TRAPPC1

TRAPPC1 (also known as MUM-2/BET5) is a core subunit of the TRAPP (transport protein particle) complex that is essential for ER-to-Golgi vesicular trafficking and autophagy in eukaryotic cells. It physically associates with TRAPPC3 (BET3) and TRAPPC6 subunits through a defined structural interface involving the α1–α2 loop of TRAPPC6, and its function in vesicular transport is conserved from yeast to humans, as demonstrated by cross-species complementation (PMID:10582700, PMID:9611195, PMID:16828797). Loss of TRAPPC1 disrupts ER homeostasis and triggers the unfolded protein response, leading to ATF4-CHOP–mediated apoptosis, Ca²⁺-dependent mitochondrial stress, and ferroptosis in multiple immune cell lineages including thymic epithelial cells, myeloid progenitors, and naive T cells, resulting in immunodeficiency and autoinflammatory disease in mice (PMID:35908180, PMID:36440617, PMID:38234007). Patient-derived TRAPPC1 variants cause membrane trafficking defects and altered Golgi morphology that are rescued by wild-type TRAPPC1, establishing TRAPPC1 deficiency as a cause of human disease affecting secretion and autophagy (PMID:39273027).

1. 1998 High

   The fundamental question of what BET5 (TRAPPC1 ortholog) does was answered: it encodes an essential component of ER-to-Golgi transport that acts in conjunction with BET3, establishing the functional partnership at the heart of the TRAPP complex.

   Evidence High-copy suppressor screen, temperature-sensitive mutant analysis, and CPY/α-factor secretion assays in yeast

   PMID:9611195

   Open questions at the time

   • No structural information on how BET5 and BET3 interact
   • Mammalian function not yet tested
   • Role in autophagy not examined
2. 1999 High

   Cross-species conservation was established: human MUM-2 (TRAPPC1) complements yeast bet5 disruption, demonstrating that the ER-to-Golgi transport function is preserved across eukaryotes and connecting this trafficking gene to a human protein.

   Evidence Heterologous yeast complementation assay with human MUM-2/TRAPPC1 cDNA in bet5 deletion strain

   PMID:10582700

   Open questions at the time

   • No mammalian loss-of-function data
   • The identity of the mammalian TRAPP complex subunits interacting with TRAPPC1 was unknown
3. 2006 High

   The structural basis for TRAPPC1 incorporation into the TRAPP complex was resolved: crystallography and binding assays revealed that TRAPPC1 binds the α1–α2 loop of TRAPPC6 paralogs within a TRAPPC3–TRAPPC6 heterodimer scaffold.

   Evidence X-ray crystallography of human BET3–TPC6B heterodimer combined with pull-down assays mapping the TRAPPC1 binding site

   PMID:16828797

   Open questions at the time

   • Full TRAPP complex architecture with TRAPPC1 not resolved
   • No mutational validation of the interface in a cellular context
4. 2014 Medium

   Structural characterization of the related Bet5 family member Tca17 revealed the conserved longin fold shared by TRAPPC1 family subunits, providing a structural framework for understanding how these subunits engage other TRAPP components.

   Evidence X-ray crystallography of yeast Tca17 at 1.8 Å resolution with structural comparison to Bet5

   PMID:24961828

   Open questions at the time

   • Structure is of a paralog, not TRAPPC1 itself
   • Functional implications of the longin fold for TRAPPC1 activity not tested directly
5. 2022 High

   The in vivo consequences of TRAPPC1 loss were defined in immune cells: conditional knockout in thymic epithelial cells and myeloid progenitors revealed that TRAPPC1 deficiency causes ER/Golgi loss, UPR activation, ATF4-CHOP–driven apoptosis, ferroptosis, and immune cell developmental failure, linking vesicular trafficking to immune homeostasis.

   Evidence TEC-specific and inducible myeloid-specific Trappc1 conditional KO mice with RNA-seq, UPR pathway analysis, apoptosis/ferroptosis assays, and bone marrow chimeras

   PMID:35908180 PMID:36440617

   Open questions at the time

   • Whether TRAPPC1-dependent ferroptosis is a direct consequence of lipid trafficking defects or secondary to ER stress is not resolved
   • Non-immune tissue phenotypes not characterized
6. 2024 High

   TRAPPC1 deficiency in naive T cells was shown to drive ferroptotic cell death and release of DAMPs and lipid oxidation products that provoke neutrophilic autoinflammation, establishing a cell-extrinsic mechanism linking T cell trafficking defects to systemic inflammatory disease.

   Evidence CD4Cre-Trappc1 conditional KO mice, adoptive transfer, RNA-seq, lipid peroxidation and intracellular Ca²⁺ assays

   PMID:38234007

   Open questions at the time

   • The specific DAMP species most responsible for neutrophil activation not identified
   • Whether pharmacological inhibition of ferroptosis prevents disease not tested
7. 2024 High

   Human TRAPPC1 variants were shown to be pathogenic: a patient-derived frameshift variant is non-functional and a deletion variant is conditional-lethal, affecting both secretion and autophagy, with trafficking defects in patient fibroblasts rescued by wild-type TRAPPC1.

   Evidence CRISPR/Cas9 humanized yeast model, secretion and autophagy assays, patient fibroblast trafficking and Golgi morphology analysis with WT rescue

   PMID:39273027

   Open questions at the time

   • Full clinical spectrum of TRAPPC1 deficiency in humans not delineated
   • Structural impact of the p.His22_Lys24del variant on TRAPP complex assembly not determined

• It remains unknown how TRAPPC1 contributes to selective versus non-selective autophagy at a mechanistic level, whether its loss differentially affects TRAPP II versus TRAPP III complex functions in mammalian cells, and what the full phenotypic spectrum of human TRAPPC1 deficiency is.
• TRAPP II vs TRAPP III specificity of TRAPPC1 not resolved
• Structural model of full mammalian TRAPP complex with TRAPPC1 not available
• Genotype-phenotype correlations in human patients not established