| Aspect | Key findings | Evidence type/method | Source (with year/URL) |
|---|---|---|---|
| Identity and domains | **mex-5** in **Caenorhabditis elegans** encodes an embryonic polarity mediator with **two tandem CCCH zinc-finger domains**, consistent with an RNA-binding protein and matching the UniProt Q9XUB2 domain context. It functions redundantly with the close paralog **mex-6**. (pqac-00000002, pqac-00000007) | Gene identification, mutant analysis, protein domain annotation from primary and review literature | Kemphues 2000, https://doi.org/10.1016/S0092-8674(00)80844-2; Rose & Gonczy 2014, https://doi.org/10.1895/wormbook.1.30.2 |
| Localization pattern in early embryo | MEX-5 is initially broadly cytoplasmic after fertilization, then becomes **anterior-enriched** by the end of the one-cell stage and is preferentially inherited by **AB**; it is also reported on **P granules/RNP assemblies** during early divisions. In par-1 mutants, the gradient is weakened or lost. (pqac-00000000, pqac-00000015, pqac-00000023) | Live imaging of GFP/Dendra-tagged proteins; endogenous protein localization; mutant phenotyping | Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012; Spilker 2009, https://doi.org/10.1534/genetics.109.106716; Rose & Gonczy 2014, https://doi.org/10.1895/wormbook.1.30.2 |
| Core molecular function: RNA binding | MEX-5/6 are **RNA-binding polarity mediators** that act largely through **translational control** and regulated association with RNA-containing complexes. Their CCCH zinc fingers are required for normal mobility/asymmetry, supporting direct functional coupling between RNA binding and polarity formation. (pqac-00000000, pqac-00000007, pqac-00000009) | Domain-function inference, mutational analysis of zinc fingers, biochemical and imaging studies | Rose & Gonczy 2014, https://doi.org/10.1895/wormbook.1.30.2; Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012 |
| Translational regulation / zif-1 pathway | MEX-5/6 bind the **zif-1 3′UTR** and act **positively** to relieve repression, antagonizing POS-1 on the same regulatory region; this enables somatic **zif-1** expression and downstream **ZIF-1–dependent degradation** of germline determinants such as **PIE-1, POS-1, and MEX-1** in somatic lineages. (pqac-00000001, pqac-00000024) | 3′UTR regulatory assays, genetic interaction analysis, developmental phenotyping summarized in review | Rose & Gonczy 2014, https://doi.org/10.1895/wormbook.1.30.2 |
| Scaffold/recruiter for polo kinases | MEX-5/6 are required for **anterior enrichment of PLK-1 and PLK-2**; the polo kinases bind MEX-5/6 via their **polo-box domains**. Mutations that disrupt PLK binding impair MEX-5 function without abolishing MEX-5 asymmetry, supporting a **scaffold/adaptor role** rather than simple localization dependence. Recent work reiterates that MEX-5 recruits **PLK-1** to the anterior cytoplasm to regulate segregation of posterior factors. (pqac-00000001, pqac-00000006) | Yeast two-hybrid interaction assays, mutant functional analysis, recent mechanistic preprint | Rose & Gonczy 2014, https://doi.org/10.1895/wormbook.1.30.2; Kim et al. 2024, https://doi.org/10.1101/2024.07.26.605193 |
| Upstream regulator: PAR-1 | **PAR-1** is the principal upstream kinase coupling cortical/cytoplasmic polarity to MEX-5 asymmetry. Posterior PAR-1 activity phosphorylates MEX-5, increases its mobility in the posterior, and thereby drives formation of the **anterior-high MEX-5 gradient**. (pqac-00000004, pqac-00000008, pqac-00000012) | In vitro kinase assays, phosphosite mapping, live photoconversion/diffusion analysis, mechanistic modeling | Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012; Folkmann & Seydoux 2019, https://doi.org/10.1242/dev.171116 |
| Antagonistic phosphatase control | A largely uniform **PP2A/LET-92** phosphatase antagonizes PAR-1-dependent phosphorylation, returning MEX-5 to slower-diffusing states; reducing PP2A activity increases mobility and weakens asymmetry. (pqac-00000009, pqac-00000022) | Okadaic acid sensitivity, let-92(RNAi), phospho-specific antibodies, reaction-diffusion modeling | Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012 |
| Key phosphorylation sites | PAR-1 phosphorylates MEX-5 at **S404** and **S458**; the **S404A/S458A double mutant** is no longer a substrate in the in vitro PAR-1 assay. **S404** is rapidly dephosphorylated in embryo extract and linked to PP2A-sensitive regulation; **S458** is also par-1/par-4 dependent and contributes to fast diffusion, but phosphorylation at S458 alone does not fully explain gradient formation. (pqac-00000008, pqac-00000009, pqac-00000012) | In vitro kinase assays with activated PAR-1, phosphosite mutagenesis, phospho-specific antibodies, embryo extracts | Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012 |
