An Optimized Single-Molecule Pull-Down Assay for Quantification of Protein Phosphorylation

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Signaling

Background:

• Phosphorylation is a critical posttranslational modification regulating protein function and cell signaling.
• Existing methods struggle to capture phosphorylation heterogeneity at the single-protein level.

Purpose of the Study:

• To adapt and optimize the single-molecule pull-down (SiMPull) assay for quantifying protein phosphorylation states.
• To enable robust analysis of phosphorylation patterns in individual membrane receptors.

Main Methods:

• Adaptation of the SiMPull assay for immunoprecipitation and single-molecule imaging of proteins on a glass coverslip.
• Optimization of glass preparation and antibody fixation protocols for enhanced data quality.
• Development of code for single-molecule data analysis to determine phosphorylation fractions.

Main Results:

• The optimized SiMPull assay provides robust quantification of phosphorylation states at the single-molecule level.
• Thousands of individual receptors can be imaged to accurately determine phosphorylation patterns.
• The protocol is effective for epidermal growth factor receptor (EGFR) phosphorylation and generalizable to other signaling molecules.

Conclusions:

• The optimized SiMPull protocol offers a powerful tool for analyzing single-molecule phosphorylation heterogeneity.
• This method advances the understanding of cell signaling regulation by providing precise phosphorylation data.
• The protocol's generalizability makes it applicable to a wide range of membrane receptors and signaling proteins.