RAB5A

TgRab5A is a Rab-GTPase that governs post-Golgi trafficking of a subset of secretory organelle proteins to micronemes and rhoptries in Toxoplasma gondii, thereby regulating biogenesis of the invasion organelles (PMID:23505371). It is activated by the guanine nucleotide exchange factor TgVps9, whose loss disrupts sorting of proteins destined for rhoptries, micronemes, and dense granules and impairs organelle maturation and secretion (PMID:27966671). TgRab5A defines an endosome-like compartment through which post-Golgi maturation proceeds: the clathrin adaptor TgAP1 acts at this Rab5A-positive compartment during conversion of preROP compartments into mature apically anchored rhoptries, and AP1 disruption misroutes microneme proteins at the trans-Golgi network (PMID:28430827). Rab5A dependence subdivides the micronemes into two biogenetically distinct sub-populations, on which the ferlin TgFER1 acts differentially to mediate trafficking from the trans-Golgi-endosomal network to the cortex and subsequent exocytic fusion (PMID:39294204). Downstream tethering is inferred to involve the HOPS/CORVET subunit TgVps11, which is required for secretory organelle biogenesis and for motility, invasion, egress, and growth (PMID:25640905). No structural model of TgRab5A or direct effector-binding data has been characterized in the available corpus.

1. 2013 Medium

   Established that a Rab-GTPase controls secretory organelle trafficking in T. gondii, answering which regulators route proteins to micronemes and rhoptries and revealing that this pathway serves only a subset of cargo.

   Evidence Rab-GTPase overexpression screen with two-colour STED super-resolution microscopy in T. gondii

   PMID:23505371

   Open questions at the time

   • No direct loss-of-function of Rab5A itself
   • Identity of the redundant, Rab5A-independent microneme targeting pathway unknown
   • No in vitro reconstitution or direct effector identification
2. 2015 Medium

   Placed a HOPS/CORVET tethering subunit downstream of Rab-GTPases in the pathway, addressing how tethering machinery contributes to organelle biogenesis and invasion phenotypes.

   Evidence Tetracycline-regulated conditional knockdown of TgVps11 with immunofluorescence, subcellular fractionation, and invasion/egress/growth assays in T. gondii

   PMID:25640905

   Open questions at the time

   • Rab5A involvement inferred from pathway context, not a direct interaction assay
   • No demonstration of physical Rab5A-Vps11 coupling
3. 2016 High

   Identified the upstream activator of Rab5A, answering how the GTPase is switched on and linking that activation to sorting, maturation, and secretion of secretory organelle proteins.

   Evidence In vitro GEF nucleotide-exchange assay toward Rab5A plus conditional loss-of-function of TgVps9 with secretory organelle phenotype readouts in T. gondii

   PMID:27966671

   Open questions at the time

   • Downstream Rab5A effectors not identified
   • Whether Vps9 acts on Rab5A exclusively or additional Rabs is unresolved
4. 2017 Medium

   Defined the functional relationship between the Rab5A-positive endosome-like compartment and clathrin adaptor sorting, showing how post-Golgi maturation of rhoptries and correct microneme routing depend on AP1 at this compartment.

   Evidence CreLox inducible knockout and ddFKBP overexpression of TgAP1 with immunofluorescence co-localisation to the Rab5A marker in T. gondii

   PMID:28430827

   Open questions at the time

   • Direct physical interaction between AP1 and Rab5A not shown
   • Molecular mechanism of preROP-to-rhoptry maturation unresolved
5. 2024 Medium

   Resolved that micronemes comprise two Rab5A/C-dependence-defined sub-populations and identified a ferlin acting differentially on them, answering how distinct microneme cargo is trafficked and exocytosed.

   Evidence Dominant-negative overexpression of TgFER1 with live imaging and microneme exocytosis assays in T. gondii

   PMID:39294204

   Open questions at the time

   • Dominant-negative rather than clean loss-of-function
   • Mechanism distinguishing Rab5A-dependent vs -independent microneme sub-populations unknown
   • Direct Rab5A-FER1 coupling not established

• The direct effectors of activated TgRab5A and the structural basis of its nucleotide switching remain unidentified.
• No direct Rab5A effector or interactor mapped biochemically
• No structural model of TgRab5A
• GTP/GDP cycling of Rab5A in vivo not directly tracked