TBC1D14 is a Rab-GAP TBC domain-containing protein that functions primarily as a RAB11-binding scaffolding protein rather than as a canonical GTPase-activating protein. Despite possessing the conserved TBC (Tre2-Bub2-Cdc16) domain, studies of mammalian orthologs demonstrate that TBC1D14 does not exhibit GAP activity toward RAB11 but instead acts as a RAB11 effector. The protein recruits the TRAPPIII tethering complex via a TRAPP-binding region, facilitating RAB1 activation and coordinating membrane exchange between RAB11-positive recycling endosomes and the early Golgi/ERGIC compartment. Through this scaffolding function, TBC1D14 regulates ATG9 trafficking and negatively regulates macroautophagy by controlling the delivery of recycling endosome-derived membranes to autophagosome formation sites. The protein localizes to recycling endosomes, the Golgi complex, and tubulo-vesicular transport intermediates. The Atlantic cod protein contains the conserved Rab-GAP TBC domain (aa 388-596) and a coiled-coil region, and is predicted to share these functions based on high sequence conservation across vertebrates.
Summary: This annotation assigns GTPase activator (GAP) activity based on the presence of the TBC domain (TreeGrafter inference). While the TBC domain is indeed a hallmark of Rab-GAP proteins, detailed studies of mammalian TBC1D14 orthologs (Lamb et al. 2016, EMBO J) show that TBC1D14 does NOT exhibit GAP activity toward its binding partner RAB11 and instead functions as a RAB11 effector/scaffold. The TBC domain may retain GAP activity toward an unidentified Rab substrate, or may have evolved a purely structural role. The IEA annotation based on domain presence alone is therefore potentially misleading, though not definitively wrong since GAP activity toward other Rab GTPases cannot be excluded.
Reason: The deep research clearly establishes that mammalian TBC1D14 does not function as a GAP for RAB11, its primary binding partner. However, GAP activity toward other Rab substrates has not been tested and cannot be excluded. The TreeGrafter inference from the TBC domain is a reasonable default but may be misleading. Without experimental data for the cod protein, the correct disposition is uncertain.
Summary: This ARBA annotation places TBC1D14 in the vacuole. In mammalian cells, TBC1D14 localizes to recycling endosomes and the Golgi complex; vacuolar localization has not been reported for TBC1D14 orthologs. The vacuole term (GO:0005773) in animal cells typically refers to lysosome-related compartments. Since fish cells are more similar to mammalian cells than to yeast in this regard, and there is no evidence placing TBC1D14 at the vacuole/lysosome, this appears to be an over-annotation likely arising from broad ARBA rule matching rather than specific functional evidence.
Reason: No evidence from mammalian ortholog studies supports vacuolar localization for TBC1D14. The protein localizes to recycling endosomes and Golgi, not to vacuoles/lysosomes. This ARBA IEA annotation appears to be an over-generalization.
Summary: This ARBA annotation assigns involvement in vesicle-mediated transport. This is well supported by ortholog studies: TBC1D14 functions at the interface of recycling endosome and Golgi trafficking, coordinates TRAPPIII-mediated RAB1 activation for membrane exchange, and regulates ATG9 vesicle cycling. The term is appropriate but broad -- TBC1D14 is specifically involved in endosomal-to-Golgi membrane trafficking and ATG9 vesicle cycling rather than vesicle-mediated transport in general.
Reason: Vesicle-mediated transport is well established for TBC1D14 through its role in coordinating membrane exchange between recycling endosomes and the Golgi, and in maintaining the ATG9 vesicle cycling pool. The term is somewhat broad but accurate.
Summary: This ARBA annotation assigns enzyme binding activity. TBC1D14 does bind enzymes -- it interacts with ULK1 (the autophagy-initiating kinase) and with small GTPases (RAB11, which has GTPase activity). However, enzyme binding (GO:0019899) is an uninformative term per GO curation guidelines, similar to protein binding. More specific terms such as small GTPase binding (GO:0031267, already annotated) better capture the actual binding interactions.
Reason: While TBC1D14 does bind enzymes (RAB11 GTPase, ULK1 kinase), this is already captured more informatively by the small GTPase binding annotation. The generic enzyme binding term adds no biological insight.
