Mechanisms for Kinase-mediated Dimerization of the Epidermal Growth Factor Receptor

We study a mechanism by which dimerization of the EGF receptor (EGFR) cytoplasmic domain is transmitted to the ectodomain. Therapeutic and other small molecule antagonists to the kinase domain that stabilize its active conformation, but not those that stabilize an inactive conformation, stabilize ectodomain dimerization. Inhibitor-induced dimerization requires an asymmetric kinase domain interface associated with activation. EGF and kinase inhibitors stimulate formation of identical dimer interfaces in the EGFR transmembrane domain, as shown by disulfide cross-linking. Disulfide cross-linking at an interface in domain IV in the ectodomain was also stimulated similarly; however, EGF but not inhibitors stimulated cross-linking in domain II. Inhibitors similarly induced noncovalent dimerization in nearly full-length, detergent-solubilized EGFR as shown by gel filtration. EGFR ectodomain deletion resulted in spontaneous dimerization, whereas deletion of exons 2–7, in which extracellular domains III and IV are retained, did not. In EM, kinase inhibitor-induced dimers lacked any well defined orientation between the ectodomain monomers. Fab of the therapeutic antibody cetuximab to domain III confirmed a variable position and orientation of this domain in inhibitor-induced dimers but suggested that the C termini of domain IV of the two monomers were in close proximity, consistent with dimerization in the transmembrane domains. The results provide insights into the relative energetics of intracellular and extracellular dimerization in EGFR and have significance for physiologic dimerization through the asymmetric kinase interface, bidirectional signal transmission in EGFR, and mechanism of action of therapeutics.     

Ligand-induced EGF receptor oligomerization is kinase-dependent and enhances internalization

The current activation model of the EGF receptor (EGFR) predicts that binding of EGF results in dimerization and oligomerization of the EGFR, leading to the allosteric activation of the intracellular tyrosine kinase. Little is known about the regulatory mechanism of receptor oligomerization. In this study, we have employed FRET between identical fluorophores (homo-FRET) to monitor the dimerization and oligomerization state of the EGFR before and after receptor activation. Our data show that, in the absence of ligand, ～40% of the EGFR molecules were present as inactive dimers or predimers. The monomer/predimer ratio was not affected by deletion of the intracellular domain. Ligand binding induced the formation of receptor oligomers, which were found in both the plasma membrane and intracellular structures. Ligand-induced oligomerization required tyrosine kinase activity and nine different tyrosine kinase substrate residues. This indicates that the binding of signaling molecules to activated EGFRs results in EGFR oligomerization. Induction of EGFR predimers or pre-oligomers using the EGFR fused to the FK506-binding protein did not affect signaling but was found to enhance EGF-induced receptor internalization. Our data show that EGFR oligomerization is the result of EGFR signaling and enhances EGFR internalization.     

Differential and synergistic effects of epidermal growth factor receptor antibodies on unliganded ErbB dimers and oligomers

Antibodies directed against the epidermal growth factor receptor (EGFR) offer a potentially powerful therapeutic approach against cancers driven by the EGFR pathway. EGFR antibodies are believed to halt cell surface activation by blocking ligand-induced receptor tyrosine kinase activation, i.e., ligand binding, a change in conformation, or the monomer-dimer transition. In this work, we demonstrate that wild-type EGFR and the truncated de2-7-EGFR (tumor-associated mutant) formed unliganded homo-oligomers and examined the effects of two clinically relevant antibodies on the conformation and quaternary state of these ligand-free EGFR oligomers on the surface of cells. The EGFR antibodies were mAb528, a ligand-blocking antibody that binds domain III, and mAb806, a conformationally sensitive antibody that binds near the dimer interface in domain II. We used a model cellular system, BaF/3 cells, with GFP-tagged receptors in the absence of interference from secreted ligands or other erbB receptor members. Different antibody-mediated effects (conformational transition, receptor cross-linking, or receptor dissociation) were distinguished by combining two complementary biophysical techniques: image correlation spectroscopy (submicrometer scale clustering) and homo-Forster resonance energy transfer (association and/or conformation on a 1-10 nm scale). mAb528 cross-linked EGFR into an inactive EGFR dimer of dimers but had no effect when added to de2-7-EGFR oligomers. mAb806 had a minor effect on EGFR dimers as expected from its poor binding to a conformationally shielded epitope on wtEGFR but bound de2-7-EGFR oligomers, causing a conformational change in the intracellular C-terminal GFP-tagged tail. The combination of the two antibodies had synergistic effects, increasing the level of cross-linking of de2-7-EGFR, but did not lead to enhanced cross-linking of EGFR. The results reveal new modes of receptor-antibody interactions for EGFR and de2-7-EGFR.     

