The EGFR extracellular segment consists of four domains (d1, violet; d2, cyan; d3, teal; d4, yellow). In the absence of a ligand, it lies in a closed monomeric configuration. Upon ligand binding (here EGF, gray) the receptor adopts an open, dimeric configuration triggering intracellular signaling. As the third domain of the EGFR is reported to hold most of the binding affinity to the EGF ligand, it was used as a template to design soluble EGF binders. A close-up view of the EGF in complex with the wild-type d3 domain (d3-WT) is shown in gray and teal, respectively (PDB:1IVO). Disulfide bridges in the d3-WT structure are shown as yellow sticks. Using the described energy function, the highest energy residues were identified. These residues were defined as mutable (shown in red) and designed using the combinatorial sampler. Two design models (dd3-1, purple; dd3-2, yellow) were finally chosen for experimental characterization.
