meg-3

id: Q9TXM1
gene_symbol: meg-3
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:6239
  label: Caenorhabditis elegans
description: 'MEG-3 (Maternal-Effect Germline defective 3) is an intrinsically disordered
  protein (IDP) that serves as the primary scaffold for P granule (germ granule) assembly
  in C. elegans embryos. MEG-3 contains a serine-rich N-terminal intrinsically disordered
  region (IDR) and a C-terminal HMG-box domain. It drives liquid-liquid phase separation
  (LLPS) in an RNA-dependent manner, forming gel-like assemblies that stabilize liquid
  PGL-3 droplets. MEG-3 establishes a posterior-rich concentration gradient that is
  anti-correlated with MEX-5, which suppresses MEG-3 granule formation by competing
  for RNA binding. MEG-3 function is regulated by phosphorylation: it is a substrate
  of kinase MBK-2/DYRK (promotes disassembly) and phosphatase PP2A/PPTR-1/2 (promotes
  assembly). MEG-3 functions redundantly with MEG-4; double mutants fail to assemble
  P granules in early embryos but remain partially fertile (~70%). MEG-3 is essential
  for efficient RNA recruitment to germ granules and transmission of maternal nuage
  to primordial germ cells.'
references:
- id: PMID:11922622
  title: Isolation of the interacting molecules with GEX-3 by a novel functional screening.
  findings:
  - statement: MEG-3 (as GEI-12) was identified as a GEX-3 interacting protein via
      yeast two-hybrid screening
    supporting_text: We identified many interacting molecules by yeast two-hybrid
      screening and could detect some functional interactions
- id: PMID:25535836
  title: Regulation of RNA granule dynamics by phosphorylation of serine-rich, intrinsically
    disordered proteins in C. elegans.
  findings:
  - statement: MEG-1 and MEG-3 are substrates of kinase MBK-2/DYRK and phosphatase
      PP2A(PPTR-1/2)
    supporting_text: "We demonstrate that MEG-1 and MEG-3 are substrates of the kinase\
      \ MBK-2/DYRK and the phosphatase PP2A(PPTR-\xBD)"
  - statement: Phosphorylation of MEGs promotes granule disassembly; dephosphorylation
      promotes assembly
    supporting_text: Phosphorylation of the MEGs promotes granule disassembly and
      dephosphorylation promotes granule assembly
  - statement: GFP-tagged MEG-3 localizes to a dynamic domain that surrounds and penetrates
      each P granule
    supporting_text: GFP-tagged MEG-3 localizes to a dynamic domain that surrounds
      and penetrates each granule
  - statement: MEG proteins are required redundantly for fertility
    supporting_text: The MEG (maternal-effect germline defective) proteins are germ
      plasm components that are required redundantly for fertility
  - statement: P granules are non-homogeneous structures whose assembly is regulated
      by phosphorylation
    supporting_text: P granules are non-homogeneous structures whose assembly in embryos
      is regulated by phosphorylation
existing_annotations:
- term:
    id: GO:0051640
    label: organelle localization
  evidence_type: IMP
  original_reference_id: PMID:25535836
  review:
    summary: MEG-3 is essential for P granule localization to the posterior of the
      embryo. PMID:25535836 demonstrates that MEG-3 forms a dynamic domain that surrounds
      and penetrates P granules, and that phosphorylation/dephosphorylation cycles
      regulate granule dynamics. This annotation captures MEG-3's role in proper localization
      of P granules, though a more specific term for P granule localization might
      be preferred.
    action: ACCEPT
    reason: MEG-3 establishes a posterior-rich concentration gradient that positions
      P granules correctly in the embryo. The Wang et al. 2014 study used lattice
      light sheet microscopy to show that GFP-tagged MEG-3 localizes to a dynamic
      domain that surrounds and penetrates each granule. The annotation accurately
      reflects MEG-3's role in controlling where P granules localize.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: GFP-tagged MEG-3 localizes to a dynamic domain that surrounds
        and penetrates each granule
- term:
    id: GO:1903863
    label: P granule assembly
  evidence_type: IGI
  original_reference_id: PMID:25535836
  review:
    summary: This is a core function annotation. MEG-3 is the primary driver of P
      granule assembly through liquid-liquid phase separation. The IGI evidence reflects
      genetic interactions with meg-4 (WBGene00016485) and other genes. MEG-3/MEG-4
      double mutants fail to assemble P granules in early embryos.
