Conservation of protein abundance patterns reveals the regulatory architecture of the EGFR-MAPK pathway

Area of Science:

• Cellular biology
• Molecular oncology
• Signal transduction

Background:

• Genetic mutations in cancer can affect cell signaling pathways.
• The impact of these mutations on the abundance of pathway proteins remains unclear.
• Understanding protein levels is crucial for deciphering cancer signaling dysregulation.

Purpose of the Study:

• To investigate whether cancer-associated genetic mutations alter cell signaling by changing protein abundance.
• To quantify the absolute abundance of key proteins in the epidermal growth factor receptor (EGFR)-mitogen-activated protein kinase (MAPK) pathway.
• To compare protein abundance across normal and cancer cell lines.

Main Methods:

• Utilized RNA sequencing and ultrasensitive targeted proteomics.
• Defined 16 core proteins and 10 feedback regulators of the EGFR-MAPK pathway.
• Quantified absolute protein abundance in normal mammary epithelial cells, breast cancer cell lines, and fibroblasts.

Main Results:

• Core MAPK pathway proteins showed highly conserved absolute abundances (50,000–70,000 copies/cell) across all cell types.
• EGFR and feedback regulators exhibited significant variability in concentration.
• Adaptor proteins (SOS1, SOS2, GAB1) were found at low abundance (2,000–5,000 copies/cell), potentially limiting MAPK signaling.
• MAPK signaling saturated at 3,000–10,000 occupied EGFRs, supporting the role of adaptors in signal regulation.

Conclusions:

• The relative stoichiometry of core MAPK pathway proteins is largely conserved across different cell types.
• Cell-specific variations in signaling are primarily driven by differences in receptor and feedback regulator abundance.
• Low adaptor protein levels relative to EGFR may explain why only a fraction of EGFR participates in rapid endocytosis, high-affinity binding, and mitogenic signaling.