TRAPPC6B

TRAPPC6B is a core subunit of the multimeric TRAPP (transport protein particle) tethering complexes required for membrane trafficking to and integrity of the Golgi apparatus (PMID:37713627, PMID:28626029). It is preferentially incorporated into the TRAPP II complex through interaction with TRAPPC3, distinguishing it from its paralogue TRAPPC6A, which partitions equally between TRAPP II and TRAPP III (PMID:37713627). Loss-of-function variants destabilize the complex: the p.Q152* truncation weakens binding to TRAPPC3 and lowers cellular levels of TRAPPC6B together with the TRAPP II-specific subunits TRAPPC9 and TRAPPC10, with a compensatory rise in TRAPPC6A (PMID:37713627). The functional consequence is slowed trafficking into the Golgi and Golgi fragmentation, both rescued by wild-type re-expression (PMID:37713627). This trafficking function is evolutionarily conserved, as human TRAPPC6B complements deletion of its yeast orthologue Trs33 (PMID:39273027). Disrupting TRAPPC6B causes a neurodevelopmental disorder featuring microcephaly, neuronal hyperexcitability, and epilepsy, established through patient variants and recapitulated by zebrafish morphants showing reduced head size and lowered seizure threshold and by neuronal Drosophila knockdown causing locomotor and wing-posture defects (PMID:37713627, PMID:28626029).

1. 2016 Medium

   Established that specific TRAPPC6B sequence variants disrupt normal transcript processing, providing the first molecular handle on how the gene is inactivated in disease.

   Evidence Minigene transfection splicing assay and RT-PCR of patient transcripts

   PMID:27569842

   Open questions at the time

   • Did not assess the consequence for TRAPP complex assembly or trafficking
   • No in vivo phenotype linked to the splicing defect
2. 2017 Medium

   Connected TRAPPC6B loss-of-function to a defined neurodevelopmental phenotype, demonstrating the gene is required for normal brain development and neuronal excitability.

   Evidence Patient fibroblast splicing assay plus zebrafish morpholino knockdown with microcephaly and seizure-threshold readouts

   PMID:28626029

   Open questions at the time

   • Morpholino knockdown susceptible to off-target effects, no rescue reported
   • Subcellular trafficking mechanism not addressed
   • Assigned to TRAPP I rather than resolving complex specificity
3. 2020 Low

   Implicated TRAPPC6B in an intracellular toxin trafficking route, hinting at the breadth of cargo whose transport depends on the protein.

   Evidence RNAi knockdown in ACHN renal epithelial cells with Shiga toxin 2a cytotoxicity readout

   PMID:32188865

   Open questions at the time

   • Single RNAi knockdown without rescue or reciprocal validation
   • No mechanistic placement within the trafficking pathway
   • Relevance to neuronal phenotype unestablished
4. 2024 High

   Resolved which TRAPP subcomplex TRAPPC6B belongs to and showed how a truncating variant destabilizes it, explaining the trafficking defect mechanistically.

   Evidence Co-IP comparing TRAPP II vs III association and against paralogue TRAPPC6A, Western blot quantification, Golgi trafficking/morphology assays with wild-type rescue in patient fibroblasts, plus Drosophila neuronal RNAi behavioral assays

   PMID:37713627

   Open questions at the time

   • No structural model of how TRAPPC6B is incorporated into TRAPP II
   • Mechanism by which TRAPP II loss produces neuronal hyperexcitability not defined
   • Identity of GTPase substrate/effectors at the Golgi not addressed
5. 2024 Medium

   Demonstrated functional conservation of TRAPPC6B from yeast to human, confirming the core trafficking role is preserved across species.

   Evidence CRISPR/Cas9 humanized yeast complementation of Trs33 deletion with growth readout

   PMID:39273027

   Open questions at the time

   • Complementation does not map the human-specific TRAPP II interactions
   • Does not address neuronal function directly

• How TRAPP II-dependent Golgi trafficking failure translates into microcephaly and neuronal hyperexcitability at the cellular level remains unresolved.
• No identified molecular link between Golgi fragmentation and neuronal excitability
• No structural basis for TRAPPC6B incorporation into TRAPP II
• Cargo spectrum requiring TRAPPC6B in neurons undefined