Ligand-independent EGFR oligomers do not rely on the active state asymmetric kinase dimer

The human Epidermal Growth Factor Receptor (EGFR/ERBB1) is a Receptor Tyrosine Kinase (RTK) that forms active oligomers in response to ligand. Much evidence indicates that EGFR/ERBB1 forms oligomers in the absence of ligand, but the structure and physiological role of these ligand-independent dimers remain unclear. We use fluorescence microscopy to measure the oligomer stability and FRET efficiency for homo- and hetero-oligomers of fluorescent-protein labeled forms of EGFR and its paralog, Human Epidermal Growth Factor Receptor 2 (HER2/ERBB2) in vesicles derived from native cell membranes. Both receptors form ligand-independent oligomers at physiological plasma membrane concentrations. Mutations introduced in the EGFR kinase region at a key interface within the active state dimer alter the FRET efficiency within ligand-independent EGFR oligomers but do not affect their stability. These results indicate that ligand-independent EGFR oligomers do not require this interface and that the inactive state ensemble is distinct from the EGFR active state ensemble.