Role of extracellular subdomains of p185c-neu and the epidermal growth factor receptor in ligand-independent association and transactivation

Treadmill Running Regulates Adult Neurogenesis, Spatial and Non-spatial Learning, Parvalbumin Neuron Activity by ErbB4 Signaling

Exercise can promote adult neurogenesis and improve symptoms associated with schizophrenia and other mental disorders via parvalbumin (PV)-positive GABAergic interneurons in the dentate gyrus ErbB4 is the receptor of neurotrophic factor neuregulin 1, expressed mostly in PV-positive interneurons. Whether ErbB4 in PV-positive neurons mediates the beneficial effect of exercise and adult neurogenesis on mental disorder needs to be further investigation. Here, we first conducted a four-week study on the effects of AG1478, an ErbB4 inhibitor, on memory and neurogenesis. AG1478 significantly impaired the performance in several memory tasks, including the T-maze, Morris water maze, and contextual fear conditioning, downregulated the expression of total ErbB4 (T-ErbB4) and the ratio of phosphate-ErbB4 (p-ErbB4) to T-ErbB4, and associated with neurogenesis impairment. Interestingly, AG1478 also appeared to decrease intracellular calcium levels in PV neurons, which could be reversed by exercise. These results suggest exercise may regulate adult neurogenesis and PV neuron activity through ErbB4 signaling. Overall, these findings provide further evidence of the importance of exercise for neurogenesis and suggest that targeting ErbB4 may be a promising strategy for improving memory and other cognitive functions in individuals with mental disorders.

The HER2 S310F Mutant Can Form an Active Heterodimer with the EGFR, Which Can Be Inhibited by Cetuximab but Not by Trastuzumab as well as Pertuzumab

G309 or S310 mutations on the HER2 extracellular domain II induce receptor activation. Clinically, S310F is most frequent among HER2 extracellular domain mutations and patients with the S310F mutation without HER2 amplification responded to trastuzumab with or without the pertuzumab combination. However, the ability of S310F mutant to form homodimers or heterodimers with wild-type HER2 and other HER receptors, or their reactivity to trastuzumab and pertuzumab treatments, has not been reported. We overexpressed S310F as well as G309A, G309E and S310Y HER2 mutants and tested their reactivity to trastuzumab and pertuzumab. All mutants reacted to trastuzumab, but S310F mutant did not react to pertuzumab along with S310Y or G309E mutants. Thereafter, we tested the effects of trastuzumab and pertuzumab on 5637 cell line expressing both wild-type HER2 and S310F mutant. The ligand-independent HER2 homodimerization blocking antibody, trastuzumab, did not inhibit the activation of the HER2 receptor, suggesting that the S310F HER2 mutant did not form homodimers or heterodimers with wild-type HER2. Because 5637 cells overexpressed the EGFR, the effects of cetuximab and gefitinib were determined, and both inhibited the activation of HER2 and significantly reduced cell growth. Because pertuzumab did not inhibit the phosphorylation of HER2 while it bound to wild-type HER2, EGFR-mediated phosphorylation is expected to occur on the S310F mutant. To confirm whether the S310F mutant HER2 retained its affinity to the EGFR, single molecule interaction analyses using TIRF microscopy were performed, which showed that S310F mutant successfully formed complexes with EGFR. In conclusion, HER2 S310F mutant can form an active heterodimer with the EGFR and it can be inhibited by cetuximab, but not by trastuzumab in combination with pertuzumab.

NK3R Mediates the EGF-Induced SLα Secretion and mRNA Expression in Grass Carp Pituitary

Epidermal growth factor (EGF) is a potent regulator of cell function in many cell types. In mammals, the EGF/EGFR system played an important role in both pituitary physiology and pathology. However, it is not clear about the pituitary action of EGF in lower vertebrates. In this study, using grass carp as a model, we found that EGF could stimulate NK3R mRNA and protein expression through pituitary ErbB1 and ErbB2 coupled to MEK/ERK and PI3K/Akt/mTOR pathways. In addition, EGF could also induce pituitary somatolactin α (SLα) secretion and mRNA expression in a dose- and time-dependent manner in vivo and in vitro. The stimulatory actions of EGF on SLα mRNA expression were also mediated by PI3K/Akt/mTOR and MEK/ERK pathways coupled to ErbB1 and ErbB2 activation. Our previous study has reported that neurokinin B (NKB) could also induce SLα secretion and mRNA expression in carp pituitary cells. In the present study, interestingly, we found that EGF could significantly enhance NKB-induced SLα mRNA expression. Further studies found that NK3R antagonist SB222200 could block EGF-induced SLα mRNA expression, indicating an NK3R requirement. Furthermore, cAMP/PKA inhibitors and PLC/PKC inhibitors could both abolish EGF- and EGF+NKB-induced SLα mRNA expression, which further supported that EGF-induced SLα mRNA expression is NK3R dependent.

