with the PVM, and becomes multiply phosphorylated in part by host- or host-activated kinases, linking it to the host cell interface.

   Evidence Monoclonal antibody screen, subcellular fractionation, IEM, phosphorylation assays with kinase inhibitors

   PMID:15470260

   Open questions at the time

   • Specific phosphosites and responsible kinases not identified
   • Functional consequence of PVM phosphorylation unknown
3. 2004 Medium

   How ROP4 is routed to rhoptries was unknown; an in-frame GFP motif-trap at the endogenous locus was correctly targeted, revealing a rhoptry-targeting signal within the ROP4 coding sequence.

   Evidence GFP motif-trap integration with FACS and fluorescence microscopy

   PMID:15279957

   Open questions at the time

   • The targeting signal domain was not delineated
   • Single method, single lab
4. 2006 Medium

   The membrane orientation of ROP2-family proteins was uncertain; differential permeabilization of ROP5 indicated ROP2 and ROP4 adopt the opposite topology, with their N-terminus facing the host cytoplasm.

   Evidence Differential permeabilization and topology-specific immunofluorescence

   PMID:16879455

   Open questions at the time

   • ROP4 orientation inferred comparatively rather than tested directly
   • Mechanism of membrane insertion not resolved
5. 2006 Medium

   Whether ROP4 engages host ligands was unknown; membrane-derived ROP4 (63 kDa) and ROP2 were shown to specifically bind human lactoferrin, indicating direct host protein recognition.

   Evidence Lactoferrin affinity chromatography with competitive inhibition and ESI/MS/MS identification

   PMID:17069806

   Open questions at the time

   • Binding domain not mapped
   • Biological role of lactoferrin binding not established
   • No reciprocal or reconstituted assay
6. 2008 Medium

   Whether rhoptry and dense granule secretory pathways converge molecularly was unknown; GRA7 was found associated with ROP2 and ROP4 in infected cells, the first reported rhoptry-dense granule protein interaction.

   Evidence Co-immunoprecipitation from infected host cells and immunofluorescence

   PMID:18809661

   Open questions at the time

   • No reciprocal pull-down or in vitro reconstitution
   • Direct vs indirect nature of the interaction unresolved
   • Functional consequence of the complex unknown
7. 2012 Medium

   The nature of ROP4's carbohydrate interactions was undefined; ROP4 was shown to bind sulfated heparin/heparan sulfate with measurable specificity and affinity, extending its host-interaction repertoire to glycosaminoglycans.

   Evidence Heparin oligosaccharide microarrays and surface plasmon resonance

   PMID:22997241

   Open questions at the time

   • Binding site not confirmed by mutagenesis
   • Physiological relevance within the vacuole not demonstrated

• The core effector or enzymatic function of ROP4 — how its PVM association, phosphorylation, and host-ligand binding integrate into parasite biology or virulence — remains unresolved.
• No knockout phenotype reported
• Responsible kinases and phosphosite function unmapped
• Biological role of lactoferrin/GAG binding untested