Residue-level determinants of angiopoietin-2 interactions with its receptor Tie2

We combined computational and experimental methods to interrogate the binding determinants of angiopoietin-2 (Ang2) to its receptor tyrosine kinase (RTK) Tie2-a central signaling system in angiogenesis, inflammation, and tumorigenesis. We used physics-based electrostatic and surface-area calculations to identify the subset of interfacial Ang2 and Tie2 residues that can affect binding directly. Using random and site-directed mutagenesis and yeast surface display (YSD), we validated these predictions and identified additional Ang2 positions that affected receptor binding. We then used burial-based calculations to classify the larger set of Ang2 residues that are buried in the Ang2 core, whose mutations can perturb the Ang2 structure and thereby affect interactions with Tie2 indirectly. Our analysis showed that the Ang2-Tie2 interface is dominated by nonpolar contributions, with only three Ang2 and two Tie2 residues that contribute electrostatically to intermolecular interactions. Individual interfacial residues contributed only moderately to binding, suggesting that engineering of this interface will require multiple mutations to reach major effects. Conversely, substitutions in substantially buried Ang2 residues were more prevalent in our experimental screen, reduced binding substantially, and are therefore more likely to have a deleterious effect that might contribute to oncogenesis. Computational analysis of additional RTK-ligand complexes, c-Kit-SCF and M-CSF-c-FMS, and comparison to previous YSD results, further show the utility of our combined methodology.