The Tumor-Suppressor Protein OPCML Potentiates Anti-EGFR- and Anti-HER2-Targeted Therapy in HER2-Positive Ovarian and Breast Cancer

Opioid-binding protein/cell adhesion molecule-like (OPCML) is a tumor-suppressor gene that is frequently inactivated in ovarian cancer and many other cancers by somatic methylation. We have previously shown that OPCML exerts its suppressor function by negatively regulating a spectrum of receptor tyrosine kinases (RTK), such as ErbB2/HER2, FGFR1, and EphA2, thus attenuating their related downstream signaling. The physical interaction of OPCML with this defined group of RTKs is a prerequisite for their downregulation. Overexpression/gene amplification of EGFR and HER2 is a frequent event in multiple cancers, including ovarian and breast cancers. Molecular therapeutics against EGFR/HER2 or EGFR only, such as lapatinib and erlotinib, respectively, were developed to target these receptors, but resistance often occurs in relapsing cancers. Here we show that, though OPCML interacts only with HER2 and not with EGFR, the interaction of OPCML with HER2 disrupts the formation of the HER2-EGFR heterodimer, and this translates into a better response to both lapatinib and erlotinib in HER2-expressing ovarian and breast cancer cell lines. Also, we show that high OPCML expression is associated with better response to lapatinib therapy in breast cancer patients and better survival in HER2-overexpressing ovarian cancer patients, suggesting that OPCML co-therapy could be a valuable sensitizing approach to RTK inhibitors. Mol Cancer Ther; 16(10); 2246-56. ©2017 AACR.

Targeting EGFR/HER2 heterodimerization with a novel anti-HER2 domain II/III antibody

HER2, a ligand-free tyrosine kinase receptor of the HER family, is frequently overexpressed in breast cancer. The anti-HER2 antibody trastuzumab has shown significant clinical benefits in metastatic breast cancer. Despite the effectiveness of trastuzumab, its efficacy remains variable and often modest. Thus, there is an urgent need to improve ErbB2-targeting therapy. Here, we describe a novel anti-HER2 antibody, 7C3, which was developed using hybridoma technique. Structural analysis confirms that the epitope of this antibody is in domain II/III of HER2. Moreover, a structural conformation change was observed in HER2 in complex with 7C3. Interestingly, this novel anti-HER2 antibody exhibits efficacy in blocking HER2/EGFR heterodimerization and signaling. The results highlight the different function role of HER2 domains and the unique potential of 7C3 to inhibit the HER2/EGFR heterodimer, which may complement current anti-HER2 treatments.

A monoclonal antibody targeting ErbB2 domain III inhibits ErbB2 signaling and suppresses the growth of ErbB2-overexpressing breast tumors

The anti-ErbB2 antibodies trastuzumab and pertuzumab in combination have recently been approved for the treatment of patients with ErbB2-positive metastatic breast cancer. Pertuzumab, which binds to ErbB2 near the center of domain II, and trastuzumab, which binds to the juxtamembrane region of ErbB2 domain IV, directly interfere with domain II- and domain IV-mediated heterodimerization contacts, respectively. In this study, we report a novel anti-ErbB2 antibody, 3E10, which binds to an epitope in domain III that appears to be located opposite to the dimerization interfaces in domain II and domain IV of ErbB2. Our data show that the 3E10 antibody inhibits ErbB2 heterodimerization via a mechanism that strikingly differs from trastuzumab and pertuzumab. It could be speculated that the 3E10 antibody may affect ErbB2 heterodimerization by causing major conformational changes of ErbB2. Furthermore, 3E10 provides synergistic inhibition of ErbB2 heterodimerization and signaling in combination with either trastuzumab or pertuzumab. The combination of these three anti-ErbB2 antibodies that have complementary mechanisms of action appears to be an extremely potent ErbB2 heterodimerization blocker. Compared with trastuzumab plus pertuzumab, the combination of trastuzumab, pertuzumab and 3E10 provides a more potent blockade of ErbB2 signaling. Consistent with this, trastuzumab plus pertuzumab plus 3E10 results in greater in vitro and in vivo antitumor activity in ErbB2-overexpressing breast tumor models, suggesting its potential use for treating ErbB2-overexpressing breast cancer.

