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Showing RPH3AL — NOC2 is a alias.

RPH3AL

Rab effector Noc2 · UniProt Q9UNE2

Length
315 aa
Mass
34.5 kDa
Annotated
2026-06-10
18 papers in source corpus 12 papers cited in narrative 12 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 5/5 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

RPH3AL (Noc2) is a Rab effector protein that coordinates multiple steps of regulated exocytosis in endocrine and exocrine secretory cells (PMID:15159548, PMID:27927751). Through an N-terminal Rab-binding domain (RBD27) whose acidic α1 cluster determines specificity, it binds the GTP-bound forms of Rab3 isoforms, Rab8A, and Rab27A/B (PMID:12578829), with Rab27A providing the primary determinant of its recruitment to dense-core/secretory granule membranes (PMID:14722103); granule association requires binding to either Rab27 or Rab3, and Noc2 is anchored stably on these membranes with little exchange to cytosol (PMID:14593078, PMID:18573236). On secretory granules Noc2 assembles distinct, spatially segregated complexes: a GTP-dependent ternary Rab2a–Noc2–Rab27a complex on perinuclear immature granules that supports proinsulin processing and granule maturation, transitioning to a binary Noc2–Rab27a complex on peripheral mature granules competent for exocytosis (PMID:27927751). Functionally, Noc2 governs secretion through several context-specific routes: in beta-cells it permits Ca²⁺-triggered insulin secretion by suppressing inhibitory Gi/o signaling, a function rescued by pertussis toxin in Noc2-knockout islets and dependent on its Rab3-binding domain (PMID:15159548); in pancreatic acinar cells it acts upstream of the agonist-induced Ca²⁺ spike rather than at the membrane-fusion step itself (PMID:22615885); and in adipocytes it negatively regulates GLUT4 exocytosis, an inhibition relieved when insulin-stimulated Rab3 GTP loading disrupts the Rab3–Noc2 complex and displaces Noc2 from the plasma membrane (PMID:26024738). Noc2 additionally interacts with the vesicle-priming factor Munc13 (PMID:14593078) and the LIM-domain cytoskeletal protein zyxin (PMID:9367993).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 1997 Medium

    Establishing what Noc2 is — a rabphilin-3A-related protein lacking C2 domains — and its first binding partner began to place it in the secretory/cytoskeletal interface.

    Evidence Cloning, yeast two-hybrid and co-IP with zyxin, overexpression in PC12 cells with growth hormone secretion readout

    PMID:9367993

    Open questions at the time
    • No Rab partner yet identified
    • Cytoplasmic localization without granule-targeting mechanism defined
    • Zyxin interaction not functionally dissected
  2. 2000 High

    Defined Noc2 as a functional Rab effector by showing it acts as a negative effector of Ca²⁺-triggered exocytosis downstream of GTP-Rab3A.

    Evidence Overexpression in permeabilized PC12 cells, nucleotide-dependent Rab3A pulldown, mutagenesis of the Rab3A-binding domain

    PMID:11134008

    Open questions at the time
    • Whether other Rabs also bind unknown
    • Endogenous loss-of-function not tested
    • Direct fusion-machinery target unidentified
  3. 2003 High

    Systematic screening mapped Noc2's Rab-binding specificity and identified the acidic α1 cluster as the recognition determinant, while linking Noc2 to the priming factor Munc13 and to functional insulin secretion.

    Evidence Cotransfection screen across 42 Rabs with mutagenesis and chimeras; protein-binding assay with Munc13, RNAi and overexpression in INS-1E beta-cells

    PMID:12578829 PMID:14593078

    Open questions at the time
    • Which Rab dominates in vivo not resolved
    • Munc13 interaction not mapped structurally
    • Mechanism of secretion regulation downstream of binding unclear
  4. 2004 High

    Resolved the in vivo recruitment determinant by showing Rab27A, not Rab3A, drives Noc2 granule targeting, redefining the domain as RBD27; parallel work extended Noc2 to epithelial Rab3B-dependent transport and to in vivo Gi/o suppression in beta-cells.

    Evidence Point/deletion mutants with localization and NPY secretion in PC12 cells; GST pulldown and VSV-G transport in Caco2 cells; Noc2-KO mice with pertussis toxin rescue and domain-specific adenoviral rescue

    PMID:14722103 PMID:15003533 PMID:15159548

    Open questions at the time
    • Molecular link between Noc2 and Gi/o signaling unresolved
    • Reconciliation of Rab3 vs Rab27 roles across cell types incomplete
    • Epithelial transport role tested only by overexpression
  5. 2006 Medium

    Demonstrated dynamic regulation of the Noc2/Rab27 complex during stimulated secretion in exocrine cells, with Rab27 translocating away from Noc2 upon agonist stimulation.

