Structural analysis of the EGFR/HER3 heterodimer reveals the molecular basis for activating HER3 mutations

Cancer-associated mutations in the catalytically inactive receptor HER3 enhance its ability to activate other members of the human epidermal growth factor receptor family. Cancer by Enhanced Allosteric Activation Cancer-associated, oncogenic mutations in kinases often result in increased catalytic activity. Although cancer-associated mutations in the HER3 member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases exist, this receptor has little, if any, catalytic activity. The four members of the EGFR family form homodimers or heterodimers; this dimerization is necessary for the kinase activity of the catalytically active members HER1, HER2, and HER4. In homodimeric structures, one kinase subunit functions as an allosteric activator and the other as the activated receiver. Littlefield et al. crystallized the kinase domain of HER1 in complex with the kinase domain of wild-type HER3 or HER3 with cancer-associated mutations. This analysis, along with in vitro biochemical kinase assays, showed that HER3 functioned as an activator in the heterodimer and that the mutations enhanced the interaction between HER3 and HER1, thereby augmenting the allosteric activator function of HER3. This study provides a rationale for targeting the heterodimer interface with HER3 in cancer associated with aberrant activity of this family of receptors. The human epidermal growth factor receptor (HER) tyrosine kinases homo- and heterodimerize to activate downstream signaling pathways. HER3 is a catalytically impaired member of the HER family that contributes to the development of several human malignancies and is mutated in a subset of cancers. HER3 signaling depends on heterodimerization with a catalytically active partner, in particular epidermal growth factor receptor (EGFR) (the founding family member, also known as HER1) or HER2. The activity of homodimeric complexes of catalytically active HER family members depends on allosteric activation between the two kinase domains. To determine the structural basis for HER3 signaling through heterodimerization with a catalytically active HER family member, we solved the crystal structure of the heterodimeric complex formed by the isolated kinase domains of EGFR and HER3. The structure showed HER3 as an allosteric activator of EGFR and revealed a conserved role of the allosteric mechanism in activation of HER family members through heterodimerization. To understand the effects of cancer-associated HER3 mutations at the molecular level, we solved the structures of two kinase domains of HER3 mutants, each in a heterodimeric complex with the kinase domain of EGFR. These structures, combined with biochemical analysis and molecular dynamics simulations, indicated that the cancer-associated HER3 mutations enhanced the allosteric activator function of HER3 by redesigning local interactions at the dimerization interface.