A Discrete Three-amino Acid Segment (LVI) at the C-terminal End of Kinase-impaired ErbB3 Is Required for Transactivation of ErbB2

This is the authors' abstract. We don't add key points for this paper.

ErbB3 is unique among other members of the receptor tyrosine kinase family of growth factor receptors in that its kinase domain is enzymatically impaired. This renders it incapable of transducing a signal in response to ligand binding. However, in conjunction with ErbB2, ErbB3 is a potent mediator of signaling by the growth factor heregulin. Heregulin binding to ErbB3 induces formation of a heterodimeric complex with ErbB2, and this results in transactivation of the ErbB2 kinase. Although interaction between the extracellular domains of these receptors is an essential part of this process, it was not clear whether interaction between the cytoplasmic domains is also necessary for transactivation. By examining the abilities of a series of cytoplasmic domain mutants of ErbB3 to activate ErbB2, we have found a discrete sequence of three amino acid residues (LVI), located at the carboxyl-terminal end of the impaired ErbB3 kinase region, that is obligatory for transactivation. We conclude that formation of a functional ErbB2-ErbB3 signaling complex requires the presence of a specific structural feature within the ErbB3 cytoplasmic domain and suggest that ErbB2 transactivation results from a physical interaction between the cytoplasmic domains of these receptors. ErbB3 is unique among other members of the receptor tyrosine kinase family of growth factor receptors in that its kinase domain is enzymatically impaired. This renders it incapable of transducing a signal in response to ligand binding. However, in conjunction with ErbB2, ErbB3 is a potent mediator of signaling by the growth factor heregulin. Heregulin binding to ErbB3 induces formation of a heterodimeric complex with ErbB2, and this results in transactivation of the ErbB2 kinase. Although interaction between the extracellular domains of these receptors is an essential part of this process, it was not clear whether interaction between the cytoplasmic domains is also necessary for transactivation. By examining the abilities of a series of cytoplasmic domain mutants of ErbB3 to activate ErbB2, we have found a discrete sequence of three amino acid residues (LVI), located at the carboxyl-terminal end of the impaired ErbB3 kinase region, that is obligatory for transactivation. We conclude that formation of a functional ErbB2-ErbB3 signaling complex requires the presence of a specific structural feature within the ErbB3 cytoplasmic domain and suggest that ErbB2 transactivation results from a physical interaction between the cytoplasmic domains of these receptors. Receptor tyrosine kinases play a pivotal role in the transduction of extracellular signals into the cells. The binding of cognate growth factors to these cell-surface receptors results in receptor oligomerization and activation of the intrinsic kinase activity (1van der Geer P. Hunter T. Lindberg R.A. Annu. Rev. Cell Biol. 1994; 10: 251-337Crossref PubMed Scopus (1261) Google Scholar, 2Heldin C.H. Ostman A. Cytokine Growth Factor Rev. 1996; 7: 3-10Crossref PubMed Scopus (74) Google Scholar). This leads to receptor phosphorylation and triggers a cascade of intracellular signaling events that ultimately elicit a variety of cellular responses such as proliferation, differentiation, survival, or migration. An extensively characterized subgroup of this receptor superfamily is the ErbB group of receptors, also known as the class I receptor tyrosine kinases. Members of this group include the epidermal growth factor receptor (EGFR 1The abbreviations used are: EGFR, epidermal growth factor receptor; EGF, epidermal growth factor; HRG, heregulin; CHAPS, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. or ErbB1), ErbB2 (also termed HER2 or Neu), ErbB3 (HER3), and ErbB4 (HER4). EGFR binds several distinct ligands including EGF and transforming growth factor-α (3Groenen L.C. Nice E.C. Burgess A.W. Growth Factors. 