EXOC6

EXOC6 (Sec15) is a subunit of the octameric exocyst tethering complex that acts as a Rab GTPase effector to couple recycling-endosome-derived vesicles to sites of polarized exocytosis at the plasma membrane (PMID:7615633, PMID:15292201). Genetic and biochemical work in yeast established Sec15 as a downstream effector of the Sec4 GTPase that functions at a late secretory step and assembles, together with Sec6 and Sec8, into a stable exocyst complex localized to sites of active exocytosis (PMID:2504727, PMID:7615633). In mammalian cells EXOC6 binds Rab11 specifically in its GTP-bound form and associates with perinuclear recycling endosomes, defining the exocyst as a Rab11 effector (PMID:15292201); this Rab11–Sec15 axis is conserved in Drosophila, where Sec15 maintains Rab11 compartment organization and drives recycling of cargoes including DE-Cadherin and the Notch ligand Delta to control epithelial junction formation and asymmetric cell fate signaling (PMID:16137928, PMID:16224820). Beyond Rab11, EXOC6 engages additional small-GTPase and regulatory inputs: it binds phospho-Rabin8 to promote Rab8 activation and ciliary membrane formation during ciliogenesis (PMID:23435566), binds Rab10-GTP to support insulin-stimulated GLUT4 translocation in adipocytes (PMID:26299925), and participates in Ca2+-induced lysosome exocytosis (PMID:34100549). Functionally, EXOC6 is required for pancreatic β-cell insulin secretion and secretory machinery expression (PMID:35336762) and for presynaptic active zone assembly and neurotransmitter release at the neuromuscular junction (PMID:38086519), reflecting a broad role in delivering vesicular cargo to specialized membrane domains.

1. 1989 High

   Established that Sec15 acts at a late secretory step as a downstream effector of a Rab-family GTPase, defining its position in the vesicle delivery pathway.

   Evidence Genetic epistasis in sec4/sec2 yeast mutants with immunofluorescence and fractionation

   PMID:2504727

   Open questions at the time

   • No molecular mechanism of how Sec4 controls Sec15 patch formation
   • Mammalian relevance not yet addressed
2. 1991 High

   Defined Sec15 as partitioning between plasma membrane and a soluble particle, with its membrane attachment regulated by other Sec proteins.

   Evidence Subcellular fractionation and sucrose gradients in sec8-9 mutant yeast

   PMID:1900300

   Open questions at the time

   • Biochemical basis of the membrane attachment/detachment switch unknown
   • Composition of the soluble particle not fully resolved
3. 1995 High

   Showed Sec15 is a stable subunit of a multiprotein exocyst complex localized to sites of exocytosis, moving from a single protein to a defined molecular machine.

   Evidence Affinity purification, gel filtration, velocity centrifugation and co-IP with genetic validation in yeast

   PMID:7615633

   Open questions at the time

   • Full octameric stoichiometry not yet defined
   • Structural architecture unknown
4. 2001 Medium

   Identified the mammalian EXOC6 homologue within a brain exocyst complex regulated by a GTPase, extending exocyst biology to mammals.

   Evidence GTP-dependent pulldown and MALDI-TOF MS from brain lysates

   PMID:11406615

   Open questions at the time

   • No reciprocal co-IP for the RalA interaction
   • Functional consequence of EXOC6 in brain not tested
5. 2004 High

   Demonstrated that mammalian EXOC6 binds Rab11 specifically in the GTP state and localizes to recycling endosomes, defining the exocyst as a Rab11 effector.

   Evidence Nucleotide-dependent co-IP with multiple Rab controls, colocalization and transferrin recycling assays

   PMID:15292201

   Open questions at the time

   • Direct vs indirect nature of the Rab11 interaction not resolved
   • Cargo specificity not defined in this system
6. 2005 High

   Linked Sec15/Rab11 recycling to delivery of specific cargoes (Delta, DE-Cadherin) controlling Notch signaling, cell fate and epithelial junctions in vivo.

   Evidence Drosophila genetic loss-of-function, colocalization, cell fate analysis and co-IP of Sec15–Rab11

   PMID:15848801 PMID:16137928 PMID:16224820

   Open questions at the time

   • Mechanism of cargo selection by Sec15 unknown
   • How exocyst recruitment is coordinated with vesicle fusion not defined
7. 2010 High

   Showed the Rab11/Sec15 recycling step is a pathogen target, with anthrax factors blocking Sec15 vesicle formation to disrupt Notch and cadherin junction signaling.

   Evidence Drosophila genetics, live Sec15-GFP imaging, RNAi and human endothelial cell assays

   PMID:20944747

   Open questions at the time

   • Direct molecular target of LF in the Sec15 step not pinpointed
   • Quantitative impact on endogenous cargo flux unmeasured
8. 2013 High

   Connected EXOC6 to ciliogenesis via a phosphorylation-dependent binding switch in Rabin8 that activates Rab8 and builds ciliary membrane.

   Evidence In vitro kinase assay, phospho-mutant analysis, co-IP, colocalization and siRNA ciliogenesis readout

   PMID:23435566

   Open questions at the time

   • Structural basis of the phospho-Rabin8/Sec15 interaction not solved
   • Whether full exocyst assembly is required at the cilium not addressed
9. 2015 Medium

   Expanded the Rab repertoire by showing EXOC6 binds Rab10-GTP to support insulin-stimulated GLUT4 exocytosis, linking metabolic signaling to the tethering machinery.

   Evidence GTP-dependent co-IP and siRNA GLUT4 translocation assay in 3T3-L1 adipocytes

   PMID:26299925

   Open questions at the time

   • Direct vs indirect Rab10 binding not established
   • Redundancy with paralogue Exoc6b not fully dissected
10. 2021 Medium

    Extended EXOC6 function to Ca2+-induced lysosome exocytosis acting with Rab11, broadening its role beyond classical recycling-to-PM delivery.

    Evidence siRNA knockdown, lysosome exocytosis assays and Rab11–Sec15 co-IP

    PMID:34100549

    Open questions at the time

    • Mechanism distinguishing lysosomal from recycling-endosome tethering unknown
    • Single-lab functional study
11. 2022 Medium

    Established a requirement for EXOC6 in pancreatic β-cell insulin secretion and expression of the secretory machinery, implicating it in glucose homeostasis.

    Evidence siRNA silencing in INS1-832/13 cells with insulin secretion assays, Western blot and RT-qPCR

    PMID:35336762

    Open questions at the time

    • Whether effects are direct trafficking defects or secondary to transcriptional changes unclear
    • Not validated in primary islets in vivo
12. 2023 Medium

    Showed Sec15 is required presynaptically for active zone assembly, synaptic morphology and neurotransmitter release, in part through BMP signaling.

    Evidence Drosophila RNAi knockdown, electrophysiology, confocal imaging and wit epistasis at the NMJ

    PMID:38086519

    Open questions at the time

    • Direct cargo mediating active zone defects not identified
    • Mechanistic link between exocyst and BMP receptor trafficking incomplete

• How EXOC6 integrates competing Rab inputs (Rab11, Rab10, Rab8 via Rabin8) to achieve cargo- and tissue-specific exocytosis, and the structural basis of these interactions, remains unresolved.
• No structural model of EXOC6 bound to its Rab partners
• Mechanism of cargo selectivity across recycling, ciliary, lysosomal and secretory routes unknown
• Human disease genetics for EXOC6 not established in this corpus