EGF receptor exposed to oxidative stress acquires abnormal phosphorylation and aberrant activated conformation that impairs canonical dimerization

Crystallographic studies have offered understanding of how receptor tyrosine kinases from the ErbB family are regulated by their growth factor ligands. A conformational change of the EGFR (ErbB1) was shown to occur upon ligand binding, where a solely ligand-mediated mode of dimerization/activation was documented. However, this dogma of dimerization/activation was revolutionized by the discovery of constitutively active ligand-independent EGFR mutants. In addition, other ligand-independent activation mechanisms may occur. We have shown that oxidative stress (ox-stress), induced by hydrogen peroxide or cigarette smoke, activates EGFR differently than its ligand, EGF, thereby inducing aberrant phosphorylation and impaired trafficking and degradation of EGFR. Here we demonstrate that ox-stress activation of EGFR is ligand-independent, does not induce "classical" receptor dimerization and is not inhibited by the tyrosine kinase inhibitor AG1478. Thus, an unprecedented, apparently activated, state is found for EGFR under ox-stress. Furthermore, this activation mechanism is temperature-dependent, suggesting the simultaneous involvement of membrane structure. We propose that ceramide increase under ox-stress disrupts cholesterol-enriched rafts leading to EGFR re-localization into the rigid, ceramide-enriched rafts. This increase in ceramide also supports EGFR aberrant trafficking to a peri-nuclear region. Therefore, the EGFR unprecedented and activated conformation could be sustained by simultaneous alterations in membrane structure under ox-stress.     

A Highlights from MBoC Selection: Enhanced dimerization drives ligand-independent activity of mutant epidermal growth factor receptor in lung cancer

Mutations within the epidermal growth factor receptor (EGFR/erbB1/Her1) are often associated with tumorigenesis. In particular, a number of EGFR mutants that demonstrate ligand-independent signaling are common in non–small cell lung cancer (NSCLC), including kinase domain mutations L858R (also called L834R) and exon 19 deletions (e.g., ΔL747-P753insS), which collectively make up nearly 90% of mutations in NSCLC. The molecular mechanisms by which these mutations confer constitutive activity remain unresolved. Using multiple subdiffraction-limit imaging modalities, we reveal the altered receptor structure and interaction kinetics of NSCLC-associated EGFR mutants. We applied two-color single quantum dot tracking to quantify receptor dimerization kinetics on living cells and show that, in contrast to wild-type EGFR, mutants are capable of forming stable, ligand-independent dimers. Two-color superresolution localization microscopy confirmed ligand-independent aggregation of EGFR mutants. Live-cell Förster resonance energy transfer measurements revealed that the L858R kinase mutation alters ectodomain structure such that unliganded mutant EGFR adopts an extended, dimerization-competent conformation. Finally, mutation of the putative dimerization arm confirmed a critical role for ectodomain engagement in ligand-independent signaling. These data support a model in which dysregulated activity of NSCLC-associated kinase mutants is driven by coordinated interactions involving both the kinase and extracellular domains that lead to enhanced dimerization.     

The epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor AG1478 increases the formation of inactive untethered EGFR dimers. Implications for combination therapy with monoclonal antibody 806

The epidermal growth factor receptor (EGFR) has at least two fundamental conformations: an inactive tethered conformation and an active untethered, ligand-bound "back-to-back" dimer, which may be part of an oligomeric complex. Monoclonal antibody (mAb) 806 is an EGFR-specific antibody that only binds a transitional form of the receptor after it untethers but before forming the back-to-back, ligated, active oligomer. We have shown that AG1478, a tyrosine kinase inhibitor of the EGFR, synergistically inhibits the growth of tumors overexpressing EGFR when used in combination with mAb 806 but the mechanism for this was not elucidated (Johns, T. G., Luwor, R. B., Murone, C., Walker, F., Weinstock, J., Vitali, A. A., Perera, R. M., Jungbluth, A. A., Stockert, E., Old, L. J., Nice, E. C., Burgess, A. W., and Scott, A. M. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 15871-15876). We now show that AG1478 increases binding of mAb 806 to the cell surface through two distinct mechanisms: an immediate effect on the conformation of EGFR and a longer term increase in cell surface under-glycosylated EGFR, an event known to increase mAb 806 reactivity. Cross-linking studies demonstrated the presence of spontaneously occurring mAb 806-reactive dimers on the surface of cells overexpressing EGFR, which are rapidly increased by AG1478. Because they react with mAb 806, these dimers must exist in a conformation distinct from the ligated back-to-back dimer. Indeed, we detected similar dimers in 293T cells expressing the EGFR lacking the small dimerization/activation arm essential to the formation of the back-to-back dimer. Thus, some of the EGFR on the cell surface of cancer cells must exist as an untethered dimer that adopts a previously unreported conformation that is inactive. This information was used to optimize the therapeutic synergy between mAb 806 and AG1478 in a xenograft model.     

Autophosphorylation of EGFR at Y954 Facilitated Homodimerization and Enhanced Downstream Signals

Asymmetric dimer formation of epidermal growth factor receptor (EGFR) is crucial for EGF-induced receptor activation. Even though autophosphorylation is important for activation, its role remains elusive in the context of regulating dimers. In this study, employing overlapping time series analysis to raster image correlation spectroscopy (RICS), we observed time-dependent transient dynamics of EGFR dimerization and found EGFR kinase activity to be essential for dimerization. As a result of which, we hypothesized that phosphorylation could influence dimerization. Evaluating this point, we observed that one of the tyrosine residues (Y954) located in the C-terminal lobe of the activator kinase domain was important to potentiate dimerization. Functional imaging to monitor Ca²⁺ and ERK signals revealed a significant role of Y954 in influencing downstream signaling cascade. Crucial for stabilization of EGFR asymmetric dimer is a “latch” formed between kinase domains of the binding partners. Because Y954 is positioned adjacent to the latch binding region on the kinase domain, we propose that phosphorylation strengthened the latch interaction. On the contrary, we identified that threonine phosphorylation (T669) in the latch domain negatively regulated EGFR dimerization and the downstream signals. Overall, we have delineated the previously anonymous role of phosphorylation at the latch interface of kinase domains in regulating EGFR dimerization.     

An allosteric mechanism for activation of the kinase domain of epidermal growth factor receptor

The mechanism by which the epidermal growth factor receptor (EGFR) is activated upon dimerization has eluded definition. We find that the EGFR kinase domain can be activated by increasing its local concentration or by mutating a leucine (L834R) in the activation loop, the phosphorylation of which is not required for activation. This suggests that the kinase domain is intrinsically autoinhibited, and an intermolecular interaction promotes its activation. Using further mutational analysis and crystallography we demonstrate that the autoinhibited conformation of the EGFR kinase domain resembles that of Src and cyclin-dependent kinases (CDKs). EGFR activation results from the formation of an asymmetric dimer in which the C-terminal lobe of one kinase domain plays a role analogous to that of cyclin in activated CDK/cyclin complexes. The CDK/cyclin-like complex formed by two kinase domains thus explains the activation of EGFR-family receptors by homo- or heterodimerization.     