1994; 11: 235-257Crossref PubMed Scopus (216) Google Scholar). ErbB3 and ErbB4 bind isoforms of the heregulin family (also designated neuregulin or Neu differentiation factor) (4Carraway III, K.L. Sliwkowski M.X. Akita R. Platko J.V. Guy P.M. Nuijens A. Diamonti A.J. Vandlen R.L. Cantley L.C. Cerione R.A. J. Biol. Chem. 1994; 269: 14303-14306Abstract Full Text PDF PubMed Google Scholar, 5Plowman G.D. Culouscou J.M. Whitney G.S. Green J.M. Carlton G.W. Foy L. Neubauer M.G. Shoyab M. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 1746-17450Crossref PubMed Scopus (700) Google Scholar). A ligand that directly binds to ErbB2 has not been identified. Nevertheless, ErbB2 plays an important role in signaling. ErbB2 is transactivated by heterodimerization with ligand-occupied EGFR, ErbB3, or ErbB4 (6Wada T. Myers J.N. Kokai Y. Brown V.I. Hamuro J. LeVea C.M. Greene M.I. Oncogene. 1990; 5: 489-495PubMed Google Scholar, 7Sliwkowski M.X. Schaefer G. Akita R.W. Lofgren J.A. Fitzpatrick V.D. Nuijens A. Fendly B.M. Cerione R.A. Vandlen R.L. Carraway III., K.L. J. Biol. Chem. 1994; 269: 14661-14665Abstract Full Text PDF PubMed Google Scholar, 8Tzahar E. Levkowitz G. Karunagaran D. Yi L. Peles E. Lavi S. Chang D. Liu N. Yayon A. Wen D. Yarden Y. J. Biol. Chem. 1994; 269: 25226-25233Abstract Full Text PDF PubMed Google Scholar, 9Karunagaran D. Tzahar E. Beerli R.R. Chen X. Graus-Porta D. Ratzkin B.J. Seger R. Hynes N.E. Yarden Y. EMBO J. 1996; 15: 254-264Crossref PubMed Scopus (592) Google Scholar). The extensive interreceptor associations that occur in the ErbB family serve to increase the repertoire of cellular responses to growth factor stimulation and to fine-tune growth factor signaling. At least 10 different homo- and heteromeric combinations of ErbB proteins have been reported (10Riese D.J.I. van Raaij T.M. Plowman G.D. Andrews G.C. Stern D.F. Mol. Cell. Biol. 1995; 15: 5770-5776Crossref PubMed Scopus (349) Google Scholar, 11Pinkas-Kramarski R. Soussan L. Waterman H. Levkowitz G. Alroy I. Klapper L. Lavi S. Seger R. Ratzkin B.J. Sela M. Yarden Y. EMBO J. 1996; 15: 2452-2467Crossref PubMed Scopus (704) Google Scholar). However, these combinations are not equally favorable. The interreceptor interactions are hierarchically organized, where ErbB2 is the preferred heteromeric partner, and it favors interaction with ErbB3 (12Tzahar E. Waterman H. Chen X. Levkowitz G. Karunagaran D. Lavi S. Ratzkin B.J. Yarden Y. Mol. Cell. Biol. 1996; 16: 5276-5287Crossref PubMed Scopus (881) Google Scholar, 13Graus-Porta D. Beerli R.R. Daly J.M. Hynes N.E. EMBO J. 1997; 16: 1647-1655Crossref PubMed Scopus (1317) Google Scholar). Cross-talk between ErbB2 and ErbB3 is especially important as the kinase of ErbB3 is dysfunctional. The impaired kinase activity has been demonstrated in several systems. It was initially reported in insect cells expressing ErbB3 (14Guy P.M. Platko J.V. Cantley L.C. Cerione R.A. Carraway III., K.L. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 8132-8136Crossref PubMed Scopus (604) Google Scholar), later by demonstrating the lack of any biological activity in cells expressing ErbB3 alone (10Riese D.J.I. van Raaij T.M. Plowman G.D. Andrews G.C. Stern D.F. Mol. Cell. Biol. 1995; 15: 5770-5776Crossref PubMed Scopus (349) Google Scholar, 11Pinkas-Kramarski R. Soussan L. Waterman H. Levkowitz G. Alroy I. Klapper L. Lavi S. Seger R. Ratzkin B.J. Sela M. Yarden Y. EMBO J. 1996; 15: 2452-2467Crossref PubMed Scopus (704) Google Scholar), and recently by biochemical analysis of the purified kinase domain (15Sierke S.L. Cheng K. Kim H.H. Koland J.G. Biochem. J. 1997; 322: 757-763Crossref PubMed Scopus (133) Google Scholar). Alterations of four amino acid residues in the kinase region that are otherwise conserved among all protein tyrosine kinases (16Hanks S.K. Quinn A.M. Hunter T. Science. 1988; 241: 42-52Crossref PubMed Scopus (4117) Google Scholar) may account for the lack of catalytic activity. ErbB2, however, is characterized by a constitutively active kinase (17Lonardo F. Di Marco E. King C.R. Pierce J.H. Segatto O. Aaronson S.A. Di Fiore P.P. New Biol. 1990; 2: 992-1003PubMed Google Scholar). The physiological significance of this heteromeric complex is emphasized by the fact that the presence of ErbB2 in ErbB3-expressing cells significantly enhances the transformation ability (18Alimandi M. Romano A. Curia M.C. Muraro R. Fedi P. Aaronson S.A. Di Fiore P.P. Kraus M.H. Oncogene. 