EXOC2

EXOC2 (Sec5) is a core subunit of the octameric exocyst complex that tethers secretory vesicles to the plasma membrane, mediating polarized exocytosis, endocytic recycling, and primary ciliogenesis across diverse cell types. Its N-terminal immunoglobulin/IPT-fold domain directly binds active RalA and RalB GTPases, competing with the exocyst subunit Exo84 for the same RalA interface, thereby enabling GTPase-dependent regulation of vesicle trafficking for cargo including cadherins, integrins, and insulin granules (PMID:12839989, PMID:15920473, PMID:16224820, PMID:23844030). Beyond canonical vesicle tethering, EXOC2 serves as a signaling scaffold: RalB-dependent Sec5–TBK1 interaction activates innate immune signaling, Sec5 binding to InsP3R on phagosomes promotes Ca²⁺ release and type I interferon responses, and Sec5 interaction with STAT6 regulates macrophage M2 polarization (PMID:18413258, PMID:29703257, PMID:36313547). Biallelic truncating EXOC2 variants in humans cause severe neurodevelopmental brain malformations with loss of exocytosis and defective Arl13b-dependent ciliogenesis (PMID:32639540).

1. 2002 Medium

   The identification of DelGEF as a nucleotide-dependent Sec5-binding partner provided early evidence that EXOC2 integrates GTPase-regulatory inputs beyond the Ral family to modulate secretion.

   Evidence Yeast two-hybrid screen plus biochemical interaction assays and siRNA knockdown measuring proteoglycan secretion in HeLa cells

   PMID:12459492

   Open questions at the time

   • DelGEF–Sec5 interaction not validated by reciprocal Co-IP or structural methods
   • physiological secretory cargo regulated by this interaction not defined
   • relationship to Ral-dependent Sec5 regulation unclear
2. 2003 High

   Structural determination of the Sec5 Ral-binding domain revealed an unprecedented immunoglobulin/IPT fold for a GTPase effector and defined the atomic basis of the RalA–Sec5 interface, establishing how the exocyst is recruited by active Ral GTPases.

   Evidence X-ray crystallography (2.1 Å) and NMR of the Sec5 RBD–RalA complex with ITC and mutagenesis validation

   PMID:12624092 PMID:12839989

   Open questions at the time

   • structure of full-length Sec5 within the assembled exocyst not resolved
   • relative contributions of RalA vs. RalB to Sec5 engagement in vivo not quantified
3. 2003 High

   Drosophila Sec5 null mutants established that Sec5 is essential for biosynthetic membrane addition and neurite outgrowth but dispensable for synaptic vesicle fusion, distinguishing exocyst-dependent from SNARE-only trafficking pathways in neurons.

   Evidence Drosophila sec5 null alleles with neuromuscular junction electrophysiology and membrane trafficking assays

   PMID:12575951 PMID:14681190

   Open questions at the time

   • whether Sec5 independence of synaptic vesicle fusion extends to mammalian neurons not tested
   • identity of the tethering factor for synaptic vesicles remains separate
4. 2005 High

   Structural and biochemical demonstration that Exo84 and Sec5 compete for the same RalA surface established a regulatory switch model in which a single Ral GTPase toggles between two exocyst sub-complexes, and genetic studies revealed Sec5's role in cadherin recycling from Rab11 endosomes and in endocytic recycling at clathrin-coated structures.

   Evidence Crystal structure of Exo84 RBD–RalA with competitive binding assays; Drosophila LOF clones with cadherin trafficking analysis; sec5 truncation allele with EM localization to clathrin pits

   PMID:15920473 PMID:15955846 PMID:16224820

   Open questions at the time

   • how competitive Ral binding to Sec5 vs. Exo84 is regulated temporally in a single trafficking event not resolved
   • direct role of Sec5 in clathrin-mediated steps vs. post-endocytic recycling not distinguished
5. 2008 Medium

   Discovery that RalB activation directs Sec5 to bind and activate TBK1 kinase revealed a non-canonical exocyst function in innate immune signaling and oncogenic cell survival, expanding Sec5's role beyond vesicle tethering.

   Evidence Co-IP, in vitro kinase assays, RNAi in mammalian cells, viral infection model

   PMID:18413258

   Open questions at the time

   • structural basis of Sec5–TBK1 interaction not determined
   • whether TBK1 activation requires assembled exocyst or only the Sec5 subunit not resolved
6. 2008 Medium

   Sec5 was shown to mediate Ral-dependent delivery of α5-integrin to focal complexes, linking exocyst trafficking to cell migration and tumor invasion.

