The tethering arm of the EGF receptor is required for negative cooperativity and signal transduction

The dependence of EGFR oligomerization on environment and structure: A camera-based N&B study

Number and brightness (N&B) analysis is a fluorescence spectroscopy technique to quantify oligomerization of the mobile fraction of proteins. Accurate results, however, rely on a good knowledge of nonfluorescent states of the fluorescent labels, especially of fluorescent proteins, which are widely used in biology. Fluorescent proteins have been characterized for confocal, but not camera-based, N&B, which allows, in principle, faster measurements over larger areas. Here, we calibrate camera-based N&B implemented on a total internal reflection fluorescence microscope for various fluorescent proteins by determining their propensity to be fluorescent. We then apply camera-based N&B in live CHO-K1 cells to determine the oligomerization state of the epidermal growth factor receptor (EGFR), a transmembrane receptor tyrosine kinase that is a crucial regulator of cell proliferation and survival with implications in many cancers. EGFR oligomerization in resting cells and its regulation by the plasma membrane microenvironment are still under debate. Therefore, we investigate the effects of extrinsic factors, including membrane organization, cytoskeletal structure, and ligand stimulation, and intrinsic factors, including mutations in various EGFR domains, on the receptor's oligomerization. Our results demonstrate that EGFR oligomerization increases with removal of cholesterol or sphingolipids or the disruption of GM3-EGFR interactions, indicating raft association. However, oligomerization is not significantly influenced by the cytoskeleton. Mutations in either I706/V948 residues or E685/E687/E690 residues in the kinase and juxtamembrane domains, respectively, lead to a decrease in oligomerization, indicating their necessity for EGFR dimerization. Finally, EGFR phosphorylation is oligomerization dependent, involving the extracellular domain (550-580 residues). Coupled with biochemical investigations, camera-based N&B indicates that EGFR oligomerization and phosphorylation are the outcomes of several molecular interactions involving the lipid content and structure of the cell membrane and multiple residues in the kinase, juxtamembrane, and extracellular domains.

Cooperativity transitions driven by higher-order oligomer formations in ligand-induced receptor dimerization

While cooperativity in ligand-induced receptor dimerization has been linked with receptor-receptor couplings via minimal representations of physical observables, effects arising from higher-order oligomer, e.g., trimer and tetramer, formations of unobserved receptors have received less attention. Here we propose a dimerization model of ligand-induced receptors in multivalent form representing physical observables under basis vectors of various aggregated receptor states. Our simulations of multivalent models not only reject Wofsy-Goldstein parameter conditions for cooperativity, but show that higher-order oligomer formations can shift cooperativity from positive to negative.

Emerging Allosteric Mechanism of EGFR Activation in Physiological and Pathological Contexts

Protein kinases are key players in signal transduction pathways where they are crafted into two functional states. In response to growth factor binding stimulus, epidermal growth factor receptor (EGFR), which is physiologically populated in an autoinhibited inactive state, becomes active. Here, we outline a simple allostery scheme to clarify how an extracellular (ligand-dependent) binding event activates the intracellular EGFR kinase domain via (dimer-dependent) asymmetric dimerization, as well as how pathologically overexpressed EGFR or constitutively active mutants, leads to oncogenic pathway activation. Our underlying allosteric activation mechanism derives from a collection of inactive versus active EGFR structural, biochemical (negatively cooperative ligand binding), and biophysical (weak coupling between extracellular and intracellular kinase dimerization) data. The emerging structural insight reveals that ligand-dependent physiological activation is an outside-in allosteric activation with strong structural coupling across the membrane. In contrast, ligand-independent pathological activation is a weak inside-out activation mediated by intracellular kinase dimerization, which is structurally accommodated by additional extracellular dimers.