Single-molecule imaging of EGFR signalling on the surface of living cells

The early events in signal transduction from the epidermal growth factor (EGF) receptor (EGFR) are dimerization and autophosphorylation of the receptor, induced by binding of EGF. Here we observe these events in living cells by visualizing single molecules of fluorescent-dye-labelled EGF in the plasma membrane of A431 carcinoma cells. Single-molecule tracking reveals that the predominant mechanism of dimerization involves the formation of a cell-surface complex of one EGF molecule and an EGFR dimer, followed by the direct arrest of a second EGF molecule, indicating that the EGFR dimers were probably preformed before the binding of the second EGF molecule. Single-molecule fluorescence-resonance energy transfer shows that EGF-EGFR complexes indeed form dimers at the molecular level. Use of a monoclonal antibody specific to the phosphorylated (activated) EGFR reveals that the EGFR becomes phosphorylated after dimerization.     

Ligand-independent dimer formation of epidermal growth factor receptor (EGFR) is a step separable from ligand-induced EGFR signaling

Dimerization and phosphorylation of the epidermal growth factor (EGF) receptor (EGFR) are the initial and essential events of EGF-induced signal transduction. However, the mechanism by which EGFR ligands induce dimerization and phosphorylation is not fully understood. Here, we demonstrate that EGFRs can form dimers on the cell surface independent of ligand binding. However, a chimeric receptor, comprising the extracellular and transmembrane domains of EGFR and the cytoplasmic domain of the erythropoietin receptor (EpoR), did not form a dimer in the absence of ligands, suggesting that the cytoplasmic domain of EGFR is important for predimer formation. Analysis of deletion mutants of EGFR showed that the region between (835)Ala and (918)Asp of the EGFR cytoplasmic domain is required for EGFR predimer formation. In contrast to wild-type EGFR ligands, a mutant form of heparin-binding EGF-like growth factor (HB2) did not induce dimerization of the EGFR-EpoR chimeric receptor and therefore failed to activate the chimeric receptor. However, when the dimerization was induced by a monoclonal antibody to EGFR, HB2 could activate the chimeric receptor. These results indicate that EGFR can form a ligand-independent inactive dimer and that receptor dimerization and activation are mechanistically distinct and separable events.     

Utilization of a receptor reserve for effective amplification of mitogenic signaling by an epidermal growth factor mutant deficient in receptor activation

The idea of a receptor reserve in mediating cellular function is well known but direct biochemical evidence has not been easy to obtain. This study stems from our results showing that L15 of epidermal growth factor (EGF) is important in both EGF receptor (EGFR) binding and activation, and the L15A analog of human EGF (hEGF) partially uncouples EGFR binding from EGFR activation (Nandagopal et al., [1996] Protein Engng 9:781-788). We address the cellular mechanism of mitogenic signal amplification by EGFR tyrosine kinase in response to L15A hEGF. L15A is partially impaired in receptor dimerization, shown by chemical cross-linking and allosteric activation of EGFR in a substrate phosphorylation assay. Immunoprecipitation experiments reveal, however, that L15A can induce EGFR autophosphorylation in intact murine keratinocytes by utilizing spare receptors, the ratio of total phosphotyrosine content per receptor being significantly lower than that elicited by wild-type. This direct biochemical evidence, based on function, of utilization of a receptor reserve for kinase stimulation suggests that an EGF variant can activate varying receptor numbers to generate the same effective response. L15A-activated receptors can stimulate mitogen-activated protein kinase (MAPK) that is important for mitogenesis. The lack of linear correlation between levels of receptor dimerization, autophosphorylation, and MAPK activation suggests that signal amplification is mediated by cooperative effects. Flow cytometric analyses show that the percentages of cells which proliferate in response to 1 nM L15A and their rate of entry into S-phase are both decreased relative to 1 nM wild-type, indicating that MAPK activation alone is insufficient for maximal stimulation of mitogenesis. Higher concentrations of L15A reverse this effect, indicating that L15A and wild-type differ in the number of receptors each activates to induce the threshold response, which may be attained by cooperative activation of receptor dimers/oligomers by van der Waal's weak forces of attraction. The maintenance of a receptor reserve underscores an effective strategy in cell survival.     

