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Abstract
The EGF receptor is a classical receptor-tyrosine kinase. In the absence of ligand, the receptor adopts a closed conformation in which the dimerization arm of subdomain II interacts with the tethering arm in subdomain IV. Following the binding of EGF, the receptor opens to form a symmetric, back-to-back dimer. Although it is clear that the dimerization arm of subdomain II is central to the formation of receptor dimers, the role of the tethering arm of subdomain IV (residues 561-585) in this configuration is not known. Here we use (125)I-EGF binding studies to assess the functional role of the tethering arm in the EGF receptor dimer. Mutation of the three major residues that contribute to tethering (D563A,H566A,K585A-EGF receptor) did not significantly alter either the ligand binding properties or the signaling properties of the EGF receptor. By contrast, breaking the Cys(558)-Cys(567) disulfide bond through double alanine replacements or deleting the loop entirely led to a decrease in the negative cooperativity in EGF binding and was associated with small changes in downstream signaling. Deletion of the Cys(571)-Cys(593) disulfide bond abrogated cooperativity, resulting in a high affinity receptor and increased sensitivity of downstream signaling pathways to EGF. Releasing the Cys(571)-Cys(593) disulfide bond resulted in extreme negative cooperativity, ligand-independent kinase activity, and impaired downstream signaling. These data demonstrate that the tethering arm plays an important role in supporting cooperativity in ligand binding. Because cooperativity implies subunit-subunit interactions, these results also suggest that the tethering arm contributes to intersubunit interactions within the EGF receptor dimer.
Highlights
Releasing the Tether—The tethering “arm” of subdomain IV of the EGF receptor has been defined as the region between residues 561 and 585 [14]
Asp563, His566, and Lys585 in subdomain IV have been identified as the main residues that participate in the tethering interactions [14]
We have recently shown that this heterogeneity can best be explained by a model that involves negative cooperativity in an aggregating system [26]
Summary
These two 558 –567 disulfide loop mutants bound EGF with good affinity and were expressed at high levels on the cell surface, both were severely impaired with respect to their ability to mediate receptor autophosphorylation. The saturation binding isotherms for the ⌬571–593-EGF receptor (Fig. 7B), in which the second disulfide loop was deleted, showed no shift with increasing concentrations of EGF receptors.
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