    Evidence Subcellular fractionation, immunoblot, and anti-RBD antibody inhibition of amylase release in permeabilized parotid acinar cells

    PMID:17067543

    Open questions at the time
    • Trigger for complex disruption not defined
    • Single-cell-type observation
    • Fate of granule-retained Noc2 after Rab27 departure unclear
  6. 2008 Medium

    Quantified Noc2's membrane dynamics, showing it forms stable rather than rapidly exchanging granule complexes, distinguishing it from rabphilin.

    Evidence FRAP of EGFP-Noc2 versus EGFP-rabphilin in PC12 cells

    PMID:18573236

    Open questions at the time
    • Structural basis of stable anchoring unknown
    • Single method, single cell type
    • Relationship of stability to secretion not tested
  7. 2012 High

    Dissected the step of action in exocrine secretion, placing Noc2 upstream of the agonist-induced Ca²⁺ spike rather than at membrane fusion itself.

    Evidence Two-photon live imaging and flash photolysis of caged calcium in Noc2-KO pancreatic acinar cells with CCK/ACh stimulation

    PMID:22615885

    Open questions at the time
    • Molecular target controlling Ca²⁺ spike generation unidentified
    • Whether this upstream role generalizes to beta-cells untested
  8. 2015 High

    Established a regulatory switch in adipocytes whereby insulin-driven Rab3 GTP loading dissolves the Rab3–Noc2 complex to relieve Noc2 inhibition of GLUT4 exocytosis.

    Evidence Bio-ATB-GTP photoaffinity GTP labeling, constitutively active/dominant-negative Rab3 mutants, Rab3 siRNA, and Noc2 localization/GLUT4 translocation assays in primary rat adipocytes

    PMID:26024738

    Open questions at the time
    • Downstream effector through which Noc2 inhibits GLUT4 trafficking unknown
    • Whether Rab27 participates in adipocytes untested
  9. 2016 High

    Unified Noc2's roles into a sequential model by demonstrating a GTP-dependent ternary Rab2a–Noc2–Rab27a complex on immature granules that mediates the transition from Rab2a-dependent biogenesis/proinsulin processing to Rab27a-dependent peripheral exocytosis.

    Evidence Reciprocal co-IP, ordered GTP-dependent binding, mutagenesis, subcellular localization, siRNA knockdown with insulin secretion and proinsulin processing readouts in beta-cells

    PMID:27927751

    Open questions at the time
    • Structural architecture of the ternary complex unresolved
    • Trigger converting ternary to binary complex undefined
    • Whether ternary complex operates outside beta-cells unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • The molecular link by which Noc2 suppresses inhibitory Gi/o signaling and controls agonist-induced Ca²⁺ generation upstream of fusion remains undefined.
  • No direct effector connecting Noc2 to Gi/o or Ca²⁺ signaling identified
  • No structural model of RBD27–Rab complexes
  • Physiological hierarchy of Rab3 vs Rab27 vs Rab2a engagement across tissues not fully reconciled

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0098772 molecular function regulator activity 3 GO:0060090 molecular adaptor activity 2
Localization
GO:0031410 cytoplasmic vesicle 3 GO:0005829 cytosol 2 GO:0005886 plasma membrane 2
Partners
RAB27ARAB2ARAB3ARAB3BRAB8AUNC13ZYX
Complex memberships
Noc2-Rab27a binary complexRab2a-Noc2-Rab27a ternary complex