    action: ACCEPT
    reason: P granule assembly is the defining core function of MEG-3. The Wang et
      al. 2014 study shows that MEG proteins are germ plasm components that are required
      redundantly for fertility and that they regulate RNA granule dynamics. The genetic
      interaction with meg-4 demonstrates functional redundancy in P granule assembly.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: The MEG (maternal-effect germline defective) proteins are germ
        plasm components that are required redundantly for fertility
- term:
    id: GO:0036093
    label: germ cell proliferation
  evidence_type: IGI
  original_reference_id: PMID:25535836
  review:
    summary: This annotation reflects a downstream phenotype of MEG-3 function rather
      than a direct molecular function. MEG-3/MEG-4 double mutants show reduced fertility
      (~70% fertile), which correlates with germ cell proliferation defects. However,
      MEG-3's primary role is in P granule assembly, not direct regulation of germ
      cell proliferation.
    action: KEEP_AS_NON_CORE
    reason: While meg-3 meg-4 double mutants show fertility defects, MEG-3's direct
      molecular function is P granule scaffold activity, not direct regulation of
      proliferation. The germ cell proliferation phenotype is a downstream consequence
      of defective P granule assembly and impaired germ plasm inheritance. This annotation
      is not incorrect but represents a non-core, secondary phenotype.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: The MEG (maternal-effect germline defective) proteins are germ
        plasm components that are required redundantly for fertility
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25535836
  review:
    summary: This annotation captures MEG-3's interactions with MBK-2 (UniProtKB:A9UJN4),
      PPTR-1 (UniProtKB:O18178), and PPTR-2 (UniProtKB:Q304E5). These are functionally
      important interactions for regulating MEG-3 phosphorylation status and P granule
      dynamics. However, 'protein binding' is too general; more specific terms should
      be used.
    action: MODIFY
    reason: The protein binding annotation is too vague. MEG-3's interactions with
      MBK-2 kinase and PP2A phosphatase regulatory subunits PPTR-1/2 are functionally
      important for its regulation. A more informative annotation would be molecular
      condensate scaffold activity (GO:0140693), which captures MEG-3's true function
      in binding and bringing together macromolecules into a phase-separated condensate.
    proposed_replacement_terms:
    - id: GO:0140693
      label: molecular condensate scaffold activity
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: "We demonstrate that MEG-1 and MEG-3 are substrates of the\
        \ kinase MBK-2/DYRK and the phosphatase PP2A(PPTR-\xBD)"
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IDA
  original_reference_id: PMID:25535836
  review:
    summary: MEG-3 localizes to the cytoplasm, specifically in association with P
      granules. This annotation is correct but very general; the more specific P granule
      localization is also annotated.
    action: ACCEPT
    reason: This is a correct but general localization annotation. MEG-3 is cytoplasmic
      and specifically associates with P granules. The study used GFP-tagged MEG-3
      to show cytoplasmic localization. While the P granule annotation is more informative,
      this broader cytoplasm annotation is not incorrect and captures the general
      cellular compartment.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: GFP-tagged MEG-3 localizes to a dynamic domain that surrounds
        and penetrates each granule
- term:
    id: GO:0043186
    label: P granule
  evidence_type: IDA
  original_reference_id: PMID:25535836
  review:
    summary: This is a core localization annotation. MEG-3 localizes to P granules
      and forms a dynamic scaffold surrounding and penetrating each granule. This
      was demonstrated by lattice light sheet microscopy of GFP-tagged MEG-3.
    action: ACCEPT
    reason: P granule localization is the key cellular component annotation for MEG-3.
      The Wang et al. 2014 study clearly demonstrates using lattice light sheet microscopy
      that GFP-tagged MEG-3 localizes to a dynamic domain that surrounds and penetrates
      each granule. MEG-3 is a core structural component of P granules.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: GFP-tagged MEG-3 localizes to a dynamic domain that surrounds
        and penetrates each granule
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:11922622
  review:
    summary: This annotation from 2002 reflects MEG-3 (as GEI-12) binding to GEX-3
      in a yeast two-hybrid screen. The physiological significance of this interaction
      is unclear from the publication abstract. GEX-3 is involved in tissue morphogenesis.
    action: MARK_AS_OVER_ANNOTATED
    reason: The Tsuboi et al. 2002 study was a methodological paper describing a novel
      screening approach, using GEX-3 as a model case to identify interacting molecules.
      While MEG-3/GEI-12 was identified as an interactor, the biological significance
      of this interaction is not established. The relevance to MEG-3's core P granule
      function is unclear. This appears to be a non-specific or weak interaction that
      may not reflect in vivo function.
    supported_by:
    - reference_id: PMID:11922622
      supporting_text: We identified many interacting molecules by yeast two-hybrid
        screening and could detect some functional interactions
- term:
    id: GO:0009792
    label: embryo development ending in birth or egg hatching
  evidence_type: IMP
  original_reference_id: PMID:11922622
  review:
    summary: This is a very broad biological process annotation. While MEG-3 mutants
      do show embryonic phenotypes (reduced fertility, P granule defects), this term
      is too general to be informative about MEG-3's specific function in P granule
      assembly.
    action: KEEP_AS_NON_CORE
    reason: The embryo development annotation is not incorrect but is too broad. MEG-3's
      primary function is in P granule assembly and germ plasm organization, which
      are specific aspects of early embryo development. The fertility defects in meg-3
      meg-4 double mutants (~70% fertility) demonstrate a role in embryonic development,
      but this annotation does not capture the specific molecular and cellular function.