1995; 10: 1813-1821PubMed Google Scholar, 19Wallasch C. Weiss F.U. Niederfellner G. Jallal B. Issing W. Ullrich A. EMBO J. 1995; 14: 4267-4275Crossref PubMed Scopus (364) Google Scholar). Inhibition of ErbB2 and ErbB3 complex formation abolishes HRG-mediated signaling (9Karunagaran D. Tzahar E. Beerli R.R. Chen X. Graus-Porta D. Ratzkin B.J. Seger R. Hynes N.E. Yarden Y. EMBO J. 1996; 15: 254-264Crossref PubMed Scopus (592) Google Scholar, 13Graus-Porta D. Beerli R.R. Daly J.M. Hynes N.E. EMBO J. 1997; 16: 1647-1655Crossref PubMed Scopus (1317) Google Scholar, 20Lewis G.D. Lofgren J.A. McMurtrey A.E. Nuijens A. Fendly B.M. Bauer K.D. Sliwkowski M.X. Cancer Res. 1996; 56: 1457-1465PubMed Google Scholar). Additionally, active ErbB2-ErbB3 receptor complexes have been seen in several mammary tumor cell lines, indicating the relevance of this heteromeric receptor aggregate in human neoplasia (18Alimandi M. Romano A. Curia M.C. Muraro R. Fedi P. Aaronson S.A. Di Fiore P.P. Kraus M.H. Oncogene. 1995; 10: 1813-1821PubMed Google Scholar, 21Schaefer G. Fitzpatrick V.D. Sliwkowski M.X. Oncogene. 1997; 15: 1385-1394Crossref PubMed Scopus (92) Google Scholar). HRG binds with low affinity to kinase-inactive ErbB3. Recruitment of ErbB2 into the HRG-ErbB3 complex leads to the formation of a high affinity HRG-binding receptor, which is capable of generating a tyrosine phosphorylation signal due to the kinase activity of ErbB2 (7Sliwkowski M.X. Schaefer G. Akita R.W. Lofgren J.A. Fitzpatrick V.D. Nuijens A. Fendly B.M. Cerione R.A. Vandlen R.L. Carraway III., K.L. J. Biol. Chem. 1994; 269: 14661-14665Abstract Full Text PDF PubMed Google Scholar). Studies on the isolated extracellular domain of ErbB3 show that ligand binding is exclusively mediated by the extracellular region of ErbB3 (22Horan T. Wen J. Arakawa T. Liu N. Brankow D. Hu S. Ratzkin B. Philo J.S. J. Biol. Chem. 1995; 270: 24604-24608Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar). Furthermore, the interaction between ErbB2 and ErbB3 upon HRG stimulation is seen with a modified version of the extracellular domain of ErbB3 containing a glycosylphosphatidylinositol moiety anchoring it to the plasma membrane (23Tzahar E. Pinkas-Kramarski R. Moyer J.D. Klapper L.N. Alroy I. Levkowitz G. Shelly M. Henis S. Eisenstein M. Ratzkin B.J. Sela M. Yarden Y. EMBO J. 1997; 6: 4938-4950Crossref Scopus (210) Google Scholar). Also, receptor IgGs consisting of the extracellular domains of ErbB3 and ErbB2 fused to an immunoglobulin Fc domain show increased HRG binding affinity compared with ErbB3 binding alone (24Fitzpatrick V.D. Pisacane P.I. Vandlen R.L. Sliwkowski M.X. FEBS Lett. 1998; 431: 102-106Crossref PubMed Scopus (64) Google Scholar). Taken together, it appears that HRG binding and the affinity shift for HRG binding in the presence of ErbB2 require only the extracellular domains of these receptors. Details of the molecular mechanism that leads to the activation of ErbB2 kinase are unknown, and in particular, the role of the intracellular domain of ErbB3 is uncertain. We questioned whether structural elements in the intracellular domain of ErbB3 were needed for the transactivation of ErbB2. In this study, we designed a series of C-terminal deletion and substitution mutants of ErbB3 and assessed the phosphorylation status of the receptors in the complex. We report here that a distinct three-amino acid segment (LVI) in the intracellular domain of ErbB3 is required for transactivation of ErbB2 and propose that this is an intermediate step between ligand-induced receptor dimerization and kinase activation. Monoclonal anti-ErbB2 antibody 3E8 has been described previously (25Fendly B.M. Winget M. Hudziak R.M. Lipari M.T. Napier M.A. Ullrich A. Cancer Res. 1990; 50: 1550-1558PubMed Google Scholar). Polyclonal rabbit anti-ErbB2 antibody was acquired from Dako Corp. (Carpinteria, CA). Polyclonal