Maize EZ1 (Q8S4P6), also known as MEZ1 (Maize Enhancer-of-zeste 1), is an Enhancer-of-zeste [E(z)]-class SET-domain histone-lysine N-methyltransferase (EC 2.1.1.356) and the catalytic-subunit candidate of plant Polycomb Repressive Complex 2 (PRC2). It is one of three maize E(z)-like genes (Mez1/Mez2/Mez3); phylogenetically Mez1 is the maize CLF-like (CURLY LEAF-like) homolog, whereas Mez2/Mez3 are EZA1/SWN-like (SWINGER-like) (Springer et al. 2002, doi:10.1104/pp.010742). The UniProt FUNCTION statement describes it as a Polycomb group protein and "Catalytic subunit of some PcG multiprotein complex, which methylates 'Lys-27' of histone H3, leading to transcriptional repression of the affected target genes". The enzyme uses S-adenosyl-L-methionine to transfer methyl groups onto Lys-27 of histone H3 (H3K27me1/2/3), establishing a repressive chromatin state (facultative heterochromatin) at target loci; in plants PRC2 is defined by this H3K27 methylation activity. The protein carries the canonical E(z)-family architecture (EZD1/EZD2, SANT, a Cys-rich CXC region, and the C-terminal SET domain "predicted to be involved in protein methylation") and acts in the nucleus on chromatin within PRC2-like assemblies that include maize homologs of FIE/MSI1/SU(Z)12-like proteins. As part of plant PRC2, MEZ1 functions in epigenetic gene silencing and developmental regulation (flowering, photoperiod response) and is notable as the only one of the three maize E(z) homologs that is imprinted, consistent with the strong association of maize H3K27me3 with imprinted (paternally expressed) genes in endosperm. Direct maize loss-of-function genetics exist for Mez2/Mez3 (whose mutants reduce H3K27me3 at a subset of loci, implying partial redundancy); a Mez1-specific knockout and in vitro biochemistry on the maize protein itself were not available in the retrieved literature, so the catalytic and complex-membership annotations rest on strong sequence/domain, phylogenetic, and orthology evidence plus the conserved plant PRC2 mechanism.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0032259 methylation | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt keyword "Methyltransferase"/"Transferase"; snapshot-only, removed in the current GOA release. EZ1/MEZ1 is genuinely a methyltransferase, but "methylation" is the generic process term that drops the substrate: the enzyme specifically performs histone H3 Lys-27 methylation. Reason: GOA's removal of this annotation was JUSTIFIED. The keyword-derived term "methylation" (GO:0032259) is the high-level parent process that simply states a methyl group is transferred, dropping all substrate specificity. EZ1/MEZ1 is an E(z)-class enzyme whose UniProt FUNCTION explicitly states it "methylates 'Lys-27' of histone H3", i.e. it performs substrate-specific histone H3K27 methylation as a SAM-dependent histone lysine methyltransferase acting on H3K27. The substrate specificity is already captured by the molecular-function terms retained in current GOA - "histone H3K27 methyltransferase activity" (GO:0046976) and "histone H3K27 trimethyltransferase activity" (GO:0140951) - and the biological role is captured by "heterochromatin formation" (GO:0031507) and "negative regulation of gene expression, epigenetic" (GO:0045814). The bare "methylation" process term therefore adds no information once these specific annotations are present. It cannot be usefully MODIFIED to a specific histone-methylation process term either, because the candidate substrate-specific process terms in GO ("histone H3-K27 methylation" GO:0070734 and "histone methylation" GO:0016571) are now obsolete - GO carries the substrate specificity on the molecular-function branch rather than as a dedicated process. Removal of the redundant generic keyword-derived term is appropriate (over-annotation). Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md its expected enzymatic role is to use S-adenosylmethionine (SAM) to methylate **histone H3 Lys-27**, producing H3K27me3 on chromatin within PRC2 file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2 catalyzes **H3K27 trimethylation (H3K27me3)** |
