TRAPPC4

TRAPPC4 is a core subunit of the TRAPP complex that functions as a guanine nucleotide exchange factor (GEF) for Rab1 GTPase, essential for ER-to-Golgi vesicular trafficking and autophagic flux; pathogenic loss of TRAPPC4 disrupts TRAPP complex assembly, delays cargo transport, and impairs autophagy (PMID:31794024). Beyond its canonical trafficking role, TRAPPC4 acts as a scaffold in recycling endosomes linking PD-L1 to RAB11, promoting RAB11-mediated recycling of PD-L1 to the tumor cell surface, with acetylation of PD-L1 enhancing this interaction (PMID:34518538, PMID:37603071). TRAPPC4 physically interacts with ERK2 and promotes ERK1/2 phosphorylation and nuclear translocation, regulating cell cycle progression and proliferation in colorectal cancer cells (PMID:21826244, PMID:23625650). TRAPPC4 also confers ferroptosis resistance by suppressing FOS-dependent TRIM55 transcription via chromatin accessibility changes, thereby preventing TRIM55-mediated ubiquitination and degradation of GPX4 (PMID:41974002).

1. 2011 Medium

   Identifying TRAPPC4 as a physical interactor of ERK2 that promotes nuclear pERK1/2 established a previously unknown link between a vesicular trafficking subunit and MAPK signaling in cancer cell proliferation.

   Evidence Yeast two-hybrid, Co-IP, and GST pull-down with knockdown/overexpression and subcellular fractionation in colorectal cancer cells

   PMID:21826244

   Open questions at the time

   • Interaction confirmed in a single lab; independent replication needed
   • Mechanism by which TRAPPC4 promotes ERK2 nuclear translocation is unresolved
   • Whether this function is independent of TRAPP complex membership is unknown
2. 2013 Medium

   Demonstrating that TRAPPC4 overexpression rescues ERK2-knockdown-induced G0/G1 arrest and modulates xenograft growth via nuclear pERK2 levels established an epistatic relationship between TRAPPC4 and ERK2 in cell cycle control.

   Evidence siRNA epistasis/rescue, FACS cell cycle analysis, and in vivo xenograft models in colorectal cancer cells

   PMID:23625650

   Open questions at the time

   • Single-lab study without independent validation
   • Direct biochemical mechanism of TRAPPC4 on ERK2 phosphorylation remains unknown
   • Relevance beyond colorectal cancer is unaddressed
3. 2020 High

   Establishing TRAPPC4 as the core TRAPP subunit required for Rab1 GEF activity, TRAPP complex stability, ER-to-Golgi trafficking, and autophagy defined its fundamental cell-biological function and linked its loss to a neurodevelopmental disease.

   Evidence Native PAGE, size exclusion chromatography, VSVG-GFP trafficking assay, lentiviral rescue, autophagy flux assays, and yeast trs23 complementation in patient fibroblasts

   PMID:31794024

   Open questions at the time

   • Structural basis for how TRAPPC4 contributes to GEF catalysis is not resolved
   • Whether TRAPP II and TRAPP III complexes are differentially affected is unclear
   • Full phenotypic spectrum of TRAPPC4 deficiency in humans remains incomplete
4. 2021 High

   Showing that TRAPPC4 scaffolds PD-L1 with RAB11 in recycling endosomes to drive PD-L1 surface recycling revealed a non-canonical trafficking role with direct implications for tumor immune evasion.

   Evidence Reciprocal Co-IP, live-cell imaging of recycling endosomes, knockdown/overexpression with surface PD-L1 readout, and in vivo tumor models

   PMID:34518538

   Open questions at the time

   • Whether TRAPPC4-mediated recycling extends to other immune checkpoint molecules is unknown
   • The structural basis of the TRAPPC4–PD-L1 interaction is unresolved
   • Contribution of TRAPP complex versus monomeric TRAPPC4 to this recycling function is unclear
5. 2023 Medium

   Demonstrating that PD-L1 acetylation enhances its interaction with TRAPPC4 and surface expression linked post-translational modification of cargo to TRAPPC4-dependent recycling.

   Evidence Co-IP and flow cytometry with HDAC inhibitor (VPA) treatment in pancreatic cancer cells

   PMID:37603071

   Open questions at the time

   • Single Co-IP method for the acetylation-dependent interaction change
   • Specific acetylation sites on PD-L1 mediating enhanced TRAPPC4 binding are not mapped
   • In vivo validation of acetylation-dependent recycling is lacking
6. 2026 High

   A CRISPR screen and multi-omic dissection revealed that TRAPPC4 confers ferroptosis resistance by closing chromatin at a TRIM55 regulatory element, reducing FOS-dependent TRIM55 transcription, and thereby preventing GPX4 ubiquitination and degradation — a mechanism entirely distinct from its trafficking roles.

   Evidence Genome-wide CRISPR screen, ATAC-seq, transcriptomics, ubiquitination assay, patient-derived organoids, CDX/PDX models, conditional knockout mice, and structure-based virtual screening in HNSCC

   PMID:41974002

   Open questions at the time

   • Mechanism by which TRAPPC4 modulates chromatin accessibility at the TRIM55 locus is not defined
   • Whether this chromatin-regulatory function operates in tissues beyond HNSCC is unknown
   • How TRAPPC4 integrates its trafficking and chromatin/ferroptosis functions remains unaddressed

• How TRAPPC4 coordinates its canonical TRAPP-dependent trafficking role with its non-canonical functions in ERK signaling, PD-L1 recycling, and chromatin-level ferroptosis regulation — and whether these represent context-dependent or concurrent activities — remains an open question.
• No structural model of TRAPPC4 within TRAPP II or TRAPP III at high resolution
• Whether monomeric TRAPPC4 acts independently of the TRAPP complex in signaling and chromatin contexts is untested
• Comprehensive interactome mapping beyond individual partner studies is lacking