Abstract
The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that couples the binding of extracellular ligands, such as EGF and transforming growth factor-α (TGF-α), to the initiation of intracellular signaling pathways. EGFR binds to EGF and TGF-α with similar affinity, but generates different signals from these ligands. To address the mechanistic basis of this phenomenon, we have carried out cryo-EM analyses of human EGFR bound to EGF and TGF-α. We show that the extracellular module adopts an ensemble of dimeric conformations when bound to either EGF or TGF-α. The two extreme states of this ensemble represent distinct ligand-bound quaternary structures in which the membrane-proximal tips of the extracellular module are either juxtaposed or separated. EGF and TGF-α differ in their ability to maintain the conformation with the membrane-proximal tips of the extracellular module separated, and this conformation is stabilized preferentially by an oncogenic EGFR mutation. Close proximity of the transmembrane helices at the junction with the extracellular module has been associated previously with increased EGFR activity. Our results show how EGFR can couple the binding of different ligands to differential modulation of this proximity, thereby suggesting a molecular mechanism for the generation of ligand-sensitive differential outputs in this receptor family.
Highlights
Human epidermal growth factor receptor (EGFR) can be activated by seven related ligands that generate different signaling outputs from the receptor
We purified full-length human EGFR expressed in a human HEK293S GnTI- cell line (Reeves et al., 2002; Qiu et al, 2009), and reconstituted the complex of EGFR with EGF into four membrane-mimetic environments: detergent micelles, lipid nanodiscs (Bayburt and Sligar, 2010), amphipols (Gohon et al, 2006), and peptidiscs (Angiulli et al, 2020; Carlson et al, 2018)
Cryo-electron microscopy (EM) single-particle analysis reveals a range of conformations for the EGF104 bound extracellular module
Summary
Human EGFR can be activated by seven related ligands that generate different signaling outputs from the receptor. While the classical mechanism for receptor tyrosine kinase activation, ligandinduced dimerization, is essential for all these ligands to activate the receptor (Kovacs et al., 2015; Leahy, 2004; Lemmon et al, 2014), it cannot explain how the extracellular module of EGFR is able to respond differently to different ligands once the receptor is dimerized (Ronan et al, 2016; Singh et al, 2016; Wilson et al, 2009). The extracellular module of EGFR consists of four domains, denoted Domains I, II, III, and IV (Figure 1). Domain II, which bridges the ligand-binding domains, contains the “dimerization arm”, a loop that mediates the principal interaction with the other subunit in an activated dimer (Dawson et al, 2005). Domains I, II, and III form a compact unit that we refer to as the ligand-binding “head”, and Domain IV forms a rigid and elongated “leg” that connects the ligand-binding head to the single transmembrane helix (Figure 1)
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