TRAPPC2

TRAPPC2 (SEDL/Sedlin) is a small conserved subunit of the TRAPP complex that functions as a structural adaptor governing assembly of higher-order TRAPP complexes that direct membrane traffic and autophagy (PMID:21858081, PMID:23465091). In mammalian cells it directly binds the TRAPPII-specific subunit TRAPPC9 and, mutually exclusively, the TRAPPIII-specific subunit TRAPPC8, thereby templating formation of either complex (PMID:21858081); its yeast ortholog Trs20 is correspondingly required for TRAPPII assembly through interaction with Trs120 and for the resulting Ypt32-directed guanine-nucleotide exchange activity (PMID:23465091), and for recruitment of core TRAPP to the pre-autophagosomal structure via the TRAPPIII subunit Trs85 (PMID:24329977). Consistent with an adaptor role, its crystal structure resembles the regulatory domains of SNAREs despite lacking sequence homology (PMID:12361953), and it interacts with the SNARE Syntaxin 5 (PMID:23898804). The protein localizes to perinuclear membranes overlapping the ER-Golgi intermediate compartment (PMID:11031107), and loss of TRAPPC2 function reduces type II collagen (COL2A1) expression and secretion in chondrocytes (PMID:37693308) and blocks both selective and non-selective autophagy in yeast (PMID:23898804, PMID:24329977). Loss-of-function mutations in TRAPPC2 cause X-linked spondyloepiphyseal dysplasia tarda (SEDT) (PMID:10431248); the surface mutation D47Y (yeast D46Y) selectively abolishes the adaptor interactions with TRAPPC9/TRAPPC8 (Trs120/Trs85) and weakens Syntaxin 5 binding while sparing core TRAPP, defining a disease mechanism centered on disrupted TRAPPII/III assembly (PMID:21858081, PMID:23465091, PMID:23898804).

1. 1999 Medium

   Establishing the genetic basis of an X-linked skeletal dysplasia first implicated this gene in vesicular transport, before any molecular function was known.

   Evidence positional cloning and mutation analysis in SEDT families with cartilage expression analysis

   PMID:10431248

   Open questions at the time

   • Inferred ER-to-Golgi transport role was not biochemically tested
   • No protein partners identified
   • No structural information
2. 2000 Medium

   Subcellular localization placed the protein at the ER-Golgi interface and showed disease mutations mislocalize it, linking trafficking compartment to function.

   Evidence transient transfection of tagged constructs in COS-7 cells with fluorescence microscopy

   PMID:11031107

   Open questions at the time

   • Partial ERGIC overlap only; precise residence not resolved
   • Overexpression of tagged constructs may not reflect endogenous distribution
   • Mechanism of C-terminal targeting unknown
3. 2001 Low

   Patient cartilage ultrastructure provided in vivo evidence that loss of function perturbs the secretory pathway in the disease-relevant cell type.

   Evidence histological/ultrastructural analysis of SEDT patient articular cartilage

   PMID:11326333

   Open questions at the time

   • Single patient pathological observation without biochemical assay
   • Cannot distinguish direct from secondary effects on ER/Golgi morphology
4. 2002 High

   The crystal structure revealed an unexpected SNARE-like fold and rationalized disease mutations as either structure-disrupting or surface interaction-disrupting, predicting an adaptor/protein-interaction function.

   Evidence X-ray crystallography at 2.4 A with disease-mutation mapping

   PMID:12361953

   Open questions at the time

   • Predicted partner protein for the D47Y surface not identified in this work
   • Functional consequence of the SNARE-like fold untested
5. 2003 Medium

   Cross-species complementation established functional conservation with yeast Trs20 and parsed which disease residues affect core TRAPP function versus other activities.

   Evidence yeast trs20 knockout complementation with multiple human disease alleles

   PMID:14597397

   Open questions at the time

   • Does not define the molecular interactions affected by non-complementing alleles
   • Yeast viability readout does not resolve which trafficking step is impaired
6. 2011 High

   Defining direct binding to TRAPPC9 and TRAPPC8 identified TRAPPC2 as the mutually exclusive adaptor for mammalian TRAPPII versus TRAPPIII assembly and pinpointed Asp47 as the critical interaction residue.

   Evidence reciprocal Co-IP of endogenous and overexpressed TRAPP subunits with disease-mutant mapping in mammalian cells

   PMID:21858081

   Open questions at the time

   • Structural basis of mutual exclusivity not resolved
   • Functional consequence for mammalian Rab-GEF activity not measured here
7. 2013 High

   Yeast work connected the adaptor role to enzymatic output, showing Trs20-Trs120 interaction is required for TRAPPII assembly and its Ypt32-GEF activity, which the D46Y mutation selectively abolishes while sparing TRAPPI.

   Evidence Co-IP and in vitro Ypt32-GEF assay with D46Y mutant in yeast

   PMID:23465091

   Open questions at the time

   • Mammalian GEF activity not directly tested
   • Whether D47Y similarly affects mammalian Rab substrates not shown here
8. 2013 High

   TRAPPC2 was linked to autophagy and to SNARE binding, with D47Y weakening Syntaxin 5 interaction and the yeast equivalent destabilizing TRAPPIII to block selective and non-selective autophagy.

   Evidence mammalian Co-IP of TRAPPC2-Syntaxin 5, yeast size-exclusion chromatography and autophagy assays with D46Y

   PMID:23898804

   Open questions at the time

   • Functional role of mammalian Syntaxin 5 binding in autophagy not established
   • Anterograde traffic was unaffected by D46Y in yeast, leaving the secretion defect mechanism in disease cells unresolved
9. 2014 High

   Reconstitution and live-cell imaging showed Trs20 mediates recruitment of core TRAPP to the pre-autophagosomal structure via Trs85, providing a spatial mechanism for its autophagy function.

   Evidence in vitro reconstitution with recombinant proteins, co-precipitation, live-cell colocalization, and autophagy assays in trs20ts yeast

   PMID:24329977

   Open questions at the time

   • PAS recruitment demonstrated in yeast; mammalian autophagosome recruitment not directly shown
   • Rab substrate engaged at the PAS not defined here
10. 2023 Medium

    Loss-of-function in chondrocytes tied TRAPPC2 directly to the disease-relevant cargo, reducing COL2A1 expression and collagen II secretion.

    Evidence siRNA knockdown in SW1353 cells and primary human chondrocytes with Western blot/ELISA, plus characterization of a nonsense variant

    PMID:37693308

    Open questions at the time

    • Whether reduced secretion reflects TRAPPII or TRAPPIII dysfunction not dissected
    • Single lab; mechanism linking TRAPP adaptor role to collagen trafficking not resolved
11. 2024 Medium

    A humanized yeast model confirmed cross-species functional replacement, providing a tractable platform for variant analysis.

    Evidence CRISPR/Cas9 humanization replacing yeast TRS20 with human TRAPPC2 and viability assay (preprint)

    PMID:bio_10.1101_2024.08.04.605925

    Open questions at the time

    • Preprint, single study
    • Does not itself define disease variant mechanisms

• How disrupted TRAPPII/III assembly in human chondrocytes mechanistically produces the specific procollagen secretion defect and skeletal phenotype remains unresolved.
• No direct demonstration that mammalian D47Y impairs Rab-GEF activity in chondrocytes
• Mammalian Rab substrate(s) downstream of TRAPPC2 not identified in the corpus
• Connection between Syntaxin 5 binding and procollagen export untested