| GO:0003682 chromatin binding | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation propagated across the E(z)/EZH phylogenetic group. As a PRC2 catalytic subunit, EZ1/MEZ1 associates with chromatin (nucleosomes) to deposit H3K27 methylation. Reason: Appropriate and well supported by conserved biology. PRC2 is a chromatin-associated nuclear complex that deposits histone methylation on nucleosomes at genomic loci, so chromatin binding is a genuine, conserved molecular activity of the E(z)/EZH family. The IBA term is at the right level of specificity and consistent with the EZ1 SANT and CXC chromatin-engaging modules described for the maize MEZ proteins. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2-dependent deposition of H3K27me3 is a chromatin-associated nuclear process file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2 complex subunits are required for nucleosome association of PRC2 |
| GO:0031507 heterochromatin formation | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation: the E(z)/PRC2 family establishes repressive (facultative heterochromatin) chromatin states via H3K27me3. This is a core biological process of EZ1/MEZ1. Reason: Core function, strongly supported by conserved plant PRC2 biology and maize-specific epigenomics. Plant PRC2 deposits H3K27 methylation that establishes a repressive chromatin state at target loci, and maize H3K27me3 marks define facultative heterochromatin and are attributed to the E(z)/PRC2 pathway. The IBA term is at an appropriate level of specificity for an E(z)-class catalytic subunit. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md Genomic distribution of maize facultative heterochromatin marked by trimethylation of H3K27 file:MAIZE/EZ1/EZ1-deep-research-falcon.md Maize H3K27me3 is tissue-variable and enriched in gene-dense chromosome arms |
| GO:0005634 nucleus | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation for nuclear localization, consistent with the UniProt subcellular location ("Nucleus") and with PRC2 being a chromatin-associated nuclear complex. Reason: Correct and well supported. The UniProt entry assigns subcellular location "Nucleus", and PRC2 is inherently a chromatin-associated nuclear complex because it deposits histone methylation on nucleosomes; genome-wide analyses report that most PRC2 proteins localize to the nucleus. Although maize-specific localization microscopy for MEZ1 was not retrieved, the conservative inference is strong. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md most PRC2 core proteins localize to the **nucleus** and physically interact to form multi-subunit complexes file:MAIZE/EZ1/EZ1-deep-research-falcon.md maize PRC2 core subunits (including E(z)/MEZ family proteins) are nuclear/chromatin-associated |
| GO:0006338 chromatin remodeling | IEA GO_REF:0000002 | MODIFY | Summary: IEA annotation from InterPro (IPR045318, EZH1/2-like). "Chromatin remodeling" is an imprecise process term for a histone methyltransferase whose action is covalent H3K27 methylation, not ATP-dependent nucleosome repositioning. Reason: The essence (EZ1 alters chromatin state) is correct, but "chromatin remodeling" (GO:0006338) conventionally denotes dynamic, often ATP-dependent reorganization of nucleosome structure, whereas EZ1/MEZ1 acts by depositing a covalent histone mark (H3K27 methylation) to establish a repressive chromatin state. The biology is more precisely captured by "heterochromatin formation" (GO:0031507), which is the established process for E(z)/PRC2-mediated H3K27me3 silencing and is already present via the IBA annotation. Modify to the more specific and accurate process term. Proposed replacements: heterochromatin formation Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2 catalyzes **H3K27 trimethylation (H3K27me3)** file:MAIZE/EZ1/EZ1-deep-research-falcon.md Genomic distribution of maize facultative heterochromatin marked by trimethylation of H3K27 |
| GO:0031519 PcG protein complex | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation: EZ1/MEZ1 is part of a Polycomb group (PcG) protein complex (PRC2). This is a core cellular-component annotation for an E(z)-class catalytic subunit. Reason: Correct and core. The UniProt FUNCTION states EZ1 is a Polycomb group protein and the catalytic subunit of "some PcG multiprotein complex"; plant E(z)-class proteins function within PRC2-like assemblies that include FIE/MSI1/SU(Z)12-like subunits. In maize, EZ1/MEZ1 is most plausibly functional as part of PRC2-like complexes. The term is at an appropriate level of specificity for complex membership. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md **ZmMEZ1** is highly expressed together with **ZmFie1, ZmEmf2b, ZmMSI1c**, supporting participation in a grain-filling PRC2 complex PMID:11950982 Polycomb group (PcG) proteins play an important role in developmental and |
