EXOC6

EXOC6 (Sec15) is a core subunit of the octameric exocyst complex that couples Rab GTPase signaling to vesicle tethering at the plasma membrane, mediating polarized exocytosis and endocytic recycling across diverse cell types. EXOC6 functions as a direct GTP-dependent effector of Rab11 and Rab10, linking recycling endosomal vesicles to membrane fusion sites to deliver cargoes including E-Cadherin, Delta/Notch ligands, GLUT4, and lysosomal contents (PMID:15292201, PMID:16224820, PMID:26299925, PMID:34100549). Its interaction with Rabin8, regulated by NDR2 phosphorylation, switches Rabin8 from lipid binding to exocyst engagement, coupling vesicle tethering to Rab8 activation and ciliogenesis (PMID:23435566). Loss of EXOC6 disrupts active zone assembly and synaptic transmission at neuromuscular junctions, impairs insulin secretion in pancreatic β-cells, and blocks Notch signaling during asymmetric cell division, underscoring its role as a central node integrating vesicle trafficking with cell polarity, signaling, and secretion (PMID:38086519, PMID:35336762, PMID:16137928).

1. 1989 High

   Establishing that Sec15 acts downstream of the Sec4 GTPase at a late secretory step resolved where in the exocytic pathway this protein functions, showing it responds to Rab-family signaling to control vesicle delivery.

   Evidence Genetic suppression analysis and overexpression phenotyping in yeast sec4 and sec2 mutant backgrounds

   PMID:2504727

   Open questions at the time

   • Mechanism of Sec4-Sec15 coupling unknown
   • No direct binding demonstrated
   • Mammalian relevance not tested
2. 1991 High

   Demonstrating that Sec15p partitions between the plasma membrane and a soluble 19.5S particle—regulated by Sec8 and Sec10—established that Sec15 cycles between a soluble complex and the membrane, defining the biophysical basis of exocyst dynamics.

   Evidence Subcellular fractionation and sucrose gradient sedimentation in sec8 and sec10 mutant yeast

   PMID:1900300

   Open questions at the time

   • Identity of 19.5S particle subunits incomplete
   • Membrane attachment mechanism unknown
3. 1995 High

   Biochemical isolation of a stable multisubunit exocyst complex containing Sec15, Sec8, and Sec6 at sites of active exocytosis established EXOC6 as an integral subunit of the exocyst tethering machinery.

   Evidence Affinity chromatography, gel filtration, sucrose gradient centrifugation, and immunofluorescence in yeast

   PMID:7615633

   Open questions at the time

   • Full octameric composition not yet defined
   • Subunit–subunit interaction hierarchy unclear
4. 2001 High

   Identifying the mammalian exocyst as a GTP-dependent effector of RalA in brain tissue extended the exocyst's regulatory logic beyond yeast Sec4 to a mammalian GTPase, positioning the complex in Ral-dependent signaling.

   Evidence GTP-dependent pulldown from rat brain lysates with MALDI-TOF MS identification

   PMID:11406615

   Open questions at the time

   • Direct EXOC6–RalA contact not resolved
   • Functional consequence of RalA–exocyst interaction not tested in vivo
5. 2004 High

   Showing that mammalian Sec15 (EXOC6) directly and specifically binds Rab11-GTP on recycling endosomes identified EXOC6 as the Rab11 effector subunit, establishing the molecular bridge between recycling vesicles and the exocyst.

   Evidence Co-immunoprecipitation with GTP/GDP-locked Rab11 mutants and colocalization in mammalian cells

   PMID:15292201

   Open questions at the time

   • Structural basis of Rab11–Sec15 interaction unknown
   • Which exocyst subunits are co-recruited unclear
6. 2005 High

   Genetic studies in Drosophila revealed that EXOC6 is required for recycling of DE-Cadherin and Delta from Rab11 endosomes to the plasma membrane, establishing its essential role in polarized cargo delivery for cell adhesion and Notch signaling.

