EXOC6B

EXOC6B encodes a subunit of the octameric exocyst complex that tethers secretory vesicles to the plasma membrane, functioning in regulated exocytosis across multiple cell types. It binds Rab10-GTP to couple insulin signaling to GLUT4 vesicle translocation in adipocytes (PMID:26299925), is required for insulin secretion and expression of exocytotic machinery components (Vamp2, Glut2, Pdx1) in pancreatic β-cells (PMID:35336762), and interacts with post-translationally modified STAT3 to direct its translocation into multivesicular endosomes for secretion, thereby modulating ERK1/2 and GSK3β signaling in embryonic stem cell self-renewal (PMID:39475099). EXOC6B is essential for primary ciliogenesis downstream of its vesicle-tethering function; biallelic loss-of-function variants in humans abrogate exocytosis and impair cilia formation, and knockdown in prechondrocytes attenuates Hedgehog signaling and disrupts chondrogenic differentiation (PMID:36150098, PMID:38305850).

1. 2001 Medium

   Identification of EXOC6B (KIAA0919) as a second mammalian Sec15 homologue within the brain exocyst complex established it as a bona fide exocyst subunit regulated by the small GTPase RalA.

   Evidence GTP-loaded RalA pulldown from rat brain extracts followed by MALDI-TOF MS identification of exocyst subunits including KIAA0919

   PMID:11406615

   Open questions at the time

   • No direct binding assay between RalA and EXOC6B specifically (pulled down as part of whole complex)
   • Functional consequence of RalA–exocyst interaction for EXOC6B-dependent trafficking not tested
2. 2015 Medium

   Demonstrating that Rab10-GTP directly binds EXOC6B and that its depletion inhibits GLUT4 translocation answered how insulin signaling engages the exocyst, identifying a specific GTPase–effector link for regulated exocytosis in adipocytes.

   Evidence Co-immunoprecipitation/pulldown with GTP-state specificity; siRNA knockdown of Exoc6/Exoc6b with GLUT4 translocation assay in 3T3-L1 adipocytes

   PMID:26299925

   Open questions at the time

   • Structural basis of Rab10–EXOC6B interaction unresolved
   • Relative contributions of EXOC6 versus EXOC6B to GLUT4 trafficking in vivo not delineated
3. 2022 Medium

   Showing that biallelic EXOC6B loss-of-function in human patient fibroblasts abrogates exocytosis and primary ciliogenesis established that exocyst-mediated vesicle tethering is required for cilia assembly and linked EXOC6B deficiency to a developmental phenotype.

   Evidence Patient-derived fibroblasts with biallelic EXOC6B variants; functional assays for exocytosis, ciliogenesis, and gene expression profiling for osteogenesis/ECM pathways

   PMID:36150098

   Open questions at the time

   • Rescue experiment to confirm causality of EXOC6B variants not reported
   • Which specific vesicle cargo is required for ciliogenesis downstream of EXOC6B remains unknown
4. 2022 Medium

   Demonstrating that Exoc6/Exoc6b knockdown in pancreatic β-cells impairs insulin secretion, insulin content, and expression of key exocytosis genes extended the exocytotic role of EXOC6B beyond adipocytes to endocrine cells.

   Evidence siRNA knockdown in INS1-832/13 cells; insulin secretion assays; RT-qPCR and Western blot for Ins1/2, Pdx1, Glut2, Vamp2

   PMID:35336762

   Open questions at the time

   • Combined Exoc6/6b knockdown makes individual contribution of EXOC6B uncertain
   • In vivo β-cell phenotype of EXOC6B loss not assessed
5. 2024 Medium

   Localizing EXOC6B to the ciliary base and showing that its depletion blocks both ciliogenesis and Hedgehog signaling in prechondrocytes defined a mechanism linking exocyst-dependent vesicle delivery to cilium-dependent developmental signaling during chondrogenesis.

   Evidence shRNA knockdown in ATDC5 cells; immunocytochemistry for ciliary-base localization; RT-qPCR/immunoblotting for Hedgehog and chondrogenic markers

   PMID:38305850

   Open questions at the time

   • Direct cargo delivered by EXOC6B to the ciliary base not identified
   • Whether Hedgehog signaling defect is solely secondary to cilia loss or involves additional EXOC6B-dependent trafficking not resolved
6. 2024 Medium

   Discovering that EXOC6B binds acetylated/phosphorylated STAT3 to mediate its sorting into multivesicular endosomes for secretion revealed a non-canonical exocyst function in signal transduction, linking EXOC6B to ERK1/2 and GSK3β regulation and embryonic stem cell self-renewal.

   Evidence Co-immunoprecipitation of STAT3 with Sec15b; site-directed mutagenesis (K177R/K180R, Y293F); live-cell imaging and fractionation for MVE localization; Sec15b knockout mice; genetic rescue

   PMID:39475099

   Open questions at the time

   • Whether STAT3–EXOC6B interaction occurs independently of the full exocyst complex is unclear
   • In vivo phenotype of Sec15b knockout mice beyond mESC self-renewal not fully described
   • Whether MVE-mediated STAT3 secretion operates in somatic cell types is unknown

• The structural basis of EXOC6B's selective interactions with Rab10, STAT3, and other cargo adaptors, the identity of the vesicle cargoes it delivers to the ciliary base, and the in vivo developmental consequences of EXOC6B loss in mammalian models remain to be determined.
• No high-resolution structure of EXOC6B or its complexes with partners
• In vivo conditional knockout phenotypes in skeletal and neuronal tissues not characterized
• Functional redundancy between EXOC6 and EXOC6B not systematically addressed