| GO:0042054 histone methyltransferase activity | IEA GO_REF:0000002 | MODIFY | Summary: IEA annotation from InterPro (IPR045318, EZH1/2-like). EZ1/MEZ1 is a histone methyltransferase, but the substrate-specific child term "histone H3K27 methyltransferase activity" (GO:0046976) is more informative and is already present. Reason: The annotation is correct but too general. EZ1/MEZ1 is an E(z)-class enzyme that specifically methylates Lys-27 of histone H3; the deep research concludes it is a SAM-dependent histone lysine methyltransferase acting on H3K27, consistent with its EC assignment in UniProt (EC 2.1.1.356) and its E(z)-class SET domain conservation. The generic "histone methyltransferase activity" should be modified to the substrate-specific "histone H3K27 methyltransferase activity" (GO:0046976), which is already annotated (IBA/IEA) and captures the true catalytic specificity. Proposed replacements: histone H3K27 methyltransferase activity Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md its expected enzymatic role is to use S-adenosylmethionine (SAM) to methylate **histone H3 Lys-27**, producing H3K27me3 on chromatin within PRC2 file:MAIZE/EZ1/EZ1-deep-research-falcon.md E(z)-family SET-domain proteins (e.g., **CLF/SWN/MEA** in Arabidopsis) as the catalytic subunits responsible for depositing H3K27me3 |
| GO:0140951 histone H3K27 trimethyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation transferred from EC 2.1.1.356 (UniProtKB-EC). This is the most specific molecular-function term and matches the UniProt catalytic activity (formation of H3K27me3). Reason: Correct and maximally specific. The UniProt CATALYTIC ACTIVITY record assigns EC 2.1.1.356 (L-lysyl(27)-[histone H3] + 3 SAM -> trimethyl-L-lysyl(27)-[histone H3]), i.e. histone H3K27 trimethyltransferase activity, and the deep research confirms EZ1 is a SAM-dependent histone lysine methyltransferase acting on H3K27 consistent with its EC assignment in UniProt. This is a core molecular function; accept as-is. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2 catalyzes **H3K27 trimethylation (H3K27me3)** file:MAIZE/EZ1/EZ1-deep-research-falcon.md its expected enzymatic role is to use S-adenosylmethionine (SAM) to methylate **histone H3 Lys-27**, producing H3K27me3 on chromatin within PRC2 |
| GO:0001222 transcription corepressor binding | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: IEA annotation transferred from the Arabidopsis ortholog (AT2G23380 = SWN, P93831) by Ensembl Compara. "Transcription corepressor binding" is a non-specific protein-binding term with no maize-specific support and is not part of the gene's core methyltransferase function. Reason: This is an orthology-transferred protein-binding annotation lacking gene-specific support in maize. The retrieved literature does not identify a specific corepressor partner of EZ1/MEZ1; the gene's well-supported molecular function is its E(z)-class H3K27 methyltransferase activity within PRC2. "Protein binding"-type terms that do not identify an informative interaction partner are discouraged and add little once the catalytic MF and PcG-complex membership are annotated. Treat as an over-annotation pending direct interaction evidence. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md E(z)-family SET-domain proteins (e.g., **CLF/SWN/MEA** in Arabidopsis) as the catalytic subunits responsible for depositing H3K27me3 |
| GO:0003727 single-stranded RNA binding | IEA GO_REF:0000107 | REMOVE | Summary: IEA annotation transferred from the Arabidopsis ortholog (AT2G23380 = SWN) by Ensembl Compara. There is no maize-specific evidence that EZ1/MEZ1 binds single-stranded RNA, and this is not part of its supported function. Reason: Unsupported orthology transfer. While PRC2 RNA association has been reported in some systems, it is mechanistically contested and there is no maize-specific evidence that EZ1/MEZ1 binds single-stranded RNA. The retrieved literature characterizes EZ1/MEZ1 strictly as a chromatin-associated, nuclear E(z)-class histone methyltransferase acting on H3K27; no RNA-binding assay supports this term for the maize protein. A speculative, ortholog-transferred ssRNA-binding MF that is not part of the gene's demonstrated function should be removed. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2-dependent deposition of H3K27me3 is a chromatin-associated nuclear process |