Allosteric regulation of epidermal growth factor (EGF) receptor ligand binding by tyrosine kinase inhibitors

The epidermal growth factor (EGF) receptor is a classical receptor tyrosine kinase with an extracellular ligand-binding domain and an intracellular kinase domain. Mutations in the EGF receptor have been shown to drive uncontrolled cell growth and are associated with a number of different tumors. Two different types of ATP-competitive EGF receptor tyrosine kinase inhibitors have been identified that bind to either the active (type I) or inactive (type II) conformation of the kinase domain. Despite the fact that both types of inhibitors block tyrosine kinase activity, they exhibit differential efficacies in different tumor types. Here, we show that in addition to inhibiting kinase activity, these inhibitors allosterically modulate ligand binding. Our data suggest that the conformations of the EGF receptor extracellular domain and intracellular kinase domain are coupled and that these conformations exist in equilibrium. Allosteric regulators, such as the small-molecule tyrosine kinase inhibitors, as well as mutations in the EGF receptor itself, shift the conformational equilibrium among the active and inactive species, leading to changes in EGF receptor-binding affinity. Our studies also reveal unexpected positive cooperativity between EGF receptor subunits in dimers formed in the presence of type II inhibitors. These findings indicate that there is strong functional coupling between the intracellular and extracellular domains of this receptor. Such coupling may impact the therapeutic synergy between small-molecule tyrosine kinase inhibitors and monoclonal antibodies in vivo.

Activation of the EGF Receptor by Ligand Binding and Oncogenic Mutations: The "Rotation Model"

The epidermal growth factor receptor (EGFR) plays vital roles in cellular processes including cell proliferation, survival, motility, and differentiation. The dysregulated activation of the receptor is often implicated in human cancers. EGFR is synthesized as a single-pass transmembrane protein, which consists of an extracellular ligand-binding domain and an intracellular kinase domain separated by a single transmembrane domain. The receptor is activated by a variety of polypeptide ligands such as epidermal growth factor and transforming growth factor α. It has long been thought that EGFR is activated by ligand-induced dimerization of the receptor monomer, which brings intracellular kinase domains into close proximity for trans-autophosphorylation. An increasing number of diverse studies, however, demonstrate that EGFR is present as a pre-formed, yet inactive, dimer prior to ligand binding. Furthermore, recent progress in structural studies has provided insight into conformational changes during the activation of a pre-formed EGFR dimer. Upon ligand binding to the extracellular domain of EGFR, its transmembrane domains rotate or twist parallel to the plane of the cell membrane, resulting in the reorientation of the intracellular kinase domain dimer from a symmetric inactive configuration to an asymmetric active form (the “rotation model”). This model is also able to explain how oncogenic mutations activate the receptor in the absence of the ligand, without assuming that the mutations induce receptor dimerization. In this review, we discuss the mechanisms underlying the ligand-induced activation of the preformed EGFR dimer, as well as how oncogenic mutations constitutively activate the receptor dimer, based on the rotation model.

A structural perspective on the regulation of the epidermal growth factor receptor

The epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that plays a critical role in the pathogenesis of many cancers. The structure of intact forms of this receptor has yet to be determined, but intense investigations of fragments of the receptor have provided a detailed view of its activation mechanism, which we review here. Ligand binding converts the receptor to a dimeric form, in which contacts are restricted to the receptor itself, allowing heterodimerization of the four EGFR family members without direct ligand involvement. Activation of the receptor depends on the formation of an asymmetric dimer of kinase domains, in which one kinase domain allosterically activates the other. Coupling between the extracellular and intracellular domains may involve a switch between alternative crossings of the transmembrane helices, which form dimeric structures. We also discuss how receptor regulation is compromised by oncogenic mutations and the structural basis for negative cooperativity in ligand binding.