      More specific terms like P granule assembly (GO:1903863) are already annotated.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: The MEG (maternal-effect germline defective) proteins are germ
        plasm components that are required redundantly for fertility
- term:
    id: GO:0140693
    label: molecular condensate scaffold activity
  evidence_type: IDA
  original_reference_id: PMID:25535836
  review:
    summary: MEG-3 is the primary scaffold protein for P granule assembly through
      liquid-liquid phase separation. It binds and brings together RNA and other P
      granule proteins (PGL-1, PGL-3) to organize the molecular condensate.
    action: NEW
    reason: MEG-3 is an excellent example of a molecular condensate scaffold. The
      term definition "Binding and bringing together two or more macromolecules in
      contact, permitting those molecules to organize as a molecular condensate" precisely
      describes MEG-3's function. The Wang et al. 2014 study shows MEG-3 localizes
      to a dynamic domain surrounding P granules and regulates their assembly through
      phosphorylation-dependent phase transitions.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: GFP-tagged MEG-3 localizes to a dynamic domain that surrounds
        and penetrates each granule
- term:
    id: GO:0060293
    label: germ plasm
  evidence_type: IDA
  original_reference_id: PMID:25535836
  review:
    summary: MEG-3 is a component of the germ plasm. P granules are germ plasm components,
      and MEG-3 localizes to and drives the assembly of P granules, which are the
      defining structures of C. elegans germ plasm.
    action: NEW
    reason: The Wang et al. 2014 study explicitly states that the MEG proteins are
      germ plasm components. Germ plasm (GO:0060293) is defined as differentiated
      cytoplasm associated with a pole of an oocyte, egg or early embryo that will
      be inherited by the cells that will give rise to the germ line. MEG-3's localization
      to P granules, which are the cytoplasmic manifestation of germ plasm in C. elegans,
      supports this annotation.
    supported_by:
    - reference_id: PMID:25535836
      supporting_text: The MEG (maternal-effect germline defective) proteins are germ
        plasm components that are required redundantly for fertility
core_functions:
- description: MEG-3 is the primary scaffold for P granule assembly. It drives liquid-liquid
    phase separation through its intrinsically disordered N-terminal region. MEG-3
    forms gel-like assemblies that stabilize liquid PGL-3 droplets within P granules.
    Its phosphorylation state, controlled by MBK-2 kinase and PP2A phosphatase, regulates
    the assembly-disassembly dynamics of P granules.
  molecular_function:
    id: GO:0140693
    label: molecular condensate scaffold activity
  directly_involved_in:
  - id: GO:1903863
    label: P granule assembly
  locations:
  - id: GO:0043186
    label: P granule
  - id: GO:0060293
    label: germ plasm
  supported_by:
  - reference_id: PMID:25535836
    supporting_text: GFP-tagged MEG-3 localizes to a dynamic domain that surrounds
      and penetrates each granule
  - reference_id: PMID:25535836
    supporting_text: Phosphorylation of the MEGs promotes granule disassembly and
      dephosphorylation promotes granule assembly
proposed_new_terms: []
suggested_questions:
- question: What is the precise mechanism by which MEG-3 IDR drives phase separation
    - does it involve specific amino acid motifs or is it a more general property
    of the disordered region?
- question: How does MEG-3 coordinate with MEG-4 in P granule assembly, and why are
    single mutants fertile while double mutants show significant fertility defects?
- question: What RNAs does MEG-3 bind, and does it show specificity for maternal germline
    mRNAs?
suggested_experiments:
- description: CLIP-seq or eCLIP to identify MEG-3 RNA targets. This would reveal
    which RNAs MEG-3 binds and whether it shows specificity for germline-enriched
    transcripts.
  hypothesis: MEG-3 preferentially binds maternal germline mRNAs to recruit them to
    P granules
- description: Structure-function analysis of MEG-3 IDR using systematic deletions.
    This would identify minimal sequences required for phase separation and P granule
    assembly.
  hypothesis: Specific sequence motifs within the IDR are required for phase separation
- description: Phospho-proteomic analysis of MEG-3 under different developmental conditions.
    This would map the phosphorylation sites regulated by MBK-2 and PP2A and correlate
    with granule dynamics.
  hypothesis: Specific phosphorylation sites control the sol-gel transition of MEG-3
tags:
- caeel-p-granules