Abstract
Ubiquitination of the epidermal growth factor receptor (EGFR) that occurs when Cbl and Grb2 bind to three phosphotyrosine residues (pY1045, pY1068 and pY1086) on the receptor displays a sharp threshold effect as a function of EGF concentration. Here we use a simple modelling approach together with experiments to show that the establishment of the threshold requires both the multiplicity of binding sites and cooperative binding of Cbl and Grb2 to the EGFR. While the threshold is remarkably robust, a more sophisticated model predicted that it could be modulated as a function of EGFR levels on the cell surface. We confirmed experimentally that the system has evolved to perform optimally at physiological levels of EGFR. As a consequence, this system displays an intrinsic weakness that causes—at the supraphysiological levels of receptor and/or ligand associated with cancer—uncoupling of the mechanisms leading to signalling through phosphorylation and attenuation through ubiquitination.
Highlights
Ubiquitination of the epidermal growth factor receptor (EGFR) that occurs when Cbl and Grb[2] bind to three phosphotyrosine residues on the receptor displays a sharp threshold effect as a function of EGF concentration
We initially developed a simple model to account for EGFR phosphorylation at 2 min of EGF stimulation, when both phosphorylation and ubiquitination take place predominantly at the PM21
At 2 min of EGF stimulation, the individual Tyr residues are phosphorylated independently of each other, since their phosphorylation dose– response curves do not change in the presence (EGFR-WT) or absence (Tyr mutants, Fig. 1b) of other phosphorylation sites
Summary
Ubiquitination of the epidermal growth factor receptor (EGFR) that occurs when Cbl and Grb[2] bind to three phosphotyrosine residues (pY1045, pY1068 and pY1086) on the receptor displays a sharp threshold effect as a function of EGF concentration. We confirmed experimentally that the system has evolved to perform optimally at physiological levels of EGFR As a consequence, this system displays an intrinsic weakness that causes—at the supraphysiological levels of receptor and/or ligand associated with cancer—uncoupling of the mechanisms leading to signalling through phosphorylation and attenuation through ubiquitination. Molecules harbouring modules binding phosphotyrosines of EGFR (pY) are recruited to the plasma membrane (PM) and activate signalling pathways leading to context-dependent biological outputs[1,2] One such molecule is the ubiquitin ligase Cbl, which binds to the EGFR via a pY-mediated mechanism and ubiquitinates the receptor[16]. Ubiquitination of an EGFR mutant (EGFR-Y1045 þ ) that contains only the phosphorylatable Tyr residue responsible for direct Cbl binding (Y1045) does not display a threshold and is significantly reduced compared with EGFR-WT (B80% reduction)[21]
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