Epidermal growth factor binding induces a conformational change in the external domain of its receptor

To study the properties of the extracellular epidermal growth factor (EGF) binding domain of the human EGF receptor, we have infected insect cells with a suitably engineered baculovirus vector containing the cDNA encoding the entire ectodomain of the parent molecule. This resulted in a correctly folded, stable, 110 kd protein which possessed an EGF binding affinity of 200 nM. The protein was routinely purified in milligram amounts from 1 litre insect cell cultures using a series of three standard chromatographic steps. The properties of the ectodomain were studied before and after the addition of different EGF ligands, using both circular dichroism and fluorescence spectroscopic techniques. A secondary structural analysis of the far UV CD spectrum of the ectodomain indicated significant proportions of alpha‐helix and beta‐sheet in agreement with a published model of the EGF receptor. The ligand additions to the receptor showed differences in both the near‐ and far‐UV CD spectra, and were similar for each ligand used, suggesting similar conformational differences between uncomplexed and complexed receptor. Steady‐state fluorescence measurements indicated that the tryptophan residues present in the ectodomain are buried and that the solvent‐accessible tryptophans in the ligands become buried on binding the receptor. The rotational correlation times measured by fluorescence anisotropy decay for the receptor‐ligand complexes were decreased from 6 to 2.5 ns in each case. This may indicate a perturbation of the tryptophan environment of the receptor on ligand binding. Ultracentrifugation studies showed that no aggregation occurred on ligand addition, so this could not explain the observed differences from CD or fluorescence.