| Quantitative diffusion measurements | Full-length MEX-5 apparent diffusion coefficients from Dendra photoconversion were ~**1.03 ± 0.11 μm²/s** anterior and **2.68 ± 0.24 μm²/s** posterior. FCS supported two effective diffusive classes: **fast ~5.15 μm²/s** and **slow ~0.086 μm²/s**; weighted averages were ~**1.40 ± 0.29 μm²/s** anterior and **3.13 ± 0.37 μm²/s** posterior. (pqac-00000009, pqac-00000010) | Photoconversion stripe assays, fluorescence correlation spectroscopy (FCS), quantitative modeling | Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012 |
| Quantitative composition of mobility states | FCS indicated the **slow:fast** MEX-5 concentration ratio is about **66:34 in the anterior** versus **50:50 in the posterior**, implying the anterior is enriched for slow, likely RNA-complexed species that dominate the visible protein gradient. (pqac-00000010, pqac-00000022) | FCS two-component fitting, reaction-diffusion interpretation | Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012 |
| Quantitative gradient timing/model | Modeling with **Dfast = 5 μm²/s** and **Dslow = 0.07 μm²/s** showed that a cytoplasmic kinase/phosphatase cycle can establish the gradient rapidly (example **~160 s** with kphos = 0.1 s⁻¹), whereas a cortical-only PAR-1 model would be much slower (**~17 min**), supporting a cytoplasmic PAR-1 activity gradient. (pqac-00000010, pqac-00000011, pqac-00000026) | Reaction-diffusion modeling anchored to in vivo diffusion measurements | Griffin 2011, https://doi.org/10.1016/j.cell.2011.08.012 |
| MAPK antagonism / genetic suppression | Wild-type embryos showed a MEX-5 anterior enrichment ratio of **2.2 ± 0.5** (n=35), **par-1** mutants **1.5 ± 0.3** (n=35), and **mpk-1; par-1** double mutants **1.8 ± 0.4** (n=35), indicating partial restoration of asymmetry. Embryonic viability was also improved in the double mutant (reported approximately **38 ± 9%** versus very low viability for par-1 alone in the cited excerpt). (pqac-00000013, pqac-00000017) | Quantitative fluorescence ratio measurements; suppressor genetics | Spilker 2009, https://doi.org/10.1534/genetics.109.106716 |
| Downstream effects on PIE-1 and P granules | par-1 mutants strongly reduce posterior **PIE-1** enrichment (**~1.8 ± 3** vs **6.1 ± 1.4** in wild type; n=39), while **mpk-1; par-1** partially restores it (**~2.5 ± 0.9**). **P granules** are absent in one-cell par-1 embryos but reappear in mpk-1; par-1 embryos, though not properly posterior-localized. MEX-5/6 are thus central to partitioning germline condensates and determinants. (pqac-00000013, pqac-00000019, pqac-00000020) | Immunofluorescence/localization phenotyping, suppressor genetics, conceptual synthesis | Spilker 2009, https://doi.org/10.1534/genetics.109.106716; Hoege & Hyman 2013, https://doi.org/10.1038/nrm3558 |
| Conceptual model: phase separation / condensates | Current understanding is that the MEX-5 gradient links membrane PAR polarity to **cytoplasmic phase behavior**: high anterior MEX-5 promotes **P-granule dissolution**, whereas low posterior MEX-5 permits **condensation/phase separation** of germ plasm assemblies. This model explains how cortical asymmetry is translated into cytoplasmic organization. (pqac-00000018, pqac-00000019, pqac-00000020, pqac-00000021) | Integrative expert review drawing on primary biophysical and developmental studies | Hoege & Hyman 2013, https://doi.org/10.1038/nrm3558; Rose & Gonczy 2014, https://doi.org/10.1895/wormbook.1.30.2 |
| Recent 2023 development | In the two-cell embryo (**P1**), re-establishment of PAR polarity involves at least **two parallel pathways**; the early pathway depends on **PAR-1, PKC-3, and downstream cytoplasmic polarity including MEX-5 and PLK-1**. PAR-2 polarization begins within about **2 min** after P0 cytokinesis and a posterior domain is present within about **4 min**. (pqac-00000027, pqac-00000029) | Live imaging and quantitative polarity timing in preprint study | Koch & Rose 2023, https://doi.org/10.1101/2022.12.15.520651 |
| Recent 2024 development | New preprint evidence extends the MEX-5-centered model by showing that **PLK-1 activity and the MEX-5–PLK-1 interaction** regulate polarization of another posterior factor, **MEX-1**. This reinforces the view of MEX-5 as a central upstream organizer/scaffold for kinase-dependent segregation of germline proteins in the zygote. (pqac-00000005, pqac-00000006) | Recent mechanistic preprint with phosphosite mutagenesis and genetic analysis of related polarity factor | Kim et al. 2024, https://doi.org/10.1101/2024.07.26.605193 |


*Table: This table summarizes experimentally supported properties of C. elegans MEX-5, including domains, localization, regulators, molecular functions, phosphosites, and key quantitative measurements. It consolidates foundational and recent evidence to support functional annotation of UniProt Q9XUB2.*