Novel Peptidomimetics for Inhibition of HER2:HER3 Heterodimerization in HER2-Positive Breast Cancer

The current approach to treating HER2-overexpressed breast cancer is the use of monoclonal antibodies or a combination of antibodies with traditional chemotherapeutic agents or kinase inhibitors. Our approach is to target clinically validated HER2 domain IV with peptidomimetics and inhibit the protein-protein interactions (PPI) of HERs. Unlike antibodies, peptidomimetics have advantages in terms of stability, modification, and molecular size. We have designed peptidomimetics (compounds 5 and 9) that bind to HER2 domain IV, inhibit protein-protein interactions, and decrease cell viability in breast cancer cells with HER2 overexpression. We have shown, using enzyme fragment complementation and proximity ligation assays, that peptidomimetics inhibit the PPI of HER2:HER3. Compounds 5 and 9 suppressed the tumor growth in a xenograft mouse model. Furthermore, we have shown that these compounds inhibit PPI of HER2:HER3 and phosphorylation of HER2 as compared to control in tissue samples derived from in vivo studies. The stability of the compounds was also investigated in mouse serum, and the compounds exhibited stability with a half-life of up to 3 h. These results suggest that the novel peptidomimetics we have developed target the extracellular domain of HER2 protein and inhibit HER2:HER3 interaction, providing a novel method to treat HER2-positive cancer.

Design, synthesis and characterization of peptidomimetic conjugate of BODIPY targeting HER2 protein extracellular domain

Among the EGFRs, HER2 is a major heterodimer partner and also has important implications in the formation of particular tumors. Interaction of HER2 protein with other EGFR proteins can be modulated by small molecule ligands and, hence, these protein-protein interactions play a key role in biochemical reactions related to control of cell growth. A peptidomimetic (compound 5-1) that binds to HER2 protein extracellular domain and inhibits protein-protein interactions of EGFRs was conjugated with BODIPY (4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene). Conjugation of BODIPY to the peptidomimetic was investigated by different approaches. The conjugate was characterized for its ability to bind to HER2 overexpressing SKBR-3 and BT-474 cells. Furthermore, cellular uptake of conjugate of BODIPY was studied in the presence of membrane tracker and Lyso tracker using confocal microscopy. Our results suggested that fluorescently labeled compound 5-7 binds to the extracellular domain and stays in the membrane for nearly 24 h. After 24 h there is an indication of internalization of the conjugate. Inhibition of protein-protein interaction and downstream signaling effect of compound 5-1 was also studied by proximity ligation assay and Western blot analysis. Results suggested that compound 5-1 inhibit protein-protein interactions of HER2-HER3 and phosphorylation of HER2 in a time-dependent manner.

Inhibition of protein-protein interaction of HER2-EGFR and HER2-HER3 by a rationally designed peptidomimetic

Protein-protein interactions (PPI) play a crucial role in many biological processes and modulation of PPI using small molecules to target hot spots has therapeutic value. As a model system we will use PPI of human epidermal growth factor receptors (EGFRs). Among the four EGFRs, EGFR-HER2 and HER2-HER3 are well known in cancer. We have designed a small molecule that is targeted to modulate HER2-mediated signaling. Our approach is novel because the small molecule designed disrupts dimerization not only of EGFR-HER2, but also of HER2-HER3. In the present study we have shown, using surface plasmon resonance analysis, that a peptidomimetic, compound 5, binds specifically to HER2 protein extracellular domain and disrupts the dimerization of EGFRs. To evaluate the effect of compound 5 on HER2 signaling in vitro, Western blot and PathHunter assays were used. Results indicated that compound 5 inhibits the phosphorylation of HER2 kinase domain and inhibits the heterodimerization in a dose-dependent manner. Molecular modeling methods were used to model the PPI of HER2-HER3 heterodimer.

