TBC1D23

TBC1D23 is a catalytically inactive TBC-domain protein that acts as a vesicle-golgin bridging adaptor mediating endosome-to-trans-Golgi network (TGN) trafficking (PMID:29084197). Its N-terminus comprises a TBC domain packed against a catalytically inactive rhodanese domain, which together form a platform that binds the coiled-coil golgins golgin-97 and golgin-245 at the TGN (PMID:32453802), while its C-terminal domain adopts a Pleckstrin Homology (PH) fold that engages PtdIns(4)P on one surface and the WASH-complex subunit FAM21 on the opposite surface to capture endosome-derived vesicles (PMID:29084197, PMID:31624125). The same C-terminal domain directly recognizes a threonine-leucine-tyrosine (TLY) motif adjacent to an acidic cluster in the cytoplasmic tails of cargo proteins such as carboxypeptidase D and syntaxin-16, conferring cargo-selective vesicle tethering (PMID:38552021), and TBC1D23 cooperates with its direct binding partner FAM91A1 to route specific cargo including KIAA0319L (PMID:37903274). Beyond trafficking, TBC1D23 directly binds LKB1 and recruits it to the Golgi to enable Golgi-localized, energy-stress-induced AMPK activation (PMID:38413626), and it sustains EGFR recycling and downstream ERK signaling to promote VEGF-C-driven lymphangiogenesis in pancreatic ductal adenocarcinoma (PMID:41920243). Correct Golgi localization is essential for all of these functions: homozygous truncating mutations that mislocalize the protein to the cytoplasm impair endosome-to-Golgi and lysosomal trafficking, disrupt cortical neuron positioning, and cause pontocerebellar hypoplasia (PMID:28823707, PMID:40581672).

1. 2012 Medium

   Established an early functional readout for TBC1D23, placing it as a RAB-GAP-domain protein that restrains innate immune signaling, before its trafficking role was understood.

   Evidence Tbc1d23 knockout mice and macrophage overexpression with epistasis mapping against MyD88, Trif, and XBP1 under TLR stimulation

   PMID:22312129

   Open questions at the time

   • Did not connect the immune phenotype to a molecular trafficking mechanism
   • Whether the TBC domain has genuine GAP catalytic activity was not resolved here
   • No structural basis for the proposed activity
2. 2017 High

   Defined TBC1D23 as a vesicle-golgin bridging adaptor, answering how endosome-derived vesicles are tethered to the trans-Golgi.

   Evidence Proximity biotinylation of golgin-captured vesicles, Co-IP, knockdown trafficking assays, and an ectopic golgin vesicle-capture system

   PMID:29084197

   Open questions at the time

   • Atomic basis of golgin and WASH-complex binding not yet resolved
   • Did not identify the cargo-recognition determinant
   • Catalytic status of the TBC domain unaddressed
3. 2017 Medium

   Linked TBC1D23 loss to a human disease mechanism by showing truncating mutations disrupt vesicle/lysosomal trafficking and neuronal positioning.

   Evidence Patient-derived fibroblast trafficking assays and in utero electroporation knockdown in mouse cortex

   PMID:28823707

   Open questions at the time

   • Did not establish why truncation abolishes function at the protein level
   • Single lab; in vivo neuronal phenotype not mechanistically dissected
4. 2019 High

   Resolved the C-terminal domain as a PH fold with two distinct binding faces, explaining how TBC1D23 simultaneously reads membrane lipid identity and the WASH complex.

   Evidence Crystal structure of the C-terminal domain, phosphoinositide binding assays, FAM21 Co-IP, and zebrafish neuronal development assays

   PMID:31624125

   Open questions at the time

   • Did not yet identify a direct cargo-binding site
   • How the N-terminus engages golgins remained structurally undefined
5. 2020 High

   Showed the N-terminal TBC-rhodanese module is catalytically inactive and instead forms a golgin-binding platform, redefining TBC1D23 as a scaffold rather than an enzyme.

   Evidence Crystal structure of the N-terminal TBC+rhodanese domains with catalytic-site versus golgin-surface mutagenesis and zebrafish developmental assays

   PMID:32453802

   Open questions at the time

   • Does not reconcile the earlier proposed RAB-GAP immune function with loss of catalysis
   • Golgin-binding surface mapped but full ternary tethering geometry unresolved
6. 2023 High

   Identified FAM91A1 as a direct structural partner, showing TBC1D23 functions within a cooperative complex to route specific cargo.

   Evidence Crystal structure of the FAM91A1-TBC1D23 complex, Co-IP, KIAA0319L trafficking assays, and zebrafish developmental assays

   PMID:37903274

   Open questions at the time

   • Cargo selectivity determinant still unidentified at this stage
   • How FAM91A1 engagement is coordinated with golgin and FAM21 binding unknown
7. 2024 High

   Established the molecular basis of cargo-selective tethering by showing the C-terminal domain reads an acidic TLY motif in cargo cytoplasmic tails.

   Evidence Binding partner screen, direct binding assays, crystal structure of the C-terminal domain bound to the acidic TLY motif, and structure-guided in vivo vesicle-capture assays

   PMID:38552021

   Open questions at the time

   • Full repertoire of TLY-motif cargo not enumerated
   • How lipid, FAM21, and cargo binding are spatially partitioned on one PH domain not fully resolved
8. 2024 Medium

   Extended TBC1D23 function beyond trafficking by showing it recruits LKB1 to the Golgi to enable compartment-specific AMPK activation.

   Evidence Co-IP of direct TBC1D23-LKB1 interaction, Golgi-targeted LKB1 rescue in deficient zebrafish, and AMPK activation assays under energy stress

   PMID:38413626

   Open questions at the time

   • Structural basis of the LKB1 interaction not determined
   • Relationship between the LKB1-AMPK role and the tethering role unclear
9. 2025 Medium

   Demonstrated that Golgi localization itself is required for function and that cytoplasmic mislocalization of a stable truncated protein is cytotoxic.

   Evidence Patient variant characterization, NMD assay, immunofluorescence localization, cell proliferation assay, and zebrafish model

   PMID:40581672

   Open questions at the time

   • Single lab; mechanism of cytoplasmic toxicity not defined
   • Does not distinguish loss-of-function from gain-of-toxicity contributions to disease
10. 2026 Medium

    Implicated TBC1D23 in disease beyond neurodevelopment by showing it sustains EGFR recycling and signaling to drive lymphangiogenesis in pancreatic cancer.

    Evidence siRNA knockdown in PDAC lines, EGFR trafficking and ERK signaling readouts, VEGF-C secretion assays, and orthotopic xenograft model

    PMID:41920243

    Open questions at the time

    • Whether EGFR is a direct TLY-motif cargo not established
    • Single-lab finding; generality across tumor types untested

• How the catalytically inactive scaffold integrates its multiple binding activities (golgin platform, lipid, FAM21, FAM91A1, cargo TLY motif, LKB1) into a single coordinated tethering and signaling event remains unresolved.
• No integrated structure of TBC1D23 simultaneously engaging vesicle and golgin
• Reconciliation of the early RAB-GAP immune function with later evidence of catalytic inactivity is incomplete
• Whether cargo-selectivity rules generalize across all TBC1D23-dependent trafficking is unknown