   Evidence RNAi, dominant-negative Sec5 mutants, integrin surface trafficking, and invasion assays in prostate cancer cells

   PMID:18697830

   Open questions at the time

   • specific vesicle population carrying integrins to focal adhesions not characterized
   • in vivo relevance for metastasis not tested
7. 2010 High

   Temperature-sensitive sec5 alleles demonstrated that Sec5 is required for cleavage furrow membrane insertion during Drosophila cellularization, establishing its role in the earliest polarized membrane addition events during epithelial morphogenesis.

   Evidence Drosophila ts allele germline clones with live imaging and Neurotactin trafficking

   PMID:20630948

   Open questions at the time

   • how Sec5 is recruited to the apical lateral membrane domain during cellularization not defined
8. 2013 Medium

   TIRF microscopy and electrophysiology showed that Sec5 localizes to insulin granules and preferentially promotes newcomer granule exocytosis, explaining how the exocyst contributes to biphasic glucose-stimulated insulin secretion.

   Evidence TIRF live imaging, patch-clamp capacitance, siRNA knockdown in pancreatic β cells

   PMID:23844030

   Open questions at the time

   • whether Sec5 interacts directly with insulin granule-resident SNAREs not tested
   • contribution relative to other exocyst subunits in β cells not compared
9. 2015 Medium

   Identification of the Sec5–GEF-H1 interaction downstream of SGK1 revealed a mechanism by which Sec5 delivers RhoA-activating GEF to peripheral focal adhesions, controlling cellular tension and fibronectin fibril formation.

   Evidence Co-IP, interaction-disrupting constructs, traction force microscopy, immunofluorescence in mesenchymal stem cells

   PMID:26359301

   Open questions at the time

   • direct binding vs. complex-mediated association between Sec5 and GEF-H1 not distinguished
   • whether this mechanism operates in non-mesenchymal cells not tested
10. 2018 Medium

    Sec5 binding to InsP3R on phagosomes was shown to promote Ca²⁺ release and recruit TBK1 for IRF-3-dependent interferon responses during fungal phagocytosis, integrating Sec5's vesicle-tethering and innate immune signaling roles on the same organelle.

    Evidence Co-IP, Ca²⁺ imaging, recombinant peptide disruption, kinase activation assay in macrophages during Candida phagocytosis

    PMID:29703257

    Open questions at the time

    • stoichiometry of the InsP3R–Sec5–TBK1 complex on phagosomes not determined
    • whether assembled exocyst is present on phagosomes or only Sec5 not clarified
11. 2020 High

    Human biallelic EXOC2 truncating variants causing severe brain malformations established EXOC2 as essential for human neurodevelopment, with patient cells showing loss of exocytosis and defective Arl13b ciliary localization, linking Sec5 to primary ciliogenesis.

    Evidence Patient-derived fibroblasts from two families, exocytosis assays, Arl13b immunofluorescence, protein expression analysis

    PMID:32639540

    Open questions at the time

    • mechanism by which Sec5 promotes Arl13b ciliary targeting not elucidated
    • whether ciliogenesis defect is cell-type specific in patient tissues unknown
12. 2022 Medium

    Sec5 interaction with STAT6 and its requirement for STAT6 phosphorylation and M2 macrophage polarization expanded Sec5's signaling scaffolding role to adaptive immune regulation.

    Evidence Co-IP, siRNA/overexpression, immunofluorescence co-localization, heterozygous Sec5-deficient mice

    PMID:36313547

    Open questions at the time

    • whether Sec5 promotes STAT6 phosphorylation by facilitating JAK access or by another mechanism not determined
    • in vivo phenotype in heterozygous mice only partially characterized
13. 2024 Medium

    EXOC2 deletion or ASO-mediated knockdown in C9ORF72-ALS/FTD neurons reduced G4C2 repeat-containing RNA and dipeptide repeat proteins, revealing an unexpected role for Sec5 in regulating pathogenic repeat RNA levels.

    Evidence CRISPR-Cas9 knockout and ASO treatment in iPSC-derived motor neurons with DPR and RNA quantification

    PMID:38935506

    Open questions at the time

    • mechanism by which EXOC2 regulates G4C2 repeat RNA (transcription, stability, or export) unknown
    • whether the effect is specific to repeat-containing transcripts or reflects general RNA metabolism changes not resolved

• Key unresolved questions include the structural organization of Sec5 within the fully assembled mammalian exocyst, how Sec5's vesicle-tethering and signaling-scaffold functions are coordinated or segregated in time and space, and the molecular mechanism by which Sec5 promotes Arl13b ciliary targeting and regulates C9ORF72 repeat RNA.
• no high-resolution structure of full-length Sec5 in assembled exocyst
• signaling vs. tethering functions not mechanistically separated
• Arl13b ciliary trafficking mechanism downstream of Sec5 not defined