A conformationally constrained peptidomimetic binds to the extracellular region of HER2 protein

Human epidermal growth factor receptor 2 (HER2) is a member of the human epidermal growth factor receptor kinases (other members include EGFR or HER1, HER3, and HER4) that are involved in signaling cascades for cell growth and differentiation. It is well established that HER2-mediated heterodimerization has important implications in cancer. Deregulation of signaling pathways and overexpression of HER2 is known to occur in cancer cells, indicating a role of HER2 in tumorigenesis. Therefore, blocking HER2-mediated signaling has potential therapeutic value. We have designed several peptidomimetics to inhibit HER2-mediated signaling for cell growth. One of the compounds (HERP5, Arg-beta Naph-Phe) exhibited antiproliferative activity with IC(50) values in the micromolar-to-nanomolar range in breast cancer cell lines. Binding of fluorescently labeled HERP5 to HER2 protein was evaluated by fluorescence assay, microscopy, and circular dichroism spectroscopy. Results indicated that HERP5 binds to the extracellular region of the HER2 protein. Structure of the peptidomimetic HERP5 was studied by NMR and molecular dynamics simulations. Based on these results a model was proposed for HER2-EGFR dimerization and possible blocking by HERP5 peptidomimetic using a protein-protein docking method.

Mechanistic and signaling analysis of Muc4-ErbB2 signaling module: new insights into the mechanism of ligand-independent ErbB2 activity

The membrane mucin Muc4 is aberrantly expressed in numerous epithelial carcinomas and is currently used as a cancer diagnostic and prognostic tool. Muc4 can also potentiate signal transduction by modulating differential ErbB2 phosphorylation in the absence and in the presence of the ErbB3 soluble ligand heregulin (HRG-beta1). These features of Muc4 suggest that Muc4 is not merely a cancer marker, but an oncogenic factor with a unique-binding/activation relationship with the receptor ErbB2. In the present study, we examined the signaling mechanisms that are associated with the Muc4-ErbB2 module by analyzing ErbB2 differential signaling in response to Muc4 expression. Our study was carried out in the A375 human melanoma and BT-474 breast cancer cell lines as our model systems. Quantitative and comparative signaling modulations were evaluated by immunoblot using phospho-specific antibodies, and densitometry analysis. Signaling complex components were identified by chemical cross-linking, fractionation by gel filtration, immunoprecipitation, and immunoblotting. Activated downstream signaling pathways were analyzed by an antibody microarray screen and immunoblot analyses. Our results indicate that Muc4 modulates ErbB2 signaling potential significantly by stabilizing and directly interacting with the ErbB2-ErbB3 heterodimer. Further analyses indicate that Muc4 promotes ErbB2 autocatalysis, but it has no effect on ErbB3 phosphorylation, although the chemical cross-linking data indicated that the signaling module is composed of Muc4, ErbB2, and ErbB3. Our microarray analysis indicates that Muc4 expression promotes cell migration by increasing the phosphorylation of the focal adhesion kinase and also through an increase in the levels of beta-catenin.

Estrogen receptor-alpha mediates the epidermal growth factor-stimulated prolactin expression and release in lactotrophs

Epidermal growth factor (EGF) is a potent regulator of cell function in many cell types. EGF-receptor (EGFR/ErbB1)-activated Erk1/2 has been reported to activate estrogen receptor (ER) in an estrogen (E2)-independent manner. In the pituitary lactotrophs, both EGF and E2 stimulate prolactin (PRL) release, but the nature of interactions between ErbB and ERalpha signaling is unknown. Our objectives were to 1) characterize EGF-induced PRL release, 2) determine whether this effect requires ERalpha, and 3) determine the molecular basis for cross talk between ErbB and ERalpha signaling pathways. Using GH3 cells, a rat lactotroph cell line, we report that EGF stimulates PRL gene expression and release in a dose- and time-dependent manner. EGF caused a rapid and robust activation of Erk1/2 via ErbB1 and induced phosphorylation of S118 on ERalpha in an Erk1/2-dependent manner. The global antiestrogen ICI 182780 and the ERalpha-specific antagonist 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylet hoxy)phenol]-1H-pyrazole dihydrochloride (MPP), but not the ERbeta-specific antagonist 4-[2-Phenyl-5,7-bis(trifluoromethyl) pyrazolo[1,5-a]pyrimidin-3-yl]phenol (PHTPP), blocked the EGF-induced PRL release, indicating an ERalpha requirement. This was further supported by using ERalpha knockdown by small interfering RNA. Because the antiestrogens did not block EGF-induced Mek-1 or Erk1/2 phosphorylation, ERalpha is placed downstream from the ErbB1-activated Erk1/2. These results provide the first evidence that ErbB1-induced PRL release is ERalpha dependent.