Summary: This TreeGrafter annotation assigns small GTPase binding, which is well supported by ortholog studies. TBC1D14 binds to the active (GTP-bound) form of RAB11, a small GTPase of the Rab family, and this interaction is central to its function in recruiting TBC1D14 to recycling endosome membranes. TBC1D14 also colocalizes with RAB1B at the Golgi. Small GTPase binding is arguably the most accurate molecular function term for TBC1D14 given that it functions as a RAB11 effector rather than a GAP.
Reason: RAB11 binding is the best-characterized molecular function of TBC1D14. The protein binds GTP-RAB11 as an effector and also associates with RAB1B. Small GTPase binding accurately captures this core activity.
Summary: This ARBA annotation places TBC1D14 at cytoplasmic vesicles. This is consistent with ortholog data showing localization to recycling endosomes (which are a subtype of cytoplasmic vesicle) and to tubulo-vesicular transport intermediates between recycling endosomes and the Golgi. The term is accurate but could be more specific -- recycling endosome (GO:0055037) would be the most informative localization term based on mammalian data.
Reason: TBC1D14 localizes to recycling endosomes, which are cytoplasmic vesicles. The annotation is correct; recycling endosome (GO:0055037) is a child of cytoplasmic vesicle (GO:0031410), so this annotation is accurate if not maximally specific.
Core Functions
TBC1D14 binds the GTP-bound (active) form of RAB11, functioning as a RAB11 effector rather than a GTPase-activating protein. This interaction recruits TBC1D14 to RAB11-positive recycling endosome membranes. Despite containing the conserved TBC domain typically associated with Rab-GAP activity, TBC1D14 does not catalyze GTP hydrolysis on RAB11 and instead serves as a scaffold that coordinates membrane trafficking events at the recycling endosome-Golgi interface.
Q: Does cod TBC1D14 retain any GAP activity toward Rab GTPases other than RAB11? Mammalian TBC1D14 does not act as a GAP for RAB11 but other Rab substrates have not been systematically tested.
Q: Is the TRAPPIII interaction conserved in teleost fish? The TRAPP-binding region (aa 120-223 in mammalian TBC1D14) mediates the key scaffolding function, but conservation of this interaction in cod has not been verified.
Suggested Experiments
Experiment: Co-immunoprecipitation or pulldown experiments with recombinant cod TBC1D14 and cod RAB11 to confirm the Rab effector interaction is conserved in teleost fish.
Hypothesis: Cod TBC1D14 binds active RAB11 as a Rab effector, similar to the mammalian ortholog.
Experiment: In vitro GAP assay testing cod TBC1D14 TBC domain against a panel of Rab GTPases to determine if it has GAP activity toward any Rab family member.
Hypothesis: The TBC domain of cod TBC1D14 may retain GAP activity toward specific Rab substrates despite lacking activity toward RAB11.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
The sequence relationship to vertebrate TBC1D14 and a verified experimental localization annotation on human TBC1D14 support inferred autophagosome association. This need not be the protein's predominant compartment.
Prediction method: ProtNLM2 · Version: UniProt 2024_06 pilot
Review rationale: The target has TBC1D14-like architecture and a strong phmmer match to reviewed bovine TBC1D14 A6H7I8 (reported score 791.8 in the raw evidence record). QuickGO records human TBC1D14 Q9P2M4 at autophagosomes with an IDA assigned by BHF-UCL from PMID:22613832, alongside a curated IBA; these are verified annotation records rather than an AI verdict. The paper emphasizes recycling endosomes and Golgi redistribution, so the localization should not be read as exclusive or constitutive autophagosome residence. The conserved sequence relationship and experimentally grounded curator annotation support broad localization transfer to cod despite the differing PANTHER subfamily label. The target itself lacks this term in cached GOA/UniProt, making this a supported additional annotation, not a target experiment or an existing target match.
Supporting Evidence:
file:GADMO/A0A8C5FPT8/A0A8C5FPT8-uniprot.txt: "ID A0A8C5FPT8_GADMO Unreviewed; 709 AA. ... DR InterPro; IPR000195; Rab-GAP-TBC_dom. ... DR PANTHER; PTHR47219:SF15; TBC1 DOMAIN FAMILY MEMBER 12 ISOFORM X1; 1. ... FT DOMAIN 388..596 ... FT /note="Rab-GAP TBC""
PMID:22613832: "TBC1D14 binds activated Rab11 but is not a GAP for Rab11"