In this R03 application, the PI will develop an independent project in the Lemmon lab aimed at understanding the relative importance of mutation-induced dimerization (versus direct activation) of the kinase domain of the Epidermal Growth Factor Receptor (EGFR) in lung cancer patients. The EGFR regulates critical cellular processes such as cell motility, metabolism, proliferation and differentiation, and is normally activated by ligand- induced dimerization. In a subset of lung cancer patients, kinase domain mutations activate EGFR in the absence of ligand. Traditionally, these mutations (such as L858R and exon 19 deletions) have been thought to bias the monomeric kinase domain to its active state – independent of dimerization. However, some recent studies suggest that kinase domain dimerization is key to mutational activation of EGFR. Moreover, several rare EGFR mutations – such as those in exon 18, or which cause kinase domain duplication (KDD) – seem to activate EGFR primarily by dimerizing its kinase domain. In this proposal, we ask whether/how common EGFR mutations promote kinase domain dimerization, and whether they are capable of ‘super-activating’ a kinase domain dimer. In addition, we address the ‘dimerization only’ activation mechanisms of exon 18 and KDD mutations. By understanding these different modes of activation, we hope to gain insight into new therapeutic approaches to inhibit EGFR by targeting the dimerization interface – which could be mutant-specific in detail. Through in vitro kinase assay approaches to establish reaction kinetics, as well as a host of biophysical methods, structural analysis, and computational tools (validated experimentally) we propose three Specific Aims: Aim 1: Understanding relative contribution of kinase domain dimerization to activation of EGFR by oncogenic mutations. Using a synthetic disulfide-linked EGFR kinase domain dimer, we will study the elusive asymmetric EGFR kinase domain dimer biochemically. We will ask whether common EGFR mutations further enhance activity of this dimer, and will compare activities of dimers induced by exon 18 and KDD mutations. Aim 2: Structural analysis and ‘in-solution’ dynamics of the EGFR kinase domain harboring exon 18 mutations or kinase domain duplications (KDD) that promote ligand-independent dimerization. Protein crystallography will be pursued to visualize the interfaces of the different kinase domain dimers – to ask whether they are identical or mutant-specific. In parallel, hydrogen-deuterium exchange mass spectrometry (HDX-MS) will be employed to map interfaces and their stability. These studies will be complemented by other biophysical studies to gain a view of dimer stability and dynamics for dimers induced by different means and mutations. Aim 3: Targeting the kinase domain dimerization interface by small molecules as disruptors of protein- protein interactions (PPIs). Finally, we will identify structural motifs that can bind to the EGFR kinase domain dime...