Precise Readout of MEK1 Proteoforms upon MAPK Pathway Modulation by Individual Ion Mass Spectrometry
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Updated: Jul 1, 2025
Selected Reaction Monitoring Mass Spectrometry for Absolute Protein Quantification
Published on: August 17, 2015
Bryon S Drown1, Raveena Gupta1, John P McGee1
1Proteomics Center of Excellence, Departments of Molecular Biosciences, Chemistry, and the Feinberg School of Medicine, Northwestern University, Evanston, Illinois 60202, United States.
Abstract:
The functions of proteins bearing multiple post-translational modifications (PTMs) are modulated by their modification patterns, yet precise characterization of them is difficult. MEK1 (also known as MAP2K1) is one such example that acts as a gatekeeper of the mitogen-activating protein kinase (MAPK) pathway and propagates signals via phosphorylation by upstream kinases. In principle, top-down mass spectrometry can precisely characterize whole MEK1 proteoforms, but fragmentation methods that would enable the site-specific characterization of labile modifications on 43 kDa protein ions result in overly dense tandem mass spectra. By using the charge-detection method called individual ion mass spectrometry, we demonstrate how complex mixtures of phosphoproteoforms and their fragment ions can be reproducibly handled to provide a "bird's eye" view of signaling activity through mapping proteoform landscapes in a pathway. Using this approach, the overall stoichiometry and distribution of 0-4 phosphorylations on MEK1 was determined in a cellular model of drug-resistant metastatic melanoma. This approach can be generalized to other multiply modified proteoforms, for which PTM combinations are key to their function and drug action.
Insights
Characterizing proteins with multiple post-translational modifications (PTMs) is challenging. This study introduces a novel mass spectrometry approach to map MEK1 phosphoproteoform landscapes, revealing signaling activity in melanoma.
Area of Science:
- Biochemistry
- Proteomics
- Cell Signaling
Background:
- Proteins with multiple post-translational modifications (PTMs) have functions modulated by their specific modification patterns.
- Precise characterization of these complex proteoforms is analytically challenging.
- MEK1 (MAP2K1) is a key kinase in the mitogen-activating protein kinase (MAPK) pathway, regulating cell signaling.
Purpose of the Study:
- To develop and apply a mass spectrometry-based method for comprehensive characterization of MEK1 phosphoproteoforms.
- To map the landscape of MEK1 phosphorylation states in a relevant biological context.
- To provide a "bird's eye" view of pathway signaling activity through proteoform analysis.
Main Methods:
- Utilized individual ion mass spectrometry, a charge-detection method, for top-down analysis of intact protein ions.
- Applied fragmentation techniques to analyze labile modifications on MEK1 proteoforms.
- Quantified the stoichiometry and distribution of MEK1 phosphorylations in a cellular model.
Main Results:
- Demonstrated reproducible handling of complex phosphoproteoform mixtures and their fragment ions.
- Successfully mapped the proteoform landscape of MEK1, revealing the distribution of 0-4 phosphorylations.
- Determined MEK1 phosphorylation patterns in a cellular model of drug-resistant metastatic melanoma.
Conclusions:
- Individual ion mass spectrometry enables detailed characterization of multiply modified proteoforms.
- This approach provides insights into signaling pathway dynamics by mapping proteoform landscapes.
- The method is generalizable to other complex proteoforms where PTM combinations are critical for function and drug response.
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