EXOC1

EXOC1 (Sec3) is a subunit of the octameric exocyst complex that serves as a spatial landmark for polarized exocytosis and actively promotes membrane fusion. In yeast, Sec3 localizes to exocytic sites independently of the secretory pathway and actin cytoskeleton, where its PH-domain N-terminus binds PtdIns(4,5)P2 and Rho-family GTPases (Cdc42, Rho1) to anchor the complex at the plasma membrane, and it is the only exocyst subunit capable of recruiting secretory vesicles when ectopically targeted (PMID:9491896, PMID:18195105, PMID:20062059, PMID:25232005). Sec3 also directly binds the t-SNARE Sso2, relieving its autoinhibition to stimulate SNARE complex assembly and membrane fusion independently of its tethering function (PMID:28112172). In mammals, EXOC1 is essential for peri-implantation development, and conditional loss-of-function studies demonstrate requirements in spermatogenesis via Rac1 inactivation and SNARE interactions, oocyte trafficking of c-KIT and GDF9, desmosome assembly, neuronal polarization, and TGF-β/Akt-dependent cell migration (PMID:26346620, PMID:33973520, PMID:39833146, PMID:19889837, PMID:34862972, PMID:31495494).

1. 1997 High

   The initial question was whether Sec3 functions in exocytosis; genetic analysis established that SEC3 is required for post-Golgi vesicle targeting or fusion at the plasma membrane and genetically interacts with cytoskeletal regulators.

   Evidence Temperature-sensitive alleles, synthetic lethality with profilin, gene dosage suppression in budding yeast

   PMID:9247645

   Open questions at the time

   • No biochemical mechanism identified
   • No distinction between tethering and fusion roles
2. 1998 High

   A central question was how exocytic sites are spatially specified; live imaging revealed that Sec3p localizes to polarized growth sites independently of the secretory pathway, actin, and septins, establishing it as a unique spatial landmark among exocyst subunits.

   Evidence GFP-Sec3 imaging across multiple yeast secretory and cytoskeletal mutants

   PMID:9491896

   Open questions at the time

   • Mechanism of membrane targeting unknown
   • Whether mammalian EXOC1 behaves identically unclear
3. 2001 Medium

   The mammalian EXOC1 ortholog was identified as a component of the exocyst complex in brain, linking it to RalA GTPase signaling, and initial characterization showed human Sec3 differs from yeast Sec3 in lacking the Rho1-binding site and displaying cytosolic localization in epithelial cells.

   Evidence GTP-dependent pulldown with MALDI-TOF MS from rat brain; yeast two-hybrid with exocyst subunits; GFP imaging in MDCK cells

   PMID:11406615 PMID:11493706

   Open questions at the time

   • Mammalian membrane targeting mechanism unresolved
   • Functional role of mammalian EXOC1 not yet tested by loss-of-function
4. 2004 High

   The question of how the exocyst assembles was addressed by showing Sec3p and Exo70p remain stably at the plasma membrane while six other subunits arrive on secretory vesicles, defining a two-pool assembly model.

   Evidence FRAP, immunogold EM, and video microscopy with actin disruption in budding yeast

   PMID:15583031

   Open questions at the time

   • Structural basis of subunit-subunit assembly not resolved
   • Whether this model applies in mammalian cells unclear
5. 2008 High

   The molecular basis of Sec3 membrane anchoring was resolved: the N-terminus directly binds PtdIns(4,5)P2 and GTP-Cdc42, and both interactions are required for polarized localization and exocytosis.

   Evidence In vitro lipid/GTPase binding assays with point mutagenesis and genetic validation in yeast

   PMID:18195105

   Open questions at the time

   • Atomic details of dual-lipid/GTPase recognition not yet available
6. 2010 High

   Structural determination of the Sec3 N-terminal domain in complex with Rho1 revealed a PH fold with a PtdIns(4,5)P2-binding basic cleft and a hydrophobic interface for Rho GTPase recognition, providing the atomic framework for membrane targeting.

   Evidence 2.6 Å crystal structure with mutagenesis validation

   PMID:20062059

   Open questions at the time

   • Full-length Sec3 structure not determined
   • No structure of the mammalian EXOC1 domain
7. 2007 Medium

   Sec3 was linked to septin-dependent polarity control: in C. albicans, Sec3p co-immunoprecipitates with septins and is required for hyphal tip growth after septin ring formation, with septin deletion epistatic to sec3Δ.

   Evidence Co-IP and genetic epistasis with fluorescence microscopy in C. albicans

   PMID:17504812

   Open questions at the time

   • Direct versus indirect septin–Sec3 interaction not distinguished
   • Relevance to mammalian septins untested
8. 2009 Medium

   Mammalian EXOC1 was found to have a specific role in desmosome assembly: Sec3 knockdown impairs desmosome but not adherens junction integrity, and Sec3 is recruited to cell–cell contacts in a cadherin-dependent manner.