Exogenous ganglioside GD1a enhances epidermal growth factor receptor binding and dimerization

Gangliosides are shed by tumor cells and can bind to normal cells in the tumor microenvironment and affect their function. Exposure of fibroblasts to exogenous gangliosides increases epidermal growth factor (EGF)-induced fibroblast proliferation and enhances EGF receptor (EGFR)-mediated activation of the mitogen-activated protein kinase signaling pathway (Li, R., Liu, Y., and Ladisch, S. (2001) J. Biol. Chem. 276, 42782-42792). Here we report that the EGFR itself is the target of this ganglioside effect: Preincubation of normal human dermal fibroblasts with G(D1a) ganglioside enhanced both EGF-induced EGFR autophosphorylation and receptor-tyrosine kinase activity. The enhancement was rapid (within 30 min), not due to alteration of time kinetics of the EGFR response to EGF, and reproduced in purified G(D1a)-enriched cell membranes isolated from ganglioside-preincubated fibroblasts. Evaluating the initial steps underlying activation, EGF binding, and EGFR dimerization, we found that G(D1a) enrichment of the cell membrane increased EGFR dimerization and the effective number of high affinity EGFR without increasing total receptor protein. Unexpectedly, G(D1a) enrichment also triggered increased EGFR dimerization in the absence of growth factor. This resulted in enhanced activation of the EGFR signal transduction cascade when EGF was added. We conclude that membrane ganglioside enrichment of normal fibroblasts (such as by tumor cell ganglioside shedding) facilitates receptor-receptor interactions (possibly by altering membrane topology), causing ligand-independent EGFR dimerization and, in turn, enhanced EGF signaling.     

Control of epidermal growth factor receptor endocytosis by receptor dimerization, rather than receptor kinase activation

Given that ligand binding is essential for the rapid internalization of epidermal growth factor receptor (EGFR), the events induced by ligand binding probably contribute to the regulation of EGFR internalization. These events include receptor dimerization, activation of intrinsic tyrosine kinase activity and autophosphorylation. Whereas the initial results are controversial regarding the role of EGFR kinase activity in EGFR internalization, more recent data suggest that EGFR kinase activation is essential for EGFR internalization. However, we have shown here that inhibition of EGFR kinase activation by mutation or by chemical inhibitors did not block EGF-induced EGFR internalization. Instead, proper EGFR dimerization is necessary and sufficient to stimulate EGFR internalization. We conclude that EGFR internalization is controlled by EGFR dimerization, rather than EGFR kinase activation. Our results also define a new role for EGFR dimerization: by itself it can drive EGFR internalization, independent of its role in the activation of EGFR kinase.     

Identification of EGF receptor C-terminal sequences 1005-1017 and di-leucine motif 1010LL1011 as essential in EGF receptor endocytosis

Most studies regarding the role of epidermal growth factor (EGF) receptor (EGFR) C-terminal domain in EGFR internalization are done in the context of EGFR kinase activation. We recently showed that EGF-induced EGFR internalization is directly controlled by receptor dimerization, rather than kinase activation. Here we studied the role of EGFR C-terminus in EGF-induced EGFR internalization with or without EGFR kinase activation. We showed that graduate truncation of EGFR from C-terminus to 1044 did not affect EGF-induced EGFR endocytosis with or without kinase activation. However, truncation to 991 or further completely inhibited EGFR endocytosis. Graduate truncation within 991-1044 progressively lower EGF-induced EGFR endocytosis with most significant effects observed for residues 1005-1017. The endocytosis patterns of mutant EGFRs are independent of EGFR kinase activation. The residues 1005-1017 were also required for EGFR internalization triggered by non-ligand-induced receptor dimerization. This indicates that residues 1005-1017 function as an internalization motif, rather than a dimerization motif, to mediate EGFR internalization. Furthermore, we showed that the di-leucine motif 1010LL1011 within this region is essential in mediating EGF-induced rapid EGFR internalization independent of kinase activation. We conclude that EGFR C-terminal sequences 1005-1017 and the 1010LL1011 motif are essential for EGF-induced EGFR endoytosis independent of EGFR kinase activation and autophosphorylation.     

