RAB11B

TgRab11B is an alveolate-specific small GTPase that mediates vesicular transport from the Golgi to the nascent inner membrane complex (IMC) of daughter cells during Toxoplasma gondii replication; conditional interference with its function causes a profound IMC biogenesis defect without affecting sub-pellicular microtubule formation, establishing these as mechanistically separable processes (PMID:20686666). TgRab11B acts upstream of the F-box scaffold protein TgFBXO1, whose recruitment to the daughter cell scaffold depends on TgRab11B function (PMID:31348812), and it is a substrate of the essential TBC-domain Rab GAP TBC9, which physically binds TgRab11B and links it to early vesicular transport and daughter IMC formation (PMID:38762629). Independently of this endogenous replicative role, host-cell Rab11B-positive recycling endosomal vesicles are selectively scavenged into the Toxoplasma parasitophorous vacuole through a Rab11-FIP-dependent mechanism, with class I FIP delivery requiring Rab11 binding (PMID:35274991).

1. 2010 High

   Established that TgRab11B is required for building the daughter cell inner membrane complex, answering how Golgi-derived vesicles reach the nascent pellicle during endodyogeny and showing that IMC membrane biogenesis is uncoupled from cytoskeletal microtubule formation.

   Evidence Conditional dominant-negative interference with fluorescence localization and genetic epistasis against sub-pellicular microtubule formation in T. gondii

   PMID:20686666

   Open questions at the time

   • Direct effectors and vesicle cargo transported by Rab11B not identified
   • GEF/GAP regulators of the GTPase cycle not defined at this stage
   • Molecular basis for targeting to the growing IMC end unknown
2. 2019 Medium

   Placed TgRab11B upstream of the scaffold F-box protein TgFBXO1, connecting the GTPase-driven trafficking step to a defined downstream effector required for daughter scaffold and IMC assembly.

   Evidence Genetic epistasis with conditional TgRab11B interference and fluorescence localization of TgFBXO1 in Rab11B-depleted parasites

   PMID:31348812

   Open questions at the time

   • Whether TgFBXO1 is a direct Rab11B effector or acts through intermediaries is unresolved
   • Single method for this specific epistasis finding
3. 2022 Medium

   Revealed a distinct host-directed role by showing host Rab11B recycling vesicles are selectively sequestered into the parasitophorous vacuole via a Rab11-FIP-dependent mechanism, defining specificity in parasite scavenging of host endosomal traffic.

   Evidence Quantitative fluorescence microscopy with host Rab11A/B depletion, FIP binding mutants, and co-localization of intra-PV vesicles

   PMID:35274991

   Open questions at the time

   • Parasite factor(s) that recruit host Rab11B/FIP vesicles to the PV not identified
   • Functional consequence of host vesicle scavenging for the parasite not established
   • This role is distinct from the endogenous TgRab11B function and their relationship is not integrated
4. 2024 Medium

   Identified the essential TBC-domain GAP TBC9 as a physical interactor of TgRab11B, providing a candidate regulator of the GTPase cycle governing daughter IMC formation and early vesicular transport.

   Evidence Pull-down assay and conditional TBC9 knockdown with phenotypic analysis of IMC formation and vesicular transport in T. gondii

   PMID:38762629

   Open questions at the time

   • TBC9 GAP activity toward Rab11B not demonstrated biochemically
   • TBC9 also binds Rab11A and Rab2, so Rab11B-specific regulation is not isolated
   • No structural detail of the interaction

• The GEF that activates TgRab11B, the direct effector repertoire beyond TgFBXO1, and the parasite factors mediating host Rab11B vesicle scavenging remain undefined.
• No GEF identified for the GTPase activation cycle
• Direct vesicle cargo and tether proteins unknown
• Mechanistic link between endogenous and host-scavenging roles unresolved