Understanding the regulation of the epidermal growth factor receptor tyrosine kinase activity

ErbB receptors are members of the receptor tyrosine kinase family and include EGFR (HER1), ErbB2 (HER2), ErbB3 (HER3), and ErbB4 (HER4). Discovered in the 1980\u27s and 90\u27s, these receptors have been rigorously studied due to their essential roles in development, homeostasis, and oncogenesis. Ligand binding to the extracellular region of ErbB receptor allosterically activates the intracellular tyrosine kinase domain, initiating downstream signal transduction cascades involved in cellular proliferation, differentiation, and anti-apoptosis. The work described here aims to understand the regulatory mechanisms of the ErbB tyrosine kinases and to investigate the nature of cancer-causing mutations in the EGFR intracellular domain. Various mutations seen in NSCLC (Non-Small Cell Lung Carcinomas) were introduced into the full-length receptor, stably expressed in Ba/F3 cells and monitored for ligand-independent activation, ligand binding properties, cellular localization, and drug resistance. These studies revealed that the mutations in the catalytic core of the kinase disrupt intermolecular autoinhibitory interactions, and lock it in an active state. These mutations thus induce constitutive receptor activation, and cause EGFR-dependent oncogenesis. In normal regulation, these autoinhibitory interactions are broken by asymmetric dimerization of the EGFR kinase domains following by ligand binding to the extracellular region. Studies of the ErbB4 tyrosine kinase domain suggest that this regulatory mechanism is universal among the ErbB receptor family members. However, the asymmetric dimer interface itself is insufficient to drive kinase dimerization and activation. Contributions from the juxtamembrane region (N-terminal to the kinase domain) are necessary for full kinase activity. Structural studies of the EGFR kinase domain revealed that the juxtamembrane region cradles the C-lobe of its dimerization partner to stabilize the kinase dimer, a configuration essential for its activation. Rare lung cancer mutations in the juxtamembrane region seem to strengthen these interactions and boost kinase activity. The propensity for the purified ErbB receptor intracellular domains to dimerize may contribute to overall receptor dimerization. Thus, dimerization of the full-length ErbB receptors appears to be a coordinated process mediated by communication and contributions from both extracellular and intracellular domains. This thesis details contribution made by the juxtamembrane region, and provides evidence for an important role of the C-terminal tail.