TRAPPC13

TRAPPC13 is a TRAPPIII-specific subunit of the mammalian TRAPP complex that, together with TRAPPC8, TRAPPC11, and TRAPPC12, contributes to a guanine nucleotide exchange factor (GEF) activity directed specifically at Rab1 and Rab43, with no detectable activity against 18 other Rabs and enhanced exchange on lipid membranes (PMID:34229011). Through this Rab1-activating function, TRAPPC13 supports ER-to-Golgi trafficking, autophagic flux, and the cellular survival response to Golgi-disrupting stress: its depletion lowers Rab1a/Rab1b activity, impairs autophagy, and increases susceptibility to Shigella flexneri infection, while paradoxically preserving the secretory pathway and viability after brefeldin A treatment in a GBF1/ARF1-dependent manner with reduced caspase activation and ER stress (PMID:28536105). The TRAPPIII-specific role in Rab1 activation is conserved in fungi, where constitutively active Rab1-GTP rescues TRAPPIII-mutant defects, and where TRAPPC13 cooperates with the core subunit Trs85 to drive autophagosome biogenesis by recruiting Atg9 to the phagophore assembly site (PMID:31869332, PMID:41134853). Beyond these functions, the structural basis of how TRAPPC13 specifically tunes Rab-site dynamics and its reported enrichment at the base of primary cilia have not been mechanistically resolved in the available corpus.

1. 2016 Low

   Established a candidate subcellular address for TRAPPC13, placing it at the ciliary base/centrosomal region and raising the question of a ciliary role distinct from bulk Golgi trafficking.

   Evidence epitope-tagged overexpression and quantitative immunofluorescence in hTERT-RPE1 cells

   PMID:27514913

   Open questions at the time

   • Overexpression-based localization only, not validated at endogenous levels
   • No functional consequence established for the ciliary localization
   • Relationship between ciliary-base pool and TRAPPIII trafficking function unknown
2. 2017 Medium

   Defined TRAPPC13 as a TRAPPIII subunit with a measurable cellular function, linking it to Rab1 activation, autophagic flux, and a survival response to Golgi stress.

   Evidence siRNA knockdown with Rab1a/Rab1b activity assays, autophagy flux, caspase/ER-stress readouts, and Shigella infectivity in human cells

   PMID:28536105

   Open questions at the time

   • Did not establish direct GEF activity biochemically
   • Mechanism by which TRAPPC13 loss preserves viability via GBF1/ARF1 not resolved
   • Single lab, knockdown-based
3. 2019 Medium

   Used fungal genetics to show that the essential role of TRAPPIII (containing the TRAPPC11/12/13 module) is activation of Rab1, and that these metazoan-type subunits are recruited via Tca17/TRAPPC2L.

   Evidence size-fractionation, MS, negative-stain EM, and constitutively-active RAB1* epistasis in Aspergillus nidulans null mutants

   PMID:31869332

   Open questions at the time

   • Specific contribution of TRAPPC13 versus other subunits to GEF activity not isolated
   • Conducted in fungal ortholog system
   • Subunit recruitment hierarchy in mammalian complex not directly tested
4. 2021 High

   Provided direct biochemical proof that the reconstituted TRAPPIII complex is a Rab1/Rab43-specific GEF and that complex-specific subunits alter dynamics at the Rab binding site.

   Evidence in vitro GEF assays against a panel of 20 Rabs, HDX-MS, EM, and membrane-reconstituted complexes

   PMID:34229011

   Open questions at the time

   • The individual catalytic contribution of TRAPPC13 within the complex was not isolated
   • No high-resolution structure of the TRAPPC13-containing complex with Rab
   • Physiological substrate selection between Rab1 and Rab43 in cells not addressed
5. 2025 Medium

   Connected TRAPPIII Rab1 activation to a concrete autophagy step, showing TRAPPC13 cooperates with Trs85 to recruit Atg9 to the phagophore assembly site.

   Evidence genetic deletion, localization, interaction assays, and constitutively-active Rab1-GTP rescue in Fusarium graminearum

   PMID:41134853

   Open questions at the time

   • Direct TRAPPC13-Atg9 interaction versus indirect recruitment not distinguished
   • Conducted in fungal ortholog system
   • Whether mammalian TRAPPC13 recruits ATG9 by the same mechanism untested

• How TRAPPC13 specifically shapes Rab-site dynamics within TRAPPIII and whether its ciliary-base localization reflects a distinct function remain unresolved.
• No structure of TRAPPC13 within the assembled mammalian TRAPPIII-Rab complex
• Functional role of ciliary-base localization uncharacterized
• Per-subunit dissection of GEF catalysis not performed