ErbB, so named after the viral oncogene product, Erb-B, of the erythroblastoma virus to which they are related. ErbB1 represents the classic EGF receptor (EGFR). The ErbB family is an evolutionary elaboration of the ancestral gene, Let-23,

present in Caenorhabditis elegans and its equivalent DER (also known as torpedo) in Drosophila melanogaster.

Here, we focus on the signal transduction pathway initiated by the binding of EGF, and other ligands of the EGF family, to receptors on fibroblasts or

epithelial cells. A number of the principles also apply for other tyrosine kinasecontaining receptors.

Tyrosine kinase-containing receptors

The ErbB receptor family and their ligands

The first RTK discovered was the EGFR.17 It was also the first receptor that provided evidence for a relationship between activating mutations (oncogenes) and cancer18 (see also Chapter 11). The EGF receptor is a

member of the ErbB family of proteins (ErbBs 1–4). ErbB2 lacks the capacity to interact with a ligand because its extracellular region exists in a fixed, unfolded conformation (reminiscent of the ligand-bound form of the EGFR).19 ErbB3 lacks kinase activity.20 These so-called non-autonomous receptors

do, however, contribute to growth factor signalling because they can form heterodimeric complexes in which ErbB2 is the preferred dimerization partner. For instance, association of ErbB2 with the EGFR (ErbB1), increases both the intensity and duration of the EGF signal, by increasing the ratio of active kinase to EGF and by inhibiting receptor uptake in clathrin-coated endocytic vesicles.21 ErbB2 can be said to act as an amplifier of the ErbB signalling network.22

Three groups of ligands have been identified that contain one or more copies of a conserved EGF domain (30–40 residues). All of these bind to two or more of the receptors (see Figure 12.2). Many of the ligands, including EGF, are expressed as large transmembrane proteins that can signal in a juxtacrine (cell–cell interaction) or paracrine mode, following proteolytic cleavage

of their extracellular domain. In the case of human EGF, only 55 residues constitute the growth factor out of more than 1200 present in its membrane tethered precursor form.

The various ligands have redundant functions, well exemplified by deletion of TGF , EGF, and amphiregulin. Single knockouts have little effect on developmental processes; indeed, its takes a triple deletion to manifest serious effects in mice.25,26 By contrast, inactivation of the EGFR results in midgestational death.27   

Cross-linking of receptors causes activation

Receptors containing tyrosine kinase come in several different forms, but they are unified by the presence of a single membrane-spanning domain and an intracellular kinase catalytic domain. The extracellular chains vary considerably, as indicated in Figure 12.3.

A general feature of these receptors is that ligand binding causes dimerization. In addition to activation by the natural peptide ligands, some (but not all) of the functions of the EGF receptor can be elicited by crosslinking with antibodies.28,29 Crosslinking is achieved in a number of ways. Platelet derived growth factor (PDGF) is itself a disulfide-linked dimeric ligand which cross-links its receptor upon binding. EGF is monomeric. Its binding causes the receptor to unfold, exposing a dimerization loop that allows the occupied monomers to recognize each other30,31 (Figure 12.4). Since truncated EGF receptors that lack the extracellular ligand-binding domain are constitutively active, it follows that the unoccupied extracellular domain acts to prevent kinase activation. The insulin receptor is a dimeric molecule, already cross-linked by default, yet it still requires the attachment of a ligand in order to become activated. This permits a lateral shift and approach of the

two transmembrane domains which brings the catalytic domains within reach of each other.32,33

The activation signals are, of course, more complicated than this. For activation of all the receptor functions, not only must the receptors be brought together as dimers, but they must also undergo appropriate conformational changes in relation to each other, in a way that increases the affinity for ATP within the catalytic domain and allows access of substrate.35–37

To complicate matters further, dimerization may also involve direct contact between the helical transmembrane structures of the receptors. Particular mutations within the transmembrane region cause constitutive dimerization and this, in the case of the ErbB2 receptor (also known as the HER-2/neu oncogene), can induce cell transformation.34,133

Adaptors are composed of protein interaction domains (SH2, SH3, PTB, etc.) and are without catalytic activity. They act to bring components of signalling pathways into close proximity. Examples are Grb2 and Drk: see page 332. By definition, effectors are catalysts, kinases, phospholipases, etc.

Ligand binding allows both kinase domains to encounter target sequences on the partner, so enabling intermolecular cross-phosphorylation (transphosphorylation) of tyrosine residues (Figure 12.4). The phosphorylated dimer then constitutes the active receptor.

Two types of phosphorylation sites can be distinguished. One concerns those tyrosine residues that, when phosphorylated, render the tyrosine kinase catalytically competent. The other concerns tyrosines, often in the region of the C-terminus that act as docking sites for other proteins. These phosphotyrosines bind adaptors and effectors bearing SH2 or PTB domains (see below and Chapter 24) to assemble receptor signalling complexes24,38 (Figure 12.5). In addition, dimerized and phosphorylated receptors have the potential to phosphorylate their targets.

Growth factor receptor dimers can further aggregate into oligomers of several hundreds or even thousands of units. This phenomenon, already

Fig 12.5  Phosphorylation and formation of receptor signalling complexes. Activated EGF and PDGF receptors associate with numerous effectors, including effector enzymes (PLC , GAP etc.), adaptor proteins

(p85 subunit of PI 3 kinase, Grb2, etc.) and transcription factors (STATs) to form receptor signalling complexes. See also Table 12.1.