A HIERARCHY OF KINASE ACTIVATOR-RECEIVER PREFERENCES MODULATES SIGNALING OUTPUTS FROM EGFR/ERBB HETERODIMERS
Ward, Matthew Douglas
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The ability of cells to interact with and adapt to their local environment is a fundamental process in biology. To accomplish this feat, cells maintain a diversity of receptors and sensors that respond specifically to different stimuli. A molecular understanding of how external signals are received and transduced into the cell is key to understanding how cells communicate, specialize and respond to their environments. In multicellular organisms, intercellular signaling plays a major role in specifying cell fate and establishing pattern during development. In the work presented here, I detail features of two signaling pathways that are vital for normal development in many animals, the Epidermal Growth Factor Receptor (EGFR/ErbB) signaling pathway and the Hedgehog (Hh) signaling pathway. Signaling in the EGFR/ErbB family involves ligand-induced, receptor-mediated dimerization that results in allosteric activation of intracellular tyrosine kinase activity. Previous work has illustrated the importance of asymmetry in receptor activation, through asymmetric ligand-binding to the extracellular domain dimer and asymmetric kinase dimer activation in the intracellular domain, but the relationship between these apparent asymmetries remained unresolved. I report cell-based stimulation assays of three EGFR/ErbB heterodimers carried out to investigate asymmetric kinase activation of single-ligand bound receptor dimers. I show that extracellular asymmetry and intracellular asymmetry are not linked. Instead, contributions from the intracellular domain alone define a hierarchy of kinase activator-receiver preference among EGFR/ErbB family members. This work helps rationalize how different ligand-binding signals are integrated within EGFR/ErbB family heterodimers and how signal diversification is maintained. In the Hh signaling pathway, ligands are received by coordinated receptors and co-receptors at the cell surface. Previous work has indicated that co-receptor engagement of Hh molecules has diverged from invertebrate models (fruit flies) to mammals, from a heparin-dependent to a calcium-dependent binding mode, respectively. To better understand the evolution of the Hh and co-receptor binding relationship, I performed crystallographic studies of mammalian Hh homologues, Sonic Hh (Shh), Indian Hh (Ihh) and Desert Hh (Dhh), and co-receptors Cdo and Boc. In collaboration with Dr. Jennifer Kavran, we showed that the binding mode in which all Hh homologues engage the co-receptors Cdo and Boc is conserved.