Middle-Down Mass Spectrometry Reveals Activity-Modifying Phosphorylation Barcode in a Class C G Protein-Coupled...

Area of Science:

• Biochemistry
• Molecular Biology
• Pharmacology

Background:

• G protein-coupled receptors (GPCRs) are crucial human membrane receptors involved in diverse physiological processes and are key therapeutic targets.
• Receptor phosphorylation regulates GPCR signaling in space and time, with distinct phosphorylation states potentially encoding unique signaling outcomes.
• Existing proteomic methods offer limited quantitative data on GPCR phosphorylation stoichiometry, isomeric states, and temporal dynamics.

Purpose of the Study:

• To develop and validate a novel middle-down proteomic strategy coupled with parallel reaction monitoring (PRM) for precise quantification of GPCR phosphorylation states.
• To characterize the phosphorylation patterns of the metabotropic glutamate receptor 2 (mGluR2) C-terminal tail.
• To investigate the impact of agonist treatment on mGluR2 phosphorylation and identify key phosphorylation sites regulating receptor sensitivity.

Main Methods:

• A novel middle-down proteomic strategy was employed for sample preparation.
• Parallel Reaction Monitoring (PRM) and tandem mass spectrometry were utilized for quantitative analysis.
• Label-free quantification was performed to determine the relative abundance of phosphorylation states.

Main Results:

• The study successfully quantified basal and agonist-induced phosphorylation at up to four simultaneous sites on the mGluR2 C-terminal tail.
• PRM identified and quantified the relative abundance of specific phosphorylation sites upon agonist treatment.
• Site-directed mutagenesis revealed that phosphorylation in specific C-terminal regions of mGluR2 modulates receptor sensitivity.

Conclusions:

• Middle-down purification followed by label-free quantification provides a powerful, quantitative, and accessible method for characterizing GPCR phosphorylation.
• This approach offers critical insights into GPCR signaling regulation and the development of novel therapeutics targeting GPCRs.
• The findings highlight the dynamic nature of mGluR2 phosphorylation and its role in receptor activation and sensitivity.