   Evidence Loss-of-function clonal analysis in Drosophila epithelial cells and sensory organ precursors with immunofluorescence for cargo trafficking

   PMID:15848801 PMID:16137928 PMID:16224820

   Open questions at the time

   • Cargo selectivity mechanism unknown
   • Whether Sec15 recognizes cargo adaptors or acts generically unresolved
7. 2010 High

   Demonstrating that anthrax toxins EF and LF convergently target the Rab11–Sec15 axis to block endocytic recycling and Notch/cadherin signaling validated this pathway as a critical node exploited by pathogens, and confirmed its conservation between Drosophila and human endothelial cells.

   Evidence Live imaging of Sec15-GFP vesicles under toxin treatment in Drosophila and human endothelial cells

   PMID:20944747

   Open questions at the time

   • Direct molecular target of LF on the Sec15 pathway not identified
   • Extent to which other exocyst subunits are affected unclear
8. 2013 High

   Revealing that NDR2 phosphorylation of Rabin8 switches its binding from phosphatidylserine to Sec15, coupling vesicle tethering to Rab8 activation and ciliogenesis, uncovered a phospho-regulated mechanism that redirects exocyst function toward ciliary membrane assembly.

   Evidence In vitro phosphorylation, GST pulldown with phospho-mimetic mutants, and ciliogenesis rescue assays

   PMID:23435566

   Open questions at the time

   • Structural basis of the phospho-switch on Rabin8–Sec15 interface unknown
   • Whether this mechanism operates in all ciliated cell types not tested
9. 2015 High

   Identifying Rab10-GTP as a second direct Rab binding partner of EXOC6, and showing that EXOC6 knockdown blocks insulin-stimulated GLUT4 translocation, established EXOC6 as a shared effector for multiple Rab GTPases with tissue-specific trafficking functions.

   Evidence Co-immunoprecipitation with GTP/GDP Rab10 mutants and siRNA knockdown with GLUT4 translocation assay in 3T3-L1 adipocytes

   PMID:26299925

   Open questions at the time

   • Whether Rab10 and Rab11 compete for the same binding site on EXOC6 unknown
   • Relative contributions of EXOC6 vs EXOC6B in adipocytes not resolved
10. 2021 Medium

    Demonstrating that EXOC6, as a Rab11 effector, is required for Ca²⁺-triggered lysosome exocytosis via a Rab11–Sec15–Rab3a cascade extended the exocyst's role beyond conventional secretion to lysosomal fusion with the plasma membrane.

    Evidence siRNA knockdown, live-cell imaging of Rab11-positive vesicle–lysosome interactions, and lysosome exocytosis/plasma membrane repair assays

    PMID:34100549

    Open questions at the time

    • How Sec15 hands off vesicles to Rab3a mechanistically undefined
    • Single study; independent replication needed
11. 2022 Medium

    Showing that EXOC6 silencing impairs insulin secretion and reduces expression of key β-cell genes (Ins1, Pdx1, Glut2, Vamp2) revealed its importance in pancreatic β-cell exocytosis beyond GLUT4 trafficking.

    Evidence siRNA knockdown in INS1-832/13 rat β-cell line with insulin secretion assay and expression analysis

    PMID:35336762

    Open questions at the time

    • Whether gene expression changes are direct or secondary to trafficking defects unknown
    • Not validated in primary human β-cells
12. 2023 Medium

    Presynaptic knockdown of Sec15 at Drosophila NMJs revealed its requirement for active zone integrity, synaptic vesicle release, and extracellular vesicle secretion, with phenotypes partly driven by aberrant BMP signaling, extending EXOC6's role to synapse organization.

    Evidence Transgenic RNAi in Drosophila NMJ with electrophysiology, immunofluorescence, extracellular vesicle assays, and BMP pathway epistasis

    PMID:38086519

    Open questions at the time

    • Mechanism by which Sec15 loss enhances BMP signaling unclear
    • Whether mammalian synapses show analogous phenotypes not tested

• A structural understanding of how EXOC6 simultaneously or sequentially engages Rab11, Rab10, and Rabin8—and how cargo selectivity is achieved—remains unresolved.
• No high-resolution structure of EXOC6 in complex with Rab partners
• Cargo adaptor recognition mechanism unknown
• How EXOC6 vs EXOC6B isoforms are differentially regulated in vivo unclear