Structures of the active HER2/HER3 receptor complex reveal dynamics at the dimerization interface induced by binding of a single ligand

The Human Epidermal Growth Factor Receptor 2 (HER2) and HER3 form a potent pro-oncogenic heterocomplex upon binding of growth factor neuregulin-1β (NRG1β). The mechanism by which HER2 and HER3 interact remains unknown in the absence of any structures of the complex. We isolated the NRG1β-bound near full-length HER2/HER3 dimer and obtained a 2.9Å cryo-electron microscopy (cryo-EM) reconstruction of the extracellular domain module which reveals unexpected dynamics at the HER2/HER3 dimerization interface. We show that the dimerization arm of NRG1β-bound HER3 is unresolved likely because the apo HER2 monomer fails to undergo a ligand-induced conformational change needed to establish a HER3 dimerization arm binding pocket. In a second structure of an oncogenic extracellular domain mutant of HER2, S310F, we observe a compensatory interaction with the HER3 dimerization arm that stabilizes the dimerization interface. We show that both HER2/HER3 and HER2-S310F/HER3 retain the capacity to bind to the HER2-directed therapeutic antibody, trastuzumab, but the mutant complex does not bind to pertuzumab. Our 3.5Å structure of the HER2-S310F/HER3/NRG1β/trastuzumab Fragment antigen binding (Fab) complex shows that the receptor dimer undergoes a conformational change to accommodate trastuzumab. Thus, like oncogenic mutations, therapeutics exploit the intrinsic dynamics of the HER2/HER3 heterodimer. The unique features of a singly liganded HER2/HER3 heterodimer underscore the allosteric sensing of ligand occupancy by the dimerization interface and explain why extracellular domains of HER2 do not homo-associate via a canonical active dimer interface.