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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0051640 organelle localization | IMP PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | ACCEPT | 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. 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. Supporting Evidence: PMID:25535836 GFP-tagged MEG-3 localizes to a dynamic domain that surrounds and penetrates each granule file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3 is maternally supplied and forms an **anterior-low/posterior-high cytoplasmic gradient**; within granules it occupies a **peri-granular domain** that surrounds and penetrates granules |
| GO:1903863 P granule assembly | IGI PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | ACCEPT | 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. 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. Supporting Evidence: PMID:25535836 The MEG (maternal-effect germline defective) proteins are germ plasm components that are required redundantly for fertility file:worm/meg-3/meg-3-deep-research-falcon.md P-granule asymmetry** in the polarized zygote depends on **RNA-induced phase separation/condensation of MEG-3**, which acts upstream of stable PGL/GLH granule retention file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3/4 act **upstream** of PGL components in zygotes: they are needed for stable, asymmetric posterior P-granule assembly and for segregation of granule contents into germline blastomeres |
| GO:0036093 germ cell proliferation | IGI PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | KEEP AS NON CORE | 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. 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. Falcon deep research reinforces that the fertility/germ-plasm roles are downstream of MEG-3's scaffold activity: MEG-3/4 act upstream of PGL components and loss of MEG-3 produces graded sterility (~30% in meg-3 meg-4; 100% in the meg-1 meg-3 meg-4 triple), indicating an overlapping, redundant germ-plasm function rather than a dedicated proliferation role. Supporting Evidence: PMID:25535836 The MEG (maternal-effect germline defective) proteins are germ plasm components that are required redundantly for fertility file:worm/meg-3/meg-3-deep-research-falcon.md meg-3 meg-4** mutants show **~30% sterility** file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3/4 act **upstream** of PGL components in zygotes: they are needed for stable, asymmetric posterior P-granule assembly and for segregation of granule contents into germline blastomeres |
| GO:0005515 protein binding | IPI PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | MODIFY | 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. 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 replacements: molecular condensate scaffold activity Supporting Evidence: PMID:25535836 We demonstrate that MEG-1 and MEG-3 are substrates of the kinase MBK-2/DYRK and the phosphatase PP2A(PPTR-½) file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3 is an experimentally identified substrate of the DYRK-family kinase **MBK-2**, and is also regulated by **PP2A phosphatase activity (PPTR-1/2-associated)** file:worm/meg-3/meg-3-deep-research-falcon.md a **C-terminal predicted ordered HMG-like motif (HMGL)** that contributes to condensation and **mediates binding to PGL-3** |
| GO:0005737 cytoplasm | IDA PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | ACCEPT | 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. 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. Supporting Evidence: PMID:25535836 GFP-tagged MEG-3 localizes to a dynamic domain that surrounds and penetrates each granule file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3 is maternally supplied and forms an **anterior-low/posterior-high cytoplasmic gradient**; within granules it occupies a **peri-granular domain** that surrounds and penetrates granules |
| GO:0043186 P granule | IDA PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | ACCEPT | 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. 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. Supporting Evidence: PMID:25535836 GFP-tagged MEG-3 localizes to a dynamic domain that surrounds and penetrates each granule file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3 contributes a distinct material phase that surrounds and interpenetrates PGL-rich regions |
| GO:0005515 protein binding | IPI PMID:11922622 Isolation of the interacting molecules with GEX-3 by a novel... | MARK AS OVER ANNOTATED | 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. 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. Notably, the falcon deep research synthesis of the primary MEG-3 literature does not mention a GEX-3/GEI-12 interaction at all; the well-supported function across studies is that MEG-3 is an RNA-condensate scaffold for embryonic germ granules, not a GEX-3 binding partner, further supporting over-annotation of this generic protein-binding term. Supporting Evidence: PMID:11922622 We identified many interacting molecules by yeast two-hybrid screening and could detect some functional interactions file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3 is best annotated as an **RNA-condensate scaffold/regulator of embryonic germ granule assembly and mRNA partitioning** rather than as an enzyme or transporter |
