SUMMARY
Epidermal Growth Factor Receptor (EGFR) signaling is supposed to be triggered by a dramatic ligand-induced conformational change of the extracellular domain from a closed, self-inhibited tethered monomer, to an open untethered state, which exposes a loop required for dimerization and activation. The ectodomain also untethers spontaneously, but the molecular mechanism is still unknown. Experiments with the mAb806 antibody have suggested the existence of a third untethered state, still uncharacterized, appearing transiently during the tethered to untethered transition, and which exposes a cryptic epitope hidden in the known structures. Here, molecular dynamics simulations of two ectodomain mutants (R84K and G39R) targeting a hinge at domain I-II interface highlight the possibility of such additional intermediate conformer. The new conformation is untethered but surprisingly compact, and originates from a large rotation of domain I that exposes the mAb806 epitope, in a similar way as the EGFRvIII deletion does. The present findings not only point to different molecular processes for ligand-dependent and spontaneous untethering, but also suggest that domain I-II mutation clusters and the EGFRvIII deletion may share a common structural mechanism, based on the removal of an steric hindrance from domain I that restricts dimerization.