| GO:0005677 chromatin silencing complex | IEA GO_REF:0000107 | ACCEPT | Summary: IEA annotation (Ensembl ortholog transfer; also present via EnsemblPlants in the UniProt record). EZ1/MEZ1 is part of a PRC2 chromatin-silencing complex that establishes repressive H3K27me3 chromatin. Reason: Consistent with the gene's role. Plant PRC2 is a chromatin-silencing complex that deposits H3K27 methylation to establish a repressive chromatin state at target loci, mediating transcriptional repression; EZ1/MEZ1 is the catalytic subunit of such a PcG complex per UniProt. This complex-membership term is appropriate and overlaps with the PcG protein complex annotation. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md classification into the four core groups (E(z), Su(z)12, ESC, p55/MSI), nuclear localization of components PMID:11950982 these genes likely play a conserved role in repressing gene expression |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000107 | MODIFY | Summary: IEA annotation (Ensembl ortholog transfer). EZ1/MEZ1 regulates transcription, but this is a very generic process term; its action is specifically epigenetic transcriptional repression. Reason: The essence is correct (EZ1 affects transcription) but the term is overly general. EZ1/MEZ1 acts by depositing repressive H3K27 methylation leading to transcriptional repression of target genes (UniProt FUNCTION), i.e. it mediates epigenetic negative regulation of gene expression. The more specific and informative term "negative regulation of gene expression, epigenetic" (GO:0045814) - which is already present as a separate IEA annotation - better captures this directional, mechanism- specific role. Modify the generic transcription-regulation term to the epigenetic negative-regulation term. Proposed replacements: negative regulation of gene expression, epigenetic Supporting Evidence: PMID:11950982 these genes likely play a conserved role in repressing gene expression file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2-mediated H3K27me3 participates in **allele-specific repression** and imprinting behavior in maize endosperm |
| GO:0009909 regulation of flower development | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: IEA annotation transferred from the Arabidopsis ortholog (SWN/CLF) by Ensembl Compara. In Arabidopsis, E(z)/PRC2 represses floral regulators via H3K27me3; this is a pleiotropic developmental role rather than the core molecular function. Reason: Plausible by orthology and consistent with the well-known role of Arabidopsis E(z) homologs (CLF/SWN) in repressing flowering-pathway genes via H3K27me3, but it is a downstream, pleiotropic developmental consequence of PRC2 silencing rather than the gene's core biochemical function. No maize-specific flower-development phenotype was retrieved for MEZ1. Retain as a non-core developmental process. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md braA.clf-1 mutants have reduced H3K27me3 at flowering integrator loci (FT, SOC1, SEP3 homologs) with increased transcript levels and accelerated flowering |
| GO:0031507 heterochromatin formation | IEA GO_REF:0000107 | ACCEPT | Summary: IEA annotation (Ensembl ortholog transfer); duplicates the IBA annotation to the same term. Heterochromatin (facultative, H3K27me3) formation is a core process for the E(z)/PRC2 family. Reason: Correct and consistent with the IBA annotation to the same term. Plant PRC2 deposits H3K27me3 to establish facultative heterochromatin, and maize H3K27me3 marks define facultative heterochromatin attributed to the E(z)/PRC2 pathway. Duplicate annotations with different evidence codes are acceptable; this IEA provides additional computational support for a core process. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md Genomic distribution of maize facultative heterochromatin marked by trimethylation of H3K27 file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2 catalyzes **H3K27 trimethylation (H3K27me3)** |
| GO:0045814 negative regulation of gene expression, epigenetic | IEA GO_REF:0000107 | ACCEPT | Summary: IEA annotation (Ensembl ortholog transfer). EZ1/MEZ1 mediates epigenetic gene silencing through H3K27 methylation. This accurately captures the directional, mechanism-specific biological role of PRC2. Reason: Accurate and informative. EZ1/MEZ1 deposits repressive H3K27 methylation leading to transcriptional repression of target genes (UniProt FUNCTION) and is a PRC2 catalytic subunit; PRC2 is the canonical machinery for epigenetic negative regulation of gene expression in plants. The term correctly captures the gene's directional repressive role and is preferable to the generic "regulation of DNA-templated transcription". Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md PRC2-mediated H3K27me3 participates in **allele-specific repression** and imprinting behavior in maize endosperm PMID:11950982 these genes likely play a conserved role in repressing gene expression |
