Cytosolic S-adenosylmethionine synthase 1 (methionine adenosyltransferase, MAT; EC 2.5.1.6) from Populus trichocarpa. METK1 catalyzes the only known biosynthetic route to S-adenosyl-L-methionine (SAM/AdoMet), condensing L-methionine with ATP and releasing triphosphate, which the same enzyme then hydrolyzes to phosphate and diphosphate. The reaction requires two divalent metal ions per subunit (Mg2+; Mn2+ or Co2+ usable in vitro) and one monovalent potassium ion that coordinate the substrates. The enzyme acts as a homotetramer in the cytoplasm. SAM is the universal activated methyl donor for methyltransferases acting on DNA, RNA, proteins, lipids and small molecules, and is also the precursor for polyamine and ethylene biosynthesis in plants, making METK1 a metabolic hub. METK1 is one of several SAM synthase paralogs in the poplar genome.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: SPKW keyword-mapping annotation (GO_REF:0000043) derived from the UniProt "Metal-binding", "Magnesium", "Cobalt" and "Potassium" keywords; present in the Sept 2025 GOA snapshot but removed from the current GOA release after the keyword2GO pipeline was retired for cellular organisms. The assertion is biochemically correct - the enzyme binds two divalent metal ions (Mg2+, with Mn2+ or Co2+ usable in vitro) and one monovalent potassium ion per subunit, both of which coordinate the substrates during catalysis. However, "metal ion binding" is an uninformative grouping term that does not distinguish a catalytically essential cofactor requirement from incidental metal contacts. More specific terms (magnesium ion binding, GO:0000287; potassium ion binding, GO:0030955) precisely capture the documented cofactor requirement. GOA's removal pruned a true-but-coarse term rather than losing biologically meaningful information. Reason: The term is correct (METK1 binds divalent metal and K+ cofactors required for catalysis) but uninformative. It is a redundant, low-information SPKW grouping term; the specific cofactor identities are better represented by the proposed replacement terms. Its removal from current GOA is justified as pruning of coarse keyword-derived annotation. Proposed replacements: magnesium ion binding potassium ion binding Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt Note=Binds 2 divalent ions per subunit. The metal ions interact CC primarily with the substrate (By similarity). Can utilize magnesium, CC manganese or cobalt (in vitro) file:POPTR/METK1/METK1-uniprot.txt Note=Binds 1 potassium ion per subunit. The potassium ion interacts CC primarily with the substrate (By similarity). file:POPTR/METK1/METK1-deep-research-falcon.md MAT binds ATP and L-methionine (requires **Mg2+** and **K+**) and proceeds via an **SN2-type** reaction where methionine sulfur attacks the **5β² carbon** of the ATP ribose; the ATP Ξ²βΞ³ bond is hydrolyzed producing **PPi and Pi**. |
| GO:0006730 one-carbon metabolic process | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: SPKW keyword-mapping annotation (GO_REF:0000043) derived from the UniProt "One-carbon metabolism" keyword; present in the Sept 2025 GOA snapshot but removed from the current GOA release after the keyword2GO pipeline was retired for cellular organisms. The annotation is biologically defensible - S-adenosylmethionine produced by METK1 is the universal activated methyl donor that supplies one-carbon (methyl) units to methyltransferase reactions across nucleic acid, protein, lipid and small-molecule metabolism. However, GO:0006730 is a broad parent process and is coarse relative to the precise child term S-adenosylmethionine biosynthetic process (GO:0006556), which is already annotated (IBA and IEA) and directly describes what METK1 does. The enzyme synthesizes the methyl-donor cofactor; it does not itself transfer one-carbon units. The term is therefore correct-but-broad and largely redundant with the more specific GO:0006556 annotation. Reason: One-carbon metabolic process is a true but broad parent of the gene's actual role; METK1 produces the SAM methyl donor that feeds one-carbon/methyl-transfer metabolism. It is retained as a non-core process annotation because the core function is more precisely captured by S-adenosylmethionine biosynthetic process (GO:0006556). Its removal from current GOA is a minor loss at most, since the specific child term remains annotated. Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt FUNCTION: Catalyzes the formation of S-adenosylmethionine from CC methionine and ATP. file:POPTR/METK1/METK1-uniprot.txt PATHWAY: Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis; CC S-adenosyl-L-methionine from L-methionine: step 1/1. file:POPTR/METK1/METK1-deep-research-falcon.md The **universal methyl donor** for methyltransferases, including those involved in **DNA/histone methylation** and **secondary metabolism**. |