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1997 Noc2 (RPH3AL) was cloned as a 302-amino-acid protein with 40.7% identity to the N-terminal region of rabphilin-3A but lacking C2 domains; it is expressed predominantly in endocrine tissues, localizes to the cytoplasm in MIN6 cells, and interacts with the LIM-domain protein zyxin (a cytoskeletal component) as shown by yeast two-hybrid and co-immunoprecipitation. Overexpression in PC12 cells enhanced high-K⁺-induced growth hormone secretion. Yeast two-hybrid screen, co-immunoprecipitation, overexpression in PC12 cells, immunoblot of subcellular fractions The Journal of biological chemistry Medium 9367993
2000 Noc2 directly inhibits Ca²⁺-triggered exocytosis in permeabilized PC12 cells in a manner dependent on specific binding to Rab3A (GTP-bound form); Rab3A-binding-deficient mutants of Noc2 lose the inhibitory effect, establishing Noc2 as a negative effector downstream of Rab3A in dense-core granule exocytosis. Overexpression in permeabilized PC12 cells, nucleotide-dependent pulldown of Rab3A, site-directed mutagenesis of Noc2 Rab3A-binding domain The Journal of biological chemistry High 11134008
2003 Noc2 binds Rab3A/B/C/D, Rab8A, and Rab27A/B (but not other Rab proteins) as determined by cotransfection assay with 42 different Rab proteins. The acidic cluster (Glu residues) in the α1 region of the Rab-binding domain is a critical determinant of Rab recognition specificity. Cotransfection assay with 42 Rab proteins, site-directed mutagenesis, chimeric protein analysis The Journal of biological chemistry High 12578829
2003 Noc2 is a potential partner of Munc13 (a vesicle-priming component) as revealed by protein-protein binding studies; granule targeting of Noc2 requires interaction with either Rab27 or Rab3 (mutants defective in binding both fail to associate with secretory granules). Overexpression of granule-targeted Noc2 inhibits hormone secretion; silencing Noc2 by RNAi impairs both early and sustained insulin secretion phases in INS-1E cells. Protein-protein binding assay (Noc2–Munc13), site-directed mutagenesis, RNA interference, overexpression in INS-1E beta-cells, hormone secretion assay Molecular endocrinology High 14593078
2004 Noc2 is recruited to dense-core vesicles in PC12 cells through specific interaction with Rab27A, not Rab3A: Rab3A-binding-defective Noc2(E51A) localizes normally to distal neurites, while the Rab27A-binding-defective double mutant Noc2(E51A/I55A) is cytosolic. The Rab-binding domain of Noc2 is redefined as RBD27. Deletion and point-mutation analysis, fluorescence localization in NGF-differentiated PC12 cells, neuropeptide Y secretion assay The Journal of biological chemistry High 14722103
2004 Noc2 is a potential Rab3B effector in epithelial cells: pulldown assay showed GTP-dependent binding of Noc2 to Rab3B; co-expression with constitutively active Rab3B recruited Noc2 from cytosol to perinuclear membranes; Noc2 overexpression inhibited basolateral cell-surface transport of VSV-G. GST pulldown, co-expression localization assay, VSV-G transport assay in Caco2 epithelial cells Biochemical and biophysical research communications Medium 15003533
2004 In Noc2 knockout mice, Ca²⁺-triggered insulin secretion is markedly impaired and fully rescued by pertussis toxin (Gi/o inhibitor), indicating Noc2 normally suppresses inhibitory Gi/o signaling in beta-cells. The Rab3-binding domain of Noc2 is required for this function, as Rab3-binding-defective Noc2 fails to restore secretion in KO islets. Noc2 KO also causes granule accumulation in exocrine cells with absent amylase secretion. Noc2 knockout mice, pertussis toxin rescue, adenovirus gene transfer of wild-type vs. mutant Noc2, Ca²⁺-triggered insulin secretion assay from isolated islets Proceedings of the National Academy of Sciences of the United States of America High 15159548
2006 In rat parotid acinar cells, Noc2 is present on secretory granule membranes bound to Rab27; upon isoproterenol stimulation, the Noc2/Rab27 complex is disrupted and Rab27 (but not Noc2) translocates to the apical plasma membrane. An anti-Noc2-RBD antibody inhibits IPR-stimulated amylase release from permeabilized acinar cells, indicating the Noc2/Rab27 complex is required for early-phase exocytosis. Subcellular fractionation, immunoblot, antibody inhibition of exocytosis in streptolysin-O-permeabilized cells Archives of biochemistry and biophysics Medium 17067543
2008 By FRAP of EGFP-tagged proteins in PC12 cells, Noc2 shows little or no exchange between secretory granules and cytosol (unlike Rabphilin, which exchanges rapidly), indicating Noc2 forms stable complexes on secretory granule membranes. Both Noc2 and Rabphilin are recruited to granules by Rab27. FRAP (fluorescence recovery after photobleaching) of EGFP-Noc2 in PC12 cells Biochemical and biophysical research communications Medium 18573236
2012 In pancreatic acinar cells from Noc2-KO mice (live two-photon imaging), Noc2 is not required for the membrane fusion step of zymogen granule exocytosis per se (flash photolysis of caged calcium showed intact fusion); instead, Noc2 is required for agonist-induced physiological Ca²⁺ spike generation, placing Noc2 upstream of the Ca²⁺ signal in exocytosis. Two-photon excitation live-cell imaging, flash photolysis of caged calcium, Noc2 knockout mice, cholecystokinin/acetylcholine stimulation PloS one High 22615885
2015 In primary rat adipocytes, insulin stimulation triggers Rab3 GTP loading, which disrupts the Rab3–Noc2 complex, displacing Noc2 from the plasma membrane. This relieves Noc2-mediated inhibition and facilitates GLUT4 translocation to the plasma membrane, placing Noc2 as a negative regulator of GLUT4 exocytosis downstream of insulin-stimulated Rab3 activation. Photoaffinity GTP labeling (Bio-ATB-GTP) in primary adipocytes, constitutively active/dominant-negative Rab3 mutants, Rab3 siRNA knockdown, Noc2 localization assay Diabetologia High 26024738
2016 Rab2a and Rab27a simultaneously bind Noc2 in a GTP-dependent manner to form a ternary Rab2a–Noc2–Rab27a complex, with Rab2a binding only after Rab27a has bound. The ternary complex localizes on perinuclear immature granules in pancreatic beta-cells, while the binary Noc2–Rab27a complex is on peripheral mature granules. Noc2 mutants defective in Rab2a or Rab27a binding fail to promote glucose-stimulated insulin secretion. Knockdown of Rab2a or Noc2 (but not Rab27a) impairs proinsulin-to-insulin processing, indicating Noc2 mediates the transition from Rab2a-dependent granule biogenesis to Rab27a-dependent exocytosis. Co-immunoprecipitation, GTP-dependent binding assay, subcellular localization in beta-cells, site-directed mutagenesis, siRNA knockdown, glucose-stimulated insulin secretion assay, proinsulin processing assay Journal of cell science High 27927751

