Research ResourceBiochemistry

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

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Sci. Signal.  12 Jul 2016:
Vol. 9, Issue 436, pp. rs6
DOI: 10.1126/scisignal.aaf0891

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Adaptors are the conductors in the signaling symphony

Just as there are rate-limiting enzymes in biochemical process, there are rate-limiting steps in cell signaling networks. These rate-limiting proteins direct the signal through specific molecular cascades to dictate the response. Shi et al. sought to identify the proteins in the epidermal growth factor receptor (EGFR) signaling network that serve as the conductors or directors of the EGF signal. They found that the abundance of most core pathway proteins was very similar between cells and rather that the very low abundance of the adaptor proteins made them rate-limiting for EGFR-MAPK pathway signaling in normal and malignant cells. The findings suggest that adaptor proteins serve as the directors of the signaling script.

Abstract

Various genetic mutations associated with cancer are known to alter cell signaling, but it is not clear whether they dysregulate signaling pathways by altering the abundance of pathway proteins. Using a combination of RNA sequencing and ultrasensitive targeted proteomics, we defined the primary components—16 core proteins and 10 feedback regulators—of the epidermal growth factor receptor (EGFR)–mitogen-activated protein kinase (MAPK) pathway in normal human mammary epithelial cells and then quantified their absolute abundance across a panel of normal and breast cancer cell lines as well as fibroblasts. We found that core pathway proteins were present at very similar concentrations across all cell types, with a variance similar to that of proteins previously shown to display conserved abundances across species. In contrast, EGFR and transcriptionally controlled feedback regulators were present at highly variable concentrations. The absolute abundance of most core proteins was between 50,000 and 70,000 copies per cell, but the adaptors SOS1, SOS2, and GAB1 were found at far lower amounts (2000 to 5000 copies per cell). MAPK signaling showed saturation in all cells between 3000 and 10,000 occupied EGFRs, consistent with the idea that adaptors limit signaling. Our results suggest that the relative stoichiometry of core MAPK pathway proteins is very similar across different cell types, with cell-specific differences mostly restricted to variable amounts of feedback regulators and receptors. The low abundance of adaptors relative to EGFR could be responsible for previous observations that only a fraction of total cell surface EGFR is capable of rapid endocytosis, high-affinity binding, and mitogenic signaling.

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