Mechanisms of activation of receptor tyrosine kinases: monomers or dimers

Receptor tyrosine kinases (RTKs) play essential roles in cellular processes, including metabolism, cell-cycle control, survival, proliferation, motility and differentiation. RTKs are all synthesized as single-pass transmembrane proteins and bind polypeptide ligands, mainly growth factors. It has long been thought that all RTKs, except for the insulin receptor (IR) family, are activated by ligand-induced dimerization of the receptors. An increasing number of diverse studies, however, indicate that RTKs, previously thought to exist as monomers, are present as pre-formed, yet inactive, dimers prior to ligand binding. The non-covalently associated dimeric structures are reminiscent of those of the IR family, which has a disulfide-linked dimeric structure. Furthermore, recent progress in structural studies has provided insight into the underpinnings of conformational changes during the activation of RTKs. In this review, I discuss two mutually exclusive models for the mechanisms of activation of the epidermal growth factor receptor, the neurotrophin receptor and IR families, based on these new insights.

Dynamic transition states of ErbB1 phosphorylation predicted by spatial stochastic modeling

ErbB1 overexpression is strongly linked to carcinogenesis, motivating better understanding of erbB1 dimerization and activation. Recent single-particle-tracking data have provided improved measures of dimer lifetimes and strong evidence that transient receptor coconfinement promotes repeated interactions between erbB1 monomers. Here, spatial stochastic simulations explore the potential impact of these parameters on erbB1 phosphorylation kinetics. This rule-based mathematical model incorporates structural evidence for conformational flux of the erbB1 extracellular domains, as well as asymmetrical orientation of erbB1 cytoplasmic kinase domains during dimerization. The asymmetric dimer model considers the theoretical consequences of restricted transactivation of erbB1 receptors within a dimer, where the N-lobe of one monomer docks with the C-lobe of the second monomer and triggers its catalytic activity. The dynamic nature of the erbB1 phosphorylation state is shown by monitoring activation states of individual monomers as they diffuse, bind, and rebind after ligand addition. The model reveals the complex interplay between interacting liganded and nonliganded species and the influence of their distribution and abundance within features of the membrane landscape.

Live-cell fluorescence imaging reveals high stoichiometry of Grb2 binding to the EGF receptor sustained during endocytosis

Activation of epidermal growth factor (EGF) receptor (EGFR) leads to its interaction with Grb2, a dual-function adapter mediating both signaling through Ras and receptor endocytosis. We used time-lapse three-dimensional imaging by spinning disk confocal microscopy to analyze trafficking of EGFR and Grb2 in living HeLa cells stimulated with low, physiological concentrations of EGFR ligands. Endogenous Grb2 was replaced in these cells by Grb2 fused to yellow fluorescent protein (YFP). After transient residence in the plasma membrane, Rhodamine-conjugated EGF (EGF-Rh) and Grb2-YFP were rapidly internalized and accumulated in endosomes. Quantitative image analysis revealed that on average two Grb2-YFP molecules were colocalized with one EGF-Rh in cells stimulated with 2 ng/ml EGF-Rh, and the excess of Grb2-YFP over EGF-Rh was even higher when a receptor-saturating concentration of EGF-Rh was used. Therefore, we hypothesize that a single EGFR molecule can be simultaneously associated with functionally distinct Grb2 interaction partners during and after endocytosis. Continuous presence of Grb2-YFP in endosomes was also observed when EGFR was activated by transforming growth factor-α and amphiregulin, suggesting that endosomal EGFRs remain ligand occupied and signaling competent, despite the fact that these growth factors are thought to dissociate from the receptor at acidic pH. The prolonged localization and activity of EGFR-Grb2 complexes in endosomes correlated with the sustained activation of extracellular stimulus-regulated kinase 1/2, suggesting that endosomal EGFRs contribute significantly to this signaling pathway. We propose that endosomal EGFRs function to extend signaling in time and space to compensate for rapid downregulation of surface EGFRs in cells with low receptor expression levels.