| GO:0009792 embryo development ending in birth or egg hatching | IMP PMID:11922622 Isolation of the interacting molecules with GEX-3 by a novel... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:25535836 The MEG (maternal-effect germline defective) proteins are germ plasm components that are required redundantly for fertility file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3/4 act **upstream** of PGL components in zygotes: they are needed for stable, asymmetric posterior P-granule assembly and for segregation of granule contents into germline blastomeres |
| GO:0140693 molecular condensate scaffold activity | IDA PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | NEW | 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. 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. Falcon deep research independently synthesizes the primary literature to the same conclusion: MEG-3 is an intrinsically disordered protein that binds RNA broadly and uses RNA-stimulated condensation to organize germ-plasm RNP condensates, recruiting maternal mRNAs by forming a gel-like RNA-rich phase on the surface of more dynamic PGL condensates, with a C-terminal HMG-like motif mediating binding to PGL-3 for co-assembly of the composite granule. Supporting Evidence: PMID:25535836 GFP-tagged MEG-3 localizes to a dynamic domain that surrounds and penetrates each granule PMID:34106046 MEG-3 is a modular protein that uses its IDR to bind RNA and its C-terminus to drive condensation. The HMGL motif mediates binding to PGL-3 and is required for co-assembly of MEG-3 and PGL-3 condensates in vivo. PMID:31975687 P granules recruit mRNAs by condensation with the disordered protein MEG-3. file:worm/meg-3/meg-3-deep-research-falcon.md an intrinsically disordered protein (IDP) that binds RNA broadly and uses RNA-stimulated condensation to spatially organize germ-plasm RNP condensates file:worm/meg-3/meg-3-deep-research-falcon.md It also recruits maternal mRNAs into granules by forming a **gel-like RNA-rich phase** on the surface of more dynamic PGL condensates |
| GO:0060293 germ plasm | IDA PMID:25535836 Regulation of RNA granule dynamics by phosphorylation of ser... | NEW | 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. 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. Supporting Evidence: PMID:25535836 The MEG (maternal-effect germline defective) proteins are germ plasm components that are required redundantly for fertility PMID:34106046 germ (P) granule assembly requires MEG-3, an intrinsically disordered protein that forms RNA-rich condensates on the surface of PGL condensates at the core of P granules file:worm/meg-3/meg-3-deep-research-falcon.md MEG-3 is maternally supplied and forms an **anterior-low/posterior-high cytoplasmic gradient**; within granules it occupies a **peri-granular domain** that surrounds and penetrates granules |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Q: Lee et al. 2020 (PMID:31975687) showed MEG-3 binds ~500 mRNAs in a sequence-independent manner favoring long, low-ribosome-occupancy embryonic transcripts. How is this RNA capture modulated by MEG-3 phosphorylation state (MBK-2 vs PP2A/PPTR-1/2), and how does posterior MEG-3 discriminate its mRNAs from the MEX-5-bound transcript pool in the anterior cytoplasm?
Experiment: Compare MEG-3 in vivo RNA binding (iCLIP/CLIP) between phosphomimetic and phospho-dead MEG-3 alleles, or in mbk-2 versus pptr-1/2 perturbed backgrounds, to test whether the MEG-3 RNA target set changes with phosphorylation state. The sequence-independent ~500-mRNA target set is already known from Lee et al. 2020 iCLIP (PMID:31975687); this follow-up instead asks how the MBK-2/PP2A assembly-disassembly switch reshapes RNA capture rather than re-identifying targets.
Hypothesis: MBK-2 phosphorylation reduces MEG-3 RNA condensation and the breadth/amount of bound mRNA, while PP2A/PPTR-1/2-mediated dephosphorylation increases RNA capture, linking the phospho-switch to mRNA recruitment.
Experiment: Spatially resolved (anterior vs posterior) profiling of MEG-3-bound versus MEX-5-bound transcripts in the polarized zygote, e.g. by region-specific crosslinking/proximity labeling, to determine whether MEG-3 captures a distinct mRNA pool from MEX-5 or simply condenses whatever RNA escapes the MEX-5 anterior sink.
Hypothesis: MEG-3 and MEX-5 compete for a shared, largely sequence-independent mRNA pool, so posterior MEG-3 enrichment of mRNAs reflects local RNA availability set by the MEX-5 gradient rather than intrinsic MEG-3 sequence specificity.
Experiment: 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
Experiment: 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
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