TRAPPC8

TRAPPC8 (yeast Trs85) is the TRAPPIII-specific subunit of the multi-subunit TRAPP tethering complex, where it couples membrane vesicle recognition to activation of the Rab GTPase Ypt1/Rab1 to drive autophagosome formation (PMID:23129774, PMID:16079147). In yeast it directly binds Atg9 and is itself a component of Atg9 vesicles, recruiting Ypt1 onto these vesicles and localizing with Ypt1 to the preautophagosomal structure in an Atg9-dependent manner (PMID:23129774); consistent with this tethering role, Trs85 is selectively required for macroautophagy, pexophagy, and the cytoplasm-to-vacuole targeting (Cvt) pathway and for organization of the PAS, without affecting ER-to-Golgi trafficking or vacuolar biogenesis (PMID:16079147, PMID:16874038). Trs85/TRAPPIII acts upstream of Ypt1-mediated vesicular trafficking more broadly, since its loss corrects endosomal mislocalization of the v-SNARE Snc1 and rescues membrane and cell-wall integrity defects in sphingolipid-deficient cells, whereas excess Ypt1 worsens them (PMID:40266832). In mammalian cells TRAPPC8 carries a C-terminal ASH domain and an N-terminal α-solenoid/TPR region and localizes to the centrosome/basal body, where it supports ciliogenesis by enabling Rabin8 centrosome targeting and by promoting OFD1–PCM1 association, an interaction held in balance with a competing TRAPPC12–OFD1 interaction (PMID:25018876, PMID:32258032). TRAPPC8 additionally binds the HPV L2 capsid protein and is required for viral endocytic entry and for maintenance of Golgi stack integrity (PMID:24244674).

1. 2005 High

   Established that Trs85 is a dedicated autophagy factor rather than a general secretion component, defining the entry point for TRAPP function in selective and bulk autophagy.

   Evidence Gene deletion with GFP-Atg8 localization, electron microscopy, and vacuolar protease maturation assays in yeast, plus parallel genetic screens in two yeast species

   PMID:16079147 PMID:16874038

   Open questions at the time

   • Molecular partner on the vesicle not yet identified
   • Did not define which Rab Trs85 acts through
   • No structural basis for autophagy-specific function
2. 2012 High

   Resolved the molecular mechanism by which Trs85 acts in autophagy: it binds Atg9 directly and recruits the Rab Ypt1 onto Atg9 vesicles, establishing TRAPPIII as a vesicle-tethering machinery for autophagosome formation.

   Evidence Atg9 vesicle purification with mass spectrometry, direct interaction/co-immunoprecipitation assays, and fluorescence localization in yeast

   PMID:23129774

   Open questions at the time

   • Structure of the Trs85-Atg9 interface not determined
   • Mechanism of Ypt1 nucleotide exchange by TRAPPIII not dissected here
   • Conservation of the Atg9 interaction in mammalian TRAPPC8 untested
3. 2013 Medium

   Extended TRAPPC8 function beyond autophagy by showing it mediates HPV endocytic entry and maintains Golgi integrity in human cells.

   Evidence Co-IP of TRAPPC8–L2, siRNA knockdown with reporter transduction and authentic HPV31 infection, and Golgi morphology imaging in HeLa/HaCaT cells

   PMID:24244674

   Open questions at the time

   • Whether the L2 interaction reflects TRAPP complex function or a moonlighting role unclear
   • How TRAPPC8 loss disperses the Golgi mechanistically not defined
   • Single lab; surface exposure of TRAPPC8 not independently confirmed
4. 2014 Medium

   Defined the domain architecture of human TRAPPC8 and placed it at the centrosome/basal body, linking it to ciliogenesis through Rabin8 targeting.

   Evidence Bioinformatic domain prediction, immunofluorescence of endogenous/exogenous protein, and siRNA knockdown with ciliogenesis and Rabin8-targeting readouts

   PMID:25018876

   Open questions at the time

   • Direct binding partner at the centrosome not identified
   • Role of the ASH domain in localization not functionally tested
   • Single-lab observation
5. 2020 Medium

   Provided a mechanism for TRAPPC8's ciliary role: it promotes OFD1–PCM1 association and competes with TRAPPC12 for OFD1 binding, explaining subunit-specific cilium-length phenotypes.

   Evidence Reciprocal Co-IP (TRAPPC8–OFD1, TRAPPC12–OFD1), siRNA knockdown, and immunofluorescence of OFD1/PCM1 colocalization and cilium length

   PMID:32258032

   Open questions at the time

   • Structural basis of the mutually inhibitory OFD1 binding not resolved
   • Whether this requires intact TRAPP complex unknown
   • Single lab
6. 2025 Medium

   Positioned Trs85/TRAPPIII upstream of Ypt1-dependent trafficking at endosomes, broadening its role beyond autophagosome formation to membrane and cell-wall homeostasis.

   Evidence Suppressor screen, gene deletion, yeGFP-Snc1 localization, Ypt1 overexpression epistasis, and stress-sensitivity assays in yeast

   PMID:40266832

   Open questions at the time

   • Direct molecular target at the endosome not identified
   • Relevance of this endosomal role to mammalian TRAPPC8 untested
   • Single lab

• Whether the autophagy/Atg9-tethering, ciliary/centrosomal, and HPV-entry functions of TRAPPC8 reflect a single TRAPPIII-dependent activity or distinct moonlighting roles remains unresolved.
• No structure of human TRAPPC8 within TRAPPIII
• Direct mammalian Atg9 interaction not demonstrated
• Mechanism connecting Golgi maintenance, ciliogenesis, and autophagy unknown