   Evidence RNAi, co-IP, and functional junction assays in epithelial cells

   PMID:19889837

   Open questions at the time

   • Cargo delivered by EXOC1-containing exocyst to desmosomes not identified
   • Whether EXOC1 acts as a full-complex subunit or sub-complex at desmosomes unclear
9. 2012 Medium

   Sec3 was shown to physically interact with the formin For3 in fission yeast and is required for actin cable polarization; human EXOC1 rescues fission yeast sec3 mutants, demonstrating functional conservation.

   Evidence Co-IP, FRAP, genetic deletion, and heterologous complementation

   PMID:22891673

   Open questions at the time

   • Binding interface between Sec3 and formin not mapped
   • Whether EXOC1 regulates mammalian formins untested
10. 2014 High

    Among all eight exocyst subunits, only Sec3 can recruit secretory vesicles when ectopically targeted to mitochondria, resolving the question of which subunit initiates vesicle tethering.

    Evidence Ectopic mitochondrial targeting assay with fluorescence microscopy in yeast

    PMID:25232005

    Open questions at the time

    • Molecular basis of Sec3's unique vesicle-recruiting capability not identified
    • Whether this applies in mammalian cells unknown
11. 2015 High

    Homozygous Exoc1 knockout in mice causes peri-implantation lethality, establishing EXOC1 as essential for early mammalian development.

    Evidence Constitutive knockout mouse with genetic rescue

    PMID:26346620

    Open questions at the time

    • Specific developmental process requiring EXOC1 at peri-implantation not identified
    • Cell-type-specific requirements not dissected
12. 2017 High

    Sec3 was shown to directly promote SNARE-mediated membrane fusion by binding t-SNARE Sso2 and relieving its autoinhibition, establishing a fusion-promoting function mechanistically separable from vesicle tethering.

    Evidence Crystal structure of Sec3–Sso2 complex, in vitro reconstituted membrane fusion assay, mutagenesis

    PMID:28112172

    Open questions at the time

    • Whether mammalian EXOC1 similarly activates cognate t-SNAREs not tested
    • Integration of tethering and fusion-promoting activities in vivo not resolved
13. 2017 Medium

    Sec3 protein turnover was found to be regulated by the ubiquitin-proteasome system via E3 ligase Pib1 and deubiquitylase Ubp3, connecting protein quality control to exocyst function.

    Evidence Extragenic suppressor screen with proteasome inhibition and genetic analysis in fission yeast

    PMID:28765280

    Open questions at the time

    • Ubiquitylation sites on Sec3 not mapped
    • Whether mammalian EXOC1 turnover is similarly regulated unknown
14. 2019 Medium

    EXOC1 was placed in TGF-β signaling: Sec3 knockdown abolishes TGF-β-stimulated Akt phosphorylation and cell migration without affecting Smad2 signaling, identifying EXOC1 as a pathway-selective mediator of non-canonical TGF-β signaling.

    Evidence siRNA knockdown with RNAi-resistant rescue, Western blot, wound healing assay in A549 cells

    PMID:31495494

    Open questions at the time

    • Mechanism by which EXOC1 selectively modulates Akt signaling unknown
    • Only tested in one cancer cell line
15. 2021 Medium

    Conditional knockout studies established EXOC1 as essential for spermatogenesis, where it promotes pseudopod formation via Rac1 inactivation and spermatocyte syncytia formation through interaction with SNARE proteins STX2 and SNAP23, and for neuronal polarization and cortical migration.

    Evidence Conditional knockout mice with Rac1 activity assays and co-IP (spermatogenesis); siRNA and in utero electroporation (neurons)

    PMID:33973520 PMID:34862972

    Open questions at the time

    • Whether EXOC1 directly inhibits Rac1 or acts through a GAP not determined
    • Cargo delivered during neuronal polarization not identified
16. 2025 Medium

    EXOC1 depletion from oocytes impairs intra-oocyte trafficking of c-KIT and GDF9, causing their cytoplasmic retention and leading to defective cyst breakdown and female infertility, extending the mammalian EXOC1 requirement to oogenesis.

    Evidence Oocyte-specific conditional knockout with immunofluorescence and fertility assays in mice

    PMID:39833146

    Open questions at the time

    • Whether EXOC1 directly traffics c-KIT/GDF9 or acts indirectly through general exocytic machinery unclear
    • Single lab finding

• Major open questions include the structure of full-length mammalian EXOC1, whether mammalian EXOC1 directly activates cognate t-SNAREs as yeast Sec3 does, the identity of specific cargo delivered by EXOC1-containing exocyst subcomplexes in different mammalian cell types, and the mechanism by which EXOC1 selectively modulates non-canonical TGF-β/Akt signaling.
• No full-length mammalian EXOC1 structure
• No in vitro reconstitution of mammalian EXOC1-SNARE activation
• Cargo specificity in distinct mammalian tissues unresolved