Bipartite tetracysteine display reveals allosteric control of ligand-specific EGFR activation

Aberrant activation of the epidermal growth factor receptor (EGFR), a prototypic receptor tyrosine kinase, is critical to the biology of many common cancers. The molecular events that define how EGFR transmits an extracellular ligand binding event through the membrane are not understood. Here we use a chemical tool, bipartite tetracysteine display, to report on ligand-specific conformational changes that link ligand binding and kinase activation for full-length EGFR on the mammalian cell surface. We discover that EGF binding is communicated to the cytosol through formation of an antiparallel coiled coil within the intracellular juxtamembrane (JM) domain. This conformational transition is functionally coupled to receptor activation by EGF. In contrast, TGFα binding is communicated to the cytosol through formation of a discrete, alternative helical interface. These findings suggest that the JM region can differentially decode extracellular signals and transmit them to the cell interior. Our results provide new insight into how EGFR communicates ligand-specific information across the membrane.

Dynamically varying interactions between heregulin and ErbB proteins detected by single-molecule analysis in living cells

Heregulin (HRG) belongs to the family of EGFs and activates the receptor proteins ErbB3 and ErbB4 in a variety of cell types to regulate cell fate. The interactions between HRG and ErbB3/B4 are important to the pathological mechanisms underlying schizophrenia and some cancers. Here, we observed the reaction kinetics between fluorescently labeled single HRG molecules and ErbB3/B4 on the surfaces of MCF-7 human breast cancer cells. The equilibrium association and the dissociation from equilibrium were also measured using single-molecule imaging techniques. The unitary association processes mirrored the EGF and ErbB1 interactions in HeLa cells [Teramura Y, et al. (2006) EMBO J 25:4215-4222], suggesting that the predimerization of the receptors, followed by intermediate formation (between the first and second ligand-binding events to a receptor dimer), accelerated the formation of doubly liganded signaling dimers of the receptor molecules. However, the dissociation analysis suggested that the first HRG dissociation from the doubly liganded dimer was rapid, but the second dissociation from the singly liganded dimer was slow. The dissociation rate constant from the liganded monomer was intermediate. The dynamic changes in the association and dissociation kinetics in relation to the dimerization of ErbB displayed negative cooperativity, which resulted in apparent low- and high-affinity sites of HRG association on the cell surface.

Quantitation of the effect of ErbB2 on epidermal growth factor receptor binding and dimerization

The epidermal growth factor (EGF) receptor is a member of the ErbB family of receptors that also includes ErbB2, ErbB3, and ErbB4. These receptors form homo- and heterodimers in response to ligand with ErbB2 being the preferred dimerization partner. Here we use (125)I-EGF binding to quantitate the interaction of the EGF receptor with ErbB2. We show that the EGFR/ErbB2 heterodimer binds EGF with a 7-fold higher affinity than the EGFR homodimer. Because it cannot bind a second ligand, the EGFR/ErbB2 heterodimer is not subject to ligand-induced dissociation caused by the negatively cooperative binding of EGF to the second site on the EGFR homodimer. This increases the stability of the heterodimer relative to the homodimer and is associated with enhanced and prolonged EGF receptor autophosphorylation. These effects are independent of the kinase activity of ErbB2 but require back-to-back dimerization of the EGF receptor with ErbB2. Back-to-back dimerization is also required for phosphorylation of ErbB2. These findings provide a molecular explanation for the apparent preference of the EGF receptor for dimerizing with ErbB2 and suggest that the phosphorylation of ErbB2 occurs largely in the context of the EGFR/ErbB2 heterodimer, rather than through lateral phosphorylation of isolated ErbB2 subunits.

