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Updated: Jun 1, 2026
In Vitro Assay to Measure Phosphatidylethanolamine Methyltransferase Activity
Published on: January 5, 2016
S-adenosylmethionine-binding properties of a bacterial phospholipid N-methyltransferase
Meriyem Aktas1, Jan Gleichenhagen, Raphael Stoll
1Ruhr-Universität Bochum, Bochum, Germany.
Abstract:
The presence of the membrane lipid phosphatidylcholine (PC) in the bacterial membrane is critically important for many host-microbe interactions. The phospholipid N-methyltransferase PmtA from the plant pathogen Agrobacterium tumefaciens catalyzes the formation of PC by a three-step methylation of phosphatidylethanolamine via monomethylphosphatidylethanolamine and dimethylphosphatidylethanolamine. The methyl group is provided by S-adenosylmethionine (SAM), which is converted to S-adenosylhomocysteine (SAH) during transmethylation. Despite the biological importance of bacterial phospholipid N-methyltransferases, little is known about amino acids critical for binding to SAM or phospholipids and catalysis. Alanine substitutions in the predicted SAM-binding residues E58, G60, G62, and E84 in A. tumefaciens PmtA dramatically reduced SAM-binding and enzyme activity. Homology modeling of PmtA satisfactorily explained the mutational results. The enzyme is predicted to exhibit a consensus topology of the SAM-binding fold consistent with cofactor interaction as seen with most structurally characterized SAM-methyltransferases. Nuclear magnetic resonance (NMR) titration experiments and (14)C-SAM-binding studies revealed binding constants for SAM and SAH in the low micromolar range. Our study provides first insights into structural features and SAM binding of a bacterial phospholipid N-methyltransferase.
Insights
Bacterial phospholipid N-methyltransferases are crucial for host-microbe interactions. This study identifies key amino acids in Agrobacterium tumefaciens PmtA essential for binding S-adenosylmethionine (SAM) and catalyzing phosphatidylcholine formation.
Area of Science:
- Microbiology
- Biochemistry
- Structural Biology
Background:
- Phosphatidylcholine (PC) is a vital membrane lipid in bacterial host-microbe interactions.
- Bacterial phospholipid N-methyltransferases (PmtA) synthesize PC via methylation of phosphatidylethanolamine using S-adenosylmethionine (SAM).
- Critical amino acids for SAM binding and catalysis in these enzymes remain largely uncharacterized.
Purpose of the Study:
- To investigate the structural features and SAM-binding mechanisms of the bacterial phospholipid N-methyltransferase PmtA from Agrobacterium tumefaciens.
- To identify key amino acid residues involved in SAM binding and catalysis.
Main Methods:
- Site-directed mutagenesis of predicted SAM-binding residues (E58, G60, G62, E84) in PmtA.
- Enzyme activity assays and S-adenosylmethionine (SAM) binding studies.
- Homology modeling and Nuclear Magnetic Resonance (NMR) titration experiments.
Main Results:
- Alanine substitutions at E58, G60, G62, and E84 significantly reduced PmtA's SAM-binding affinity and enzymatic activity.
- Homology modeling supported the observed mutational effects, predicting a conserved SAM-binding fold.
- NMR and binding studies determined low micromolar binding constants for SAM and S-adenosylhomocysteine (SAH).
Conclusions:
- Specific amino acids (E58, G60, G62, E84) are critical for SAM binding and catalysis in A. tumefaciens PmtA.
- The findings provide the first insights into the structural basis of SAM binding for bacterial phospholipid N-methyltransferases.
- This research lays the groundwork for understanding and potentially manipulating bacterial membrane lipid synthesis.
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