| GO:0046976 histone H3K27 methyltransferase activity | IEA GO_REF:0000107 | ACCEPT | Summary: IEA annotation (Ensembl ortholog transfer). This is the substrate-specific molecular function of EZ1/MEZ1 and a core annotation. Reason: Core molecular function, correctly specified. EZ1/MEZ1 is an E(z)-class enzyme that methylates Lys-27 of histone H3 (UniProt FUNCTION) and is a SAM-dependent histone lysine methyltransferase acting on H3K27 consistent with its EC assignment in UniProt and its E(z)-class SET domain conservation. This is the central activity of the gene product; accept. (The narrower trimethyltransferase term GO:0140951 is also present and likewise accepted.) Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md its expected enzymatic role is to use S-adenosylmethionine (SAM) to methylate **histone H3 Lys-27**, producing H3K27me3 on chromatin within PRC2 file:MAIZE/EZ1/EZ1-deep-research-falcon.md E(z)-family SET-domain proteins (e.g., **CLF/SWN/MEA** in Arabidopsis) as the catalytic subunits responsible for depositing H3K27me3 |
| GO:0048586 regulation of long-day photoperiodism, flowering | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: IEA annotation transferred from the Arabidopsis ortholog by Ensembl Compara (also reflected in the UniProt EnsemblPlants GO line). A pleiotropic, photoperiod-dependent developmental role inferred from orthology, not the core function. Reason: A plausible orthology-based developmental role: Arabidopsis E(z)/PRC2 represses flowering-time regulators (e.g. FLC/FT module) via H3K27me3, and photoperiod-dependent flowering is one downstream output. This is a pleiotropic, indirect developmental consequence of PRC2-mediated silencing rather than the gene's core biochemical function, and no maize-specific photoperiod phenotype was retrieved for MEZ1. Retain as non-core. Supporting Evidence: file:MAIZE/EZ1/EZ1-deep-research-falcon.md CLF as a “major H3K27 methyltransferase regulating flowering time” file:MAIZE/EZ1/EZ1-deep-research-falcon.md braA.clf-1 mutants have reduced H3K27me3 at flowering integrator loci (FT, SOC1, SEP3 homologs) with increased transcript levels and accelerated flowering |
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Download this section (compressed HTML)Q: Does purified maize MEZ1 (EZ1) have intrinsic H3K27 methyltransferase activity in a reconstituted maize PRC2, and what is its product specificity (mono/di/tri-methyl) at H3K27?
Suggested experts: Nathan M. Springer
Q: What is the division of labor between the three maize E(z) homologs (Mez1/clf-like vs Mez2/Mez3/SWN-like) in establishing H3K27me3 at distinct genomic loci, and is Mez1 uniquely required at imprinted/endosperm loci given that it is the only imprinted homolog?
Suggested experts: Nathan M. Springer, Irina Makarevitch
Experiment: Generate maize mez1 (single) and mez1 mez2 mez3 (higher-order) loss-of-function alleles and profile genome-wide H3K27me3 by ChIP-seq across tissues (including endosperm) with matched RNA-seq, to define Mez1-dependent H3K27me3 domains and de-repressed target genes.
Hypothesis: Mez1 (the clf-like homolog) contributes non-redundantly to a subset of H3K27me3 domains not covered by Mez2/Mez3, including imprinted loci in endosperm.
Type: loss-of-function genetics with ChIP-seq/RNA-seq
Experiment: Reconstitute a maize PRC2 from recombinant MEZ1 plus maize FIE/MSI1/SU(Z)12-like subunits and assay SAM-dependent methyltransferase activity on recombinant nucleosomes, measuring methylation state at H3K27 by quantitative mass spectrometry.
Hypothesis: MEZ1 is catalytically active as an H3K27 methyltransferase only (or much more efficiently) within an assembled PRC2 complex, consistent with the conserved requirement of non-catalytic PRC2 subunits for activity.
Type: in vitro reconstituted histone methyltransferase assay
Experiment: Test MEZ1 substrate and residue specificity using SET-domain catalytic point mutants and a panel of histone H3 peptides/nucleosomes carrying K27 versus other lysine substrates, quantifying product methylation states.
Hypothesis: The conserved E(z) SET domain of MEZ1 confers strict H3K27 specificity, and catalytic-site mutations abolish methyltransferase activity without affecting PRC2 assembly.
Type: structure-function mutagenesis and enzyme specificity assay
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