A simple toolset to identify endogenous post-translational modifications for a target protein: a snapshot of the EGFR signaling pathway.

Identification of a novel post-translational modification (PTM) for a target protein, defining its physiologic role, and studying its potential crosstalk with other PTMs is a challenging process. A set of highly sensitive tools termed Signal-Seeker kits was developed, which enables rapid and simple detection of post-translational modifications on any target protein.  The methodology for these tools utilizes affinity purification of modified proteins from a cell or tissue lysate and immunoblot analysis. These tools utilize a single lysis system that is effective at identifying endogenous, dynamic PTM changes, as well as the potential crosstalk between PTMs. As a proof-of-concept experiment, the acetylation, tyrosine phosphorylation, SUMOylation 2/3, and ubiquitination profiles of the EGFR - Ras - c-Fos axis were examined in response to EGF stimulation. All 10 previously identified PTMs of this signaling axis were confirmed using these tools, and it also identified acetylation as a novel modification of c-Fos. This axis in the EGF/EGFR signaling pathway was chosen because it is a well-established signaling pathway with proteins localized in the membrane, cytoplasmic, and nuclear compartments that ranged in abundance from 4.18x108 (EGFR) to 1.35x104 (c-Fos) molecules per A431 cell. These tools enabled the identification of low abundance PTMs, such as c-Fos Ac, at 17 molecules per cell. These studies highlight how pervasive PTMs are, and how stimulants like EGF induce multiple PTM changes on downstream signaling axis. Identification of endogenous changes and potential crosstalk between multiple PTMs for a target protein or signaling axis will provide regulatory mechanistic insight to investigators.