Source papers

Stage 0 corpus · 18 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2003 Distinct Rab binding specificity of Rim1, Rim2, rabphilin, and Noc2. Identification of a critical determinant of Rab3A/Rab27A recognition by Rim2. The Journal of biological chemistry 159 12578829
2004 Rabphilin and Noc2 are recruited to dense-core vesicles through specific interaction with Rab27A in PC12 cells. The Journal of biological chemistry 81 14722103
1997 Noc2, a putative zinc finger protein involved in exocytosis in endocrine cells. The Journal of biological chemistry 74 9367993
2003 The Rab-binding protein Noc2 is associated with insulin-containing secretory granules and is essential for pancreatic beta-cell exocytosis. Molecular endocrinology (Baltimore, Md.) 66 14593078
2004 Noc2 is essential in normal regulation of exocytosis in endocrine and exocrine cells. Proceedings of the National Academy of Sciences of the United States of America 64 15159548
2010 A manual collection of Syt, Esyt, Rph3a, Rph3al, Doc2, and Dblc2 genes from 46 metazoan genomes--an open access resource for neuroscience and evolutionary biology. BMC genomics 47 20078875
2000 A direct inhibitory role for the Rab3-specific effector, Noc2, in Ca2+-regulated exocytosis in neuroendocrine cells. The Journal of biological chemistry 46 11134008
2016 Rab2a and Rab27a cooperatively regulate the transition from granule maturation to exocytosis through the dual effector Noc2. Journal of cell science 24 27927751
1999 Cloning of a human ortholog (RPH3AL) of (RNO)Rph3al from a candidate 17p13.3 medulloblastoma tumor suppressor locus. Genomics 20 10395805
2008 The Rab27 effector Rabphilin, unlike Granuphilin and Noc2, rapidly exchanges between secretory granules and cytosol in PC12 cells. Biochemical and biophysical research communications 18 18573236
2006 Functional involvement of Noc2, a Rab27 effector, in rat parotid acinar cells. Archives of biochemistry and biophysics 18 17067543
2015 Insulin regulates Rab3-Noc2 complex dissociation to promote GLUT4 translocation in rat adipocytes. Diabetologia 14 26024738
2012 A novel function of Noc2 in agonist-induced intracellular Ca2+ increase during zymogen-granule exocytosis in pancreatic acinar cells. PloS one 9 22615885
2006 Cellular expression of Noc2, a Rab effector protein, in endocrine and exocrine tissues in the mouse. Histochemistry and cell biology 8 16835753
2009 Immunohistochemical expression of rabphilin-3A-like (Noc2) in normal and tumor tissues of human endocrine pancreas. Biotechnic & histochemistry : official publication of the Biological Stain Commission 7 19212825
2004 Identification and characterization of Noc2 as a potential Rab3B effector protein in epithelial cells. Biochemical and biophysical research communications 7 15003533
2005 Assay of the Rab-binding specificity of rabphilin and Noc2: target molecules for Rab27. Methods in enzymology 6 16473612
2005 Physical and functional interaction of noc2/rab3 in exocytosis. Methods in enzymology 5 16473607

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