Negative co-operativity in the EGF receptor

Scatchard analyses of the binding of EGF (epidermal growth factor) to its receptor (EGFR) yield concave up Scatchard plots, indicative of some type of heterogenity in ligand-binding affinity. This was typically interpreted as being due to the presence of two independent binding sites: one of high affinity representing ≤10% of the receptor population, and one of low affinity making up the bulk of the receptors. However, the concept of two independent binding sites is difficult to reconcile with the X-ray structures of the dimerized EGFR that show symmetrical binding of the two ligands. A new approach to the analysis of 125I-EGF-binding data combined with the structure of the singly-occupied Drosophila EGFR have now shown that this heterogeneity is due to the presence of negative co-operativity in the EGFR. Concerns that negative co-operativity precludes ligand-induced dimerization of the EGFR confuse the concepts of linkage and co-operativity. Linkage refers to the effect of ligand on the assembly of dimers, whereas co-operativity refers to the effect of ligand binding to one subunit on ligand binding to the other subunit within a preassembled dimer. Binding of EGF to its receptor is positively linked with dimer assembly, but shows negative co-operativity within the dimer.

Mechanistic insights into the activation of oncogenic forms of EGF receptor

Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase that is commonly activated by mutation in non-small cell lung cancer. The mechanism of this oncogenic activation is not completely understood, but in contrast to that of the wild-type EGFR, it is proposed to be independent of kinase domain dimerization. Mechanistic studies on EGFR have mainly relied on cell-based assays or isolated kinase domain measurements. Here we show, using purified, near full-length human EGFR proteins (tEGFRs), that two oncogenic mutants are fully active independently of EGF and highly resistant to the therapeutic and endogenous inhibitors cetuximab, lapatinib and MIG6. Based on the pattern of inhibition and the effects of additional asymmetric kinase dimer interface mutations, we propose that these oncogenic EGFR mutants drive and strongly depend on the formation of the asymmetric kinase dimer for activation, which has implications for drug design and cancer treatment strategies.

The membrane-proximal intracellular domain of the epidermal growth factor receptor underlies negative cooperativity in ligand binding

The binding of EGF induces dimerization of its receptor, leading to the stimulation of its intracellular tyrosine kinase activity. Kinase activation occurs within the context of an asymmetric dimer in which one kinase domain serves as the activator for the other kinase domain but is not itself activated. How ligand binding is related to the formation and dynamics of this asymmetric dimer is not known. The binding of EGF to its receptor is negatively cooperative--that is, EGF binds with lower affinity to the second site on the dimer than to the first site on the dimer. In this study, we analyzed the binding of (125)I-EGF to a series of EGF receptor mutants in the intracellular juxtamembrane domain and demonstrate that the most membrane-proximal portion of this region plays a significant role in the genesis of negative cooperativity in the EGF receptor. The data are consistent with a model in which the binding of EGF to the first site on the dimer induces the formation of one asymmetric kinase dimer. The binding of EGF to the second site is required to disrupt the initial asymmetric dimer and allow the formation of the reciprocal asymmetric dimer. Thus, some of the energy of binding to the second site is used to reorient the first asymmetric dimer, leading to a lower binding affinity and the observed negative cooperativity.

Human epidermal growth factor receptor (EGFR) aligned on the plasma membrane adopts key features of Drosophila EGFR asymmetry

The ability of epidermal growth factor receptor (EGFR) to control cell fate is defined by its affinity for ligand. Current models suggest that ligand-binding heterogeneity arises from negative cooperativity in signaling receptor dimers, for which the asymmetry of the extracellular region of the Drosophila EGFR has recently provided a structural basis. However, no asymmetry is apparent in the isolated extracellular region of the human EGFR. Human EGFR also differs from the Drosophila EGFR in that negative cooperativity is found only in full-length receptors in cells. To gain structural insights into the human EGFR in situ, we developed an approach based on quantitative Förster resonance energy transfer (FRET) imaging, combined with Monte Carlo and molecular dynamics simulations, to probe receptor conformation in epithelial cells. We experimentally demonstrate a high-affinity ligand-binding human EGFR conformation consistent with the extracellular region aligned flat on the plasma membrane. We explored the relevance of this conformation to ligand-binding heterogeneity and found that the asymmetry of this structure shares key features with that of the Drosophila EGFR, suggesting that the structural basis for negative cooperativity is conserved from invertebrates to humans but that in human EGFR the extracellular region asymmetry requires interactions with the plasma membrane.