| GO:0006556 S-adenosylmethionine biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: S-adenosylmethionine biosynthesis is the direct pathway for methionine adenosyltransferase. Reason: The enzyme forms S-adenosyl-L-methionine from L-methionine and ATP. Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt PATHWAY: Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis file:POPTR/METK1/METK1-deep-research-falcon.md The primary function inferred for Populus METK1 is to make **SAM**, the central activated methyl donor and branch-point metabolite |
| GO:0004478 methionine adenosyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Methionine adenosyltransferase activity is the core molecular function. Reason: The UniProt entry assigns EC 2.5.1.6 and the reaction for SAM synthesis. Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt FUNCTION: Catalyzes the formation of S-adenosylmethionine from CC methionine and ATP. file:POPTR/METK1/METK1-uniprot.txt EC=2.5.1.6 file:POPTR/METK1/METK1-deep-research-falcon.md Plant MAT/SAMS catalyzes **L-methionine + ATP β S-adenosyl-L-methionine (SAM/AdoMet) + PPi + Pi** |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: ATP binding is required for the reaction but is less informative than methionine adenosyltransferase activity. Reason: Retain as a co-substrate binding feature, not as the core function. Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt Reaction=L-methionine + ATP + H2O = S-adenosyl-L-methionine |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Cytoplasm/cytosol localization is appropriate for this soluble AdoMet synthase. Reason: UniProt places the enzyme in the cytoplasm. Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm |
| GO:0006556 S-adenosylmethionine biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: S-adenosylmethionine biosynthesis is the direct pathway for methionine adenosyltransferase. Reason: The enzyme forms S-adenosyl-L-methionine from L-methionine and ATP. Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt PATHWAY: Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis file:POPTR/METK1/METK1-deep-research-falcon.md The primary function inferred for Populus METK1 is to make **SAM**, the central activated methyl donor and branch-point metabolite |
| GO:0004478 methionine adenosyltransferase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Methionine adenosyltransferase activity is the core molecular function. Reason: The UniProt entry assigns EC 2.5.1.6 and the reaction for SAM synthesis. Supporting Evidence: file:POPTR/METK1/METK1-uniprot.txt FUNCTION: Catalyzes the formation of S-adenosylmethionine from CC methionine and ATP. file:POPTR/METK1/METK1-uniprot.txt EC=2.5.1.6 file:POPTR/METK1/METK1-deep-research-falcon.md Plant MAT/SAMS catalyzes **L-methionine + ATP β S-adenosyl-L-methionine (SAM/AdoMet) + PPi + Pi** |
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Download this section (compressed HTML)Q: Are METK1 and the other Populus SAM synthase paralogs functionally redundant, or do they supply distinct methylation, polyamine and ethylene biosynthetic demands in specific tissues (e.g. xylem during lignification)?
Q: Is METK1 expression coordinated with lignin biosynthesis, given that SAM is the methyl donor for caffeic acid O-methyltransferase and related monolignol-pathway methyltransferases in poplar wood formation?
Experiment: Generate METK1-specific knockdown/CRISPR lines and quantify SAM and S-adenosylhomocysteine pools, transmethylation flux, and methylation-dependent phenotypes (DNA methylation, lignin content/composition) relative to paralog-specific lines.
Type: targeted functional assay
Experiment: Recombinantly express and purify METK1 to confirm EC 2.5.1.6 activity and measure the kinetic dependence on Mg2+ and K+, validating the predicted metal cofactor requirements directly in the poplar enzyme.
Type: enzyme kinetics / biochemical characterization
Experiment: Profile METK1 transcript and protein abundance across tissues and developmental stages (especially developing xylem) to test coordination with lignin biosynthesis and other SAM-dependent pathways.
Type: expression profiling
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