All EGF(ErbB) receptors have preformed homo- and heterodimeric structures in living cells

The epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases, also known as ErbB or HER, plays crucial roles in the development of multicellular organisms. Mutations and over-expression of the ErbB receptors have been implicated in a variety of human cancers. It is widely thought that the ErbB receptors are located in the plasma membrane, and that ligand binding to the monomeric form of the receptors induces its dimeric form for activation. However, it still remains controversial whether prior to ligand binding the receptors exist as monomers or dimers on the cell surface. Using bimolecular fluorescence complementation (BiFC) assays in the present study, we demonstrate that in the absence of bound ligand, all the ErbB family members have preformed, yet inactive, homo- and heterodimers on the cell surface, except for ErbB3 homodimers and heterodimers with cleavable ErbB4, which exist primarily in the nucleus. BiFC assays of the dimerization have also suggested that the ligand-independent dimerization of the ErbB receptors occurs in the endoplasmic reticulum (ER) before newly synthesized receptor molecules reach the cell surface. Based on BiFC and mammalian two-hybrid assays, it is apparent that the intracellular domains of the receptors are responsible for the spontaneous dimer formation. These provide new insights into an understanding of transmembrane signal transduction mediated by the ErbB family members, and are relevant to the development of anti-cancer drugs.

Gefitinib-sensitive EGFR lacking residues 746-750 exhibits hypophosphorylation at tyrosine residue 1045, hypoubiquitination, and impaired endocytosis

Gefitinib-sensitive nonsmall cell lung cancers (NSCLC) are characterized by somatic mutations in the kinase domain of epidermal growth factor receptor (EGFR). The mutant EGFR forms are reported to mediate characteristic signal transduction pathways that are different from those mediated by the wild-type EGFR and are involved in transformation in vivo. We have examined signal transduction pathways initiated from a frequently identified gefitinib-sensitizing mutant EGFR lacking residues 746-750 by employing a mouse fibroblast cell line that is free of endogenous EGFR and transiently transfected COS-7 cells. Upon EGF stimulation, the deletion-mutant EGFR mediated prolonged downstream signals. The analysis of the phosphotyrosine patterns of the receptor revealed that the deletion-mutant EGFR lacked phosphorylation at tyrosine residue 1045, which is the major binding site of Cbl. The EGF-induced endocytosis of the deletion-mutant EGFR was impaired. The ubiquitination and downregulation of the deletion-mutant EGFR were also reduced. On the other hand, another mutant, EGFR, possessing a L858R substitution, exhibited phosphorylation at 1045 and its downstream signalings were not prolonged. These data suggest that the signal transduction pathways initiated from these mutant forms are different, and that impaired endocytosis might be responsible for the prolonged signals mediated by the deletion-mutant EGFR.

Targeted antireceptor therapy with monoclonal antibodies leads to the formation of inactivated tetrameric forms of ErbB receptors

mAbs capable of disabling heterodimeric kinase complexes of the epidermal growth factor receptor (EGFR) and human EGFR type 2/neu have therapeutic relevance to various human cancers. In this study, we demonstrate that in addition to the dimer, EGFR and human EGFR type 2 can associate as homo- and heterotetramers. EGF-induced phosphorylation of the tetramers was significantly lower than that of the dimers, indicating that the tetrameric receptor complexes have impaired signaling activity. Targeting v-erb-b2 erythroblastic leukemia viral oncogene homolog (erbB) receptors with mAbs promoted erbB tetrameric assembly, suggesting that a component of the antitumor activity may be mediated by the ability of Abs to shift the equilibrium from active dimeric to impaired tetrameric receptor complex states. This study suggests a novel therapeutic approach to disable signaling of erbB and potentially other receptors in tumors by biologic agents capable of inducing receptor tetramerization.

