RAB18

PF3D7_0807300 (RAB18) is one of eleven Rab GTPase genes in Plasmodium falciparum and is transcribed in infected erythrocytes, placing it within the parasite's Rab-family membrane-trafficking machinery (PMID:12657463). When expressed as a GFP fusion in P. falciparum schizonts, RAB18 gives a diffuse perinuclear fluorescence pattern distinct from the punctate rhoptry localisation of PfRab1A, and attempts to generate a dominant-negative S24N variant failed to yield transformants (PMID:27348424). Consistent with a perinuclear role, RAB18 was recovered as an interacting partner of the perinuclear membrane-trafficking myosin PfMyoF in pull-down experiments (PMID:42141018). In P. yoelii ookinetes, overexpressed N-terminally tagged RAB18 is cytoplasmic and does not co-localise with the dynein heavy chain DHC3 at subpellicular microtubules, distinguishing it from Rab11A/Rab11B and excluding it from the SPMT-dynein transport complex (PMID:39366980). A comparative-genomic inference links the presence of a RAB18 orthologue to organisms bearing a parasitophorous vacuole membrane, hinting at a possible role in PVM-directed traffic (PMID:18468471). Beyond these localisation and association findings, no direct functional mechanism, GTPase-cycle regulator, or trafficking substrate has been experimentally established for RAB18 in the available corpus.

1. 2003 Low

   Establishing whether RAB18 is an expressed member of the parasite Rab repertoire was the first step in defining it as a candidate trafficking regulator rather than a silent locus.

   Evidence RT-PCR transcription testing and phylogenetic analysis of the P. falciparum Rab family

   PMID:12657463

   Open questions at the time

   • No subcellular localisation or functional assay performed on RAB18 itself
   • Membership in the Rab family inferred bioinformatically, not biochemically
   • GTPase activity not demonstrated
2. 2008 Low

   Comparative genomics addressed where RAB18 might act by correlating its phyletic distribution with the presence of a parasitophorous vacuole membrane, generating a testable hypothesis of PVM-directed traffic.

   Evidence Reciprocal BLAST across Apicomplexa noting loss of the RAB18 orthologue in PVM-lacking Theileria and Babesia

   PMID:18468471

   Open questions at the time

   • Purely computational inference with no experimental validation
   • No direct demonstration of RAB18 at or trafficking to the PVM
3. 2016 Low

   Direct visualisation asked where RAB18 resides in the parasite, showing a diffuse perinuclear distribution distinct from other Rabs and suggesting an essential locus given the failed dominant-negative.

   Evidence Fluorescence microscopy of GFP-RAB18 in transgenic P. falciparum schizonts; attempted S24N dominant-negative mutagenesis

   PMID:27348424

   Open questions at the time

   • Overexpressed fusion protein, not endogenous tagging
   • Diffuse pattern not resolved to a specific organelle
   • Failed dominant-negative leaves essentiality unproven
4. 2024 Low

   Co-localisation testing in ookinetes clarified which transport machinery RAB18 does NOT belong to, excluding it from the SPMT-dynein complex that recruits Rab11A/Rab11B.

   Evidence IFA of N-terminally tagged RAB18 overexpressed in P. yoelii ookinetes, compared against DHC3-6HA

   PMID:39366980

   Open questions at the time

   • Negative co-localisation result using overexpressed rather than endogenous protein
   • Cross-species (P. yoelii) finding may not transfer to P. falciparum
   • Positive functional partner not identified in this study
5. 2026 Low

   A pull-down began to define RAB18's physical partners, linking it to the perinuclear myosin PfMyoF and providing the first candidate mechanistic connection consistent with its perinuclear localisation.

   Evidence Pull-down/Co-IP identifying RAB18 as a PfMyoF-interacting protein in P. falciparum

   PMID:42141018

   Open questions at the time

   • Single pull-down without reciprocal confirmation
   • No functional consequence of the interaction demonstrated
   • Directionality and stoichiometry of the RAB18-PfMyoF association unknown

• The core mechanism of RAB18 remains open: no GTPase-cycle regulators, effectors, cargo, or defined membrane compartment have been experimentally established, and its role in PVM-directed traffic is untested.
• GTPase activity and nucleotide-cycle regulators (GEF/GAP) unidentified
• No effector or cargo defined
• Functional significance of the PfMyoF interaction and the proposed PVM role untested