Functional and structural stability of the epidermal growth factor receptor in detergent micelles and phospholipid nanodiscs

Cellular signaling mediated by the epidermal growth factor receptor (EGFR or ErbB) family of receptor tyrosine kinases plays an important role in regulating normal and oncogenic cellular physiology. While structures of isolated EGFR extracellular domains and intracellular protein tyrosine kinase domains have suggested mechanisms for growth factor-mediated receptor dimerization and allosteric kinase domain activation, understanding how the transmembrane and juxtamembrane domains contribute to transmembrane signaling requires structural studies on intact receptor molecules. In this report, recombinant EGFR constructs containing the extracellular, transmembrane, juxtamembrane, and kinase domains are overexpressed and purified from human embryonic kidney 293 cell cultures. The oligomerization state, overall structure, and functional stability of the purified EGF-bound receptor are characterized in detergent micelles and phospholipid bilayers. In the presence of EGF, catalytically active EGFR dimers can be isolated by gel filtration in dodecyl maltoside. Visualization of the dimeric species by negative stain electron microscopy and single particle averaging reveals an overall structure of the extracellular domain that is similar to previously published crystal structures and is consistent with the C-termini of domain IV being juxtaposed against one another as they enter the transmembrane domain. Although detergent-soluble preparations of EGFR are stable as dimers in the presence of EGF, they exhibit differential functional stability in Triton X-100 versus dodecyl maltoside. Furthermore, the kinase activity can be significantly stabilized by reconstituting purified EGF-bound EGFR dimers in phospholipid nanodiscs or vesicles, suggesting that the environment around the hydrophobic transmembrane and amphipathic juxtamembrane domains is important for stabilizing the tyrosine kinase activity in vitro.     

Structural basis for stem cell factor-KIT signaling and activation of class III receptor tyrosine kinases

Stem cell factor (SCF) binds to and activates the KIT receptor, a class III receptor tyrosine kinase (RTK), to stimulate diverse processes including melanogenesis, gametogenesis and hematopoeisis. Dysregulation of KIT activation is associated with many cancers. We report a 2.5 A crystal structure of the functional core of SCF bound to the extracellular ligand-binding domains of KIT. The structure reveals a 'wrapping' SCF-recognition mode by KIT, in which KIT adopts a bent conformation to facilitate each of its first three immunoglobulin (Ig)-like domains to interact with SCF. Three surface epitopes on SCF, an extended loop, the B and C helices, and the N-terminal segment, contact distinct KIT domains, with two of the epitopes undergoing large conformational changes upon receptor binding. The SCF/KIT complex reveals a unique RTK dimerization assembly, and a novel recognition mode between four-helix bundle cytokines and Ig-family receptors. It serves as a framework for understanding the activation mechanisms of class III RTKs.     

Dimerization-induced activation of soluble insulin/IGF-1 receptor kinases: an alternative mechanism of activation.

To study the role of kinase dimerization in the activation of the insulin receptor (IR) and the insulin-like growth factor receptor-1 (IGF-1R), we have cloned, expressed, and purified monomeric and dimeric forms of the corresponding soluble kinase domains via the baculovirus expression system. Dimerization of the kinases was achieved by fusion of the kinase domains to the homodimeric glutathione S-transferase (GST). Kinetic analyses revealed that kinase dimerization results in substantial increases (10-100-fold) in the phosphotransferase activity in both the auto- and substrate phosphorylation reactions. Furthermore, kinase dimerization rendered the autophosphorylation reaction concentration-independent. However, whereas dimerization was required for the rapid autophosphorylation of the kinases, it was not essential for the enhanced kinase activity in substrate phosphorylation reactions. Comparison of HPLC-phosphopeptide maps of the monomeric and dimeric kinases revealed that dimerization leads to an increased phosphorylation of the regulatory activation loop of the kinases, strongly suggesting that bis- and trisphosphorylation of the activation loop are mediated by transphosphorylation within the kinase dimers. Most strikingly, limited proteolysis revealed that GST-mediated dimerization by itself had a major impact on the conformation of the activation loop by stabilizing a conformation that corresponds to the active, phosphorylated form of the kinase. Thus, in analogy to the insulin/IGF-1-ligated holoreceptors, the dimeric GST-kinases are primed to rapid autophosphorylation by an increase in the local concentration of both phosphoryl donor and phosphoryl acceptor sites and by a dimerization-induced conformational change of the activation loop that leads to an efficient transphosphorylation of the regulatory tyrosine residues.     

