ErbB2 resembles an autoinhibited invertebrate epidermal growth factor receptor

Abstract

The orphan receptor tyrosine kinase ErbB2 (also known as HER2 or Neu) transforms cells when overexpressed, and it is an important therapeutic target in human cancer. Structural studies have suggested that the oncogenic (and ligand-independent) signalling properties of ErbB2 result from the absence of a key intramolecular 'tether' in the extracellular region that autoinhibits other human ErbB receptors, including the epidermal growth factor (EGF) receptor. Although ErbB2 is unique among the four human ErbB receptors, here we show that it is the closest structural relative of the single EGF receptor family member in Drosophila melanogaster (dEGFR). Genetic and biochemical data show that dEGFR is tightly regulated by growth factor ligands, yet a crystal structure shows that it, too, lacks the intramolecular tether seen in human EGFR, ErbB3 and ErbB4. Instead, a distinct set of autoinhibitory interdomain interactions hold unliganded dEGFR in an inactive state. All of these interactions are maintained (and even extended) in ErbB2, arguing against the suggestion that ErbB2 lacks autoinhibition. We therefore suggest that normal and pathogenic ErbB2 signalling may be regulated by ligands in the same way as dEGFR. Our findings have important implications for ErbB2 regulation in human cancer, and for developing therapeutic approaches that target novel aspects of this orphan receptor.

Figure 1

ErbB receptor autoinhibition. a, The unliganded hEGFR extracellular region adopts a tethered structure (left), burying its dimerization arm (green) in autoinhibitory domain II/IV interactions. Domains I, II, III and IV are blue, green, yellow and red respectively. Binding of EGF (magenta) to domains I and III stabilizes extended s-hEGFR, exposing the dimerization arm (centre) to promote receptor dimerization (right). Most of domain IV was missing from extended s-hEGFR, structures, and was added to the centre and right-hand panels using the domain IV structure of tethered s-hEGFR (left). b, Surface representation of a monomer from the EGF-bound s-hEGFR dimer (PDB ID 1ivo). c, sErbB2 (PDB ID 1n8z: shown in surface representation) adopts an extended configuration similar to an activated s-hEGFR monomer. d, Even in its inactive, unliganded state, s-dEGFRΔV is completely extended and closely resembles both sErbB2 and activated s-hEGFR.

Figure 2

The unactivated dEGFR extracellular region closely resembles sErbB2. a, Global superimposition of inactive s-dEGFRΔV (red) and sErbB2 (cyan) illustrates their conformational similarity. Direct domain I-III interactions (more extensive in sErbB2 than in s-dEGFR) help stabilize the extended configuration in both receptors (Supplementary Fig. 5) and block ligand-binding sites. b, Low-resolution molecular envelopes from small-angle X-ray scattering (SAXS) studies of s-dEGFRΔV (left) and s-dEGFR (right), with maximum molecular dimensions (D_max) marked (see Supplementary Table 2). The s-dEGFRΔV envelope readily accommodates the crystallographic model. In intact s-dEGFR, domain V (orange) appears simply to add to the maximum dimension. Domain V and the domain IV C-terminus (poorly defined in our crystal structure) were modelled using s-hEGFR domain IV as template. In the right-hand panel, the three most C-terminal terminal disulphide-linked modules of domain V have been removed. The fact that these are not accommodated by the SAXS envelope suggests flexibility at the C-terminus.

Figure 3

Ligand binding breaks autoinhibitory domain I/II interactions common to s-dEGFR, s-hEGFR and sErbB2. a, Superposition of inactive s-hEGFR (grey) on s-dEGFRΔV (red) and sErbB2 (cyan) using domain I as reference. The eight disulphide-bonded modules (m1-m8) that define domain II are labelled, as is the dimerization arm – located almost identically in all three structures. Domain III of inactive s-hEGFR is removed for clarity. b, A similar overlay of active s-hEGFR (green) and inactive s-dEGFRΔV (red) highlights dimerization arm reorientation upon ligand binding. The structures overlay very well in modules m1-m4 of domain II, but deviate significantly at the m4/m5 linkage (green arrow) because of a ligand-induced bend. c-d, Model for activation of dEGFR (and ErbB2) by wedging an EGF-like ligand (blue) between domains I and III. Forcing domains I and III apart disrupts all direct domain I/III interactions, as well as a set of domain I/II contacts that normally maintain domain II in an inactive conformation (residues shown in space-filling representation: see Supplementary Fig. 6). In EGF-bound s-hEGFR (d), the side-chains shown in green space-filling representation no longer interact, and domain II is bent.