The extracellular domain of p185(c-neu) induces density-dependent inhibition of cell growth in malignant mesothelioma cells and reduces growth of mesothelioma in vivo

EGFR is involved in the density-dependent inhibition of cell growth, while coexpression of EGFR with erbB2 can render normal cells transformed. In this study, we have examined the effect of a species of p185 that contains the transmembrane domain and the extracellular domain of p185(c-neu), on growth properties of a human malignant mesothelioma cell line that coexpresses EGFR and erbB2. The ectodomain form of p185(c-neu) enhanced density-dependent inhibition of cell growth and we found that p21 induction appeared to be responsible for this inhibitory effect. Previously, the extracellular domain species was shown to suppress the transforming abilities of EGFR and p185(c-neu/erbB2) in a dominant-negative manner. The ability of this subdomain to affect tumor growth is significant, as it reduced in vivo tumor growth. Unexpectedly, we found that the domain did not abrogate all of EGFR functions. We noted that EGFR-induced density-dependent inhibition of cell growth was retained. Tyrosine kinase inhibitors of EGFR did not cause density-dependent inhibition of cell growth of malignant mesothelioma cells. Therefore, simultaneously inhibiting the malignant phenotype and inducing density-dependent inhibition of cell growth in malignant mesothelioma cells by the extracellular domain of p185(c-neu) may represent an important therapeutic advance.

Differential localization of ErbB receptor ensembles influences their signaling in hippocampal neurons

Our studies indicate that ErbB complexes participate in both survival and synaptic plasticity signals of hippocampal neurons but in a manner that depends on the subcellular localization of the receptor ensembles. Using dissociated hippocampal cultures, we found that neurons, rather than glial cells, are the primary targets of ErbB receptor ligands such as epidermal growth factor and heregulin. Further investigation demonstrated that ErbB receptors distribute differentially in hippocampal neurons with the epidermal growth factor receptor confined to neural cell bodies and the p185(c-neu) and ErbB4 receptors distributed to both neural soma and neurites. Activation of ErbB receptor and downstream signaling molecules were observed in neurites only after heregulin stimulation. The receptor complex which mediated neurite located signals was the p185(c-neu)/ErbB4 heterodimer. Colocalization of p185(c-neu), but not epidermal growth factor receptor, with postsynaptic density protein 95 suggests that the heregulin signaling contributes to synapse specific activities. However, the epidermal growth factor receptor complex mediates physiological survival signals, as neuronal survival was enhanced by epidermal growth factor, rather than heregulin. Collectively, these studies indicate that different ErbB ensembles localize to different locations on the neuron to mediate distinct signals and functions.

Herpes Simplex Virus 1 Amplicon Vector-Mediated siRNA Targeting Epidermal Growth Factor Receptor Inhibits Growth of Human Glioma Cells in Vivo

In primary glioblastomas and other tumor types, the epidermal growth factor receptor (EGFR) is frequently observed with alterations, such as amplification, structural rearrangements, or overexpression of the gene, suggesting an important role in glial tumorigenesis and progression. In this study, we investigated whether posttranscriptional gene silencing by vector-mediated RNAi to inhibit EGFR expression can reduce the growth of cultured human gli36 glioma cells. To "knock down" EGFR expression, we have created HSV-1-based amplicons that contain the RNA polymerase III-dependent H1 promoter to express double-stranded hairpin RNA directed against EGFR at two different locations (pHSVsiEGFR I and pHSVsiEGFR II). We demonstrate that both pHSVsiEGFR I and pHSVsiEGFR II mediated knock-down of transiently transfected full-length EGFR or endogenous EGFR in a dose-dependent manner. The knock-down of EGFR resulted in the growth inhibition of human glioblastoma (gli36-luc) cells both in culture and in athymic mice in vivo. Cell cycle analysis and annexin V staining revealed that siRNA-mediated suppression of EGFR induced apoptosis. Overall HSV-1 amplicons can mediate efficient and specific posttranscriptional gene silencing.

Molecular modeling of nearly full-length ErbB2 receptor

Members of the epidermal growth factor receptor family play important roles in various cellular processes, both in physiological and in pathological conditions. Dimerization and autophosphorylation of these receptor tyrosine kinases are key events of signal transduction. Details of the molecular events of the signaling are not entirely known. To facilitate the understanding of receptor structure and function at the molecular level, a molecular model was built for the nearly full-length ErbB2 dimer. Modeling was based on the x-ray or nuclear-magnetic resonance structures of extracellular, transmembrane, and intracellular domains. The extracellular domain was positioned above the cell membrane based on the distance determined from experimentally measured fluorescence resonance energy transfer. Favorable dimerization interactions are predicted for the extracellular, transmembrane, and protein kinase domains in the model of a nearly full-length dimer of ErbB2, which may act in a coordinated fashion in ErbB2 homodimerization, and also in heterodimers of ErbB2 with other members of the ErbB family.