Structure of the epidermal growth factor receptor kinase domain alone and in complex with a 4-anilinoquinazoline inhibitor

The crystal structure of the kinase domain from the epidermal growth factor receptor (EGFRK) including forty amino acids from the carboxyl-terminal tail has been determined to 2.6-A resolution, both with and without an EGFRK-specific inhibitor currently in Phase III clinical trials as an anti-cancer agent, erlotinib (OSI-774, CP-358,774, Tarceva(TM)). The EGFR family members are distinguished from all other known receptor tyrosine kinases in possessing constitutive kinase activity without a phosphorylation event within their kinase domains. Despite its lack of phosphorylation, we find that the EGFRK activation loop adopts a conformation similar to that of the phosphorylated active form of the kinase domain from the insulin receptor. Surprisingly, key residues of a putative dimerization motif lying between the EGFRK domain and carboxyl-terminal substrate docking sites are found in close contact with the kinase domain. Significant intermolecular contacts involving the carboxyl-terminal tail are discussed with respect to receptor oligomerization.     

Ligand-induced structural transitions in ErbB receptor extracellular domains

Crystallographic studies showed that epidermal growth factor (EGF) receptor activation involves major domain rearrangements. Without bound ligand, the extracellular region of the receptor (sEGFR) adopts a "tethered" configuration with its dimerization site occluded by apparently autoinhibitory intramolecular interactions. Ligand binding causes the receptor to become "extended," breaking the tether and exposing the dimerization site. Using small-angle X-ray scattering (SAXS), we confirm that the tethered and extended conformations are also adopted in solution, and we describe low-resolution molecular envelopes for an intact sEGFR dimer. We also use SAXS to monitor directly the transition from a tethered to extended configuration in the monomeric extracellular regions of ErbB3 and a dimerization-defective EGFR mutant. Finally, we show that mutating every intramolecular tether interaction in sEGFR does not greatly alter its conformation. These findings explain why tether mutants fail to activate EGF receptor and provide new insight into regulation of ErbB receptor conformation.     

Antibody-induced dimerization activates the epidermal growth factor receptor tyrosine kinase

The relationship between epidermal growth factor receptor (EGF-R) protein tyrosine kinase activation and ligand-induced receptor dimerization was investigated using several bivalent anti-EGF-R antibodies directed against various receptor epitopes. In A431 membrane preparations and permeabilized cells, all antibodies were able to activate the EGF-R tyrosine kinase, as measured by EGF-R autophosphorylation and phosphorylation of other substrates on tyrosine residues. EGF-R tyrosine kinase activation correlated strongly with the induction of EGF-R dimerization. (i) Both processes specifically occurred in a narrow antibody concentration range; (ii) both processes required the presence of detergent; and (iii) both processes depended on antibody bivalence since monovalent Fab fragments were inactive yet regained full activity after cross-linking by a second bivalent antibody. These data demonstrate that antibody bivalence is essential and sufficient for EGF-R activation and that activation occurs regardless of the EGF-R epitope recognized. Finally, EGF-R dimerization was shown not to depend on receptor autophosphorylation since it still occurred in the absence of ATP. Also, partial inhibition of the tyrosine kinase activity by the specific EGF-R tyrosine kinase inhibitor tyrphostin AG 213 did not affect formation of EGF-R dimers. Taken together these results demonstrate that induction of EGF-R dimerization is sufficient and in case of antibody action, essential, for activation of the EGF-R tyrosine kinase and thus provide strong support for an intermolecular mechanism of EGF-R tyrosine kinase activation.