rabbit anti-ErbB3 (C-17) and rabbit anti-ErbB4 antibodies were purchased from Santa Cruz Biotechnology (Santa Cruz, CA). Anti-phosphotyrosine antibody conjugated to horseradish peroxidase was obtained from Transduction Laboratories (Lexington, KY). Monoclonal antibody 5B6 raised against the herpes simplex glycoprotein D signal sequence has been described elsewhere (26Paborsky L.R. Fendly B.M. Fisher K.L. Lawn R.M. Marks B.J. McCray G. Tate K.M. Vehar G.A. Gorman C.M. Protein Eng. 1990; 3: PubMed Scopus Google Scholar). The domain of was in and as described previously (7Sliwkowski M.X. Schaefer G. Akita R.W. Lofgren J.A. Fitzpatrick V.D. Nuijens A. Fendly B.M. Cerione R.A. Vandlen R.L. Carraway III., K.L. J. Biol. Chem. 1994; 269: 14661-14665Abstract Full Text PDF PubMed Google Scholar, Sliwkowski M.X. Akita R.W. J. D. N. H. G.D. Vandlen R.L. Science. PubMed Scopus Google Scholar). The domain of was used in all and designated cells and cells were in a of and modified with and cells were the obtained from were for and were A cell that human ErbB3 was by a of containing the of This cell was with the ErbB2 purchased from were with of and of to the and were to analysis was to C-terminal ErbB3 or mutants J.D. R.A. PubMed Scopus Google Scholar). receptor were from a C.M. McCray G. Protein Eng. 1990; 2: Scholar). The signal of ErbB2 and ErbB3 were with the herpes simplex signal sequence of glycoprotein D for analysis P.M. Science. PubMed Scopus Google Scholar). The sequence of was into the of at the end of the signal sequence was by A of was used as an containing the sequence for amino of herpes simplex glycoprotein D. ErbB2 was into the and of the the glycoprotein sequence of The of into the of A was at the end of signal The sequence was with and in into of the C-terminal mutants were obtained by a of the amino acid deletion and substitution mutants were that the amino acid or the designated amino acid with of receptor was obtained by a modified for cells were in cells were from in were at a of in of of binding modified and were with of and a of was on for from cells were a and on a to and was as by the were a cells were in cells were with modified and for were with the of for at the was and were by of and was to a and a were with antibody conjugated to horseradish peroxidase and were with cells were in containing and with in the presence or of was at for was by to a of and it to for at were on and complexes were by were on cells or on cells with cells were for and with or alone for at or cells were in CHAPS, and 10 were with anti-ErbB2 or anti-ErbB3 complexes were purified by on protein protein and were to In the of analysis with antibody was as described whether activation of the ErbB2 kinase by ErbB3 requires a specific interaction with the cytoplasmic domain of ErbB3, we a version of ErbB3. This designated the cytoplasmic domain of the The functional of were by expressing the receptor in cells. of at the cell was by binding analysis cells not bind lack ErbB3 and cells an with a of This was to cells that were with and a binding affinity of (7Sliwkowski M.X. Schaefer G. Akita R.W. Lofgren J.A. Fitzpatrick V.D. Nuijens A. Fendly B.M. Cerione R.A. Vandlen R.L. Carraway III., K.L. J. Biol. Chem. 1994; 269: 14661-14665Abstract Full Text PDF PubMed Google Scholar). whether deletion of the cytoplasmic domain with the ability of the receptor to with ErbB2, cells were with and and were A of was in cells expressing The of this was with the of the In a molecular complex was also The of this molecular complex was that ErbB3 was with ErbB2 the of with ErbB2, proteins were a antibody against ErbB2. in in a to that seen in and was not directly to ErbB2, these that the receptor complexes in the and cells ErbB2. the high molecular complex receptors and these complexes for an ErbB2-ErbB3 receptor that a complex was The at and in and ErbB3 and which were with ErbB2, not to Furthermore, these that the interaction of with ErbB2 was to that of the The results in show that deletion of the cytoplasmic region of ErbB3 not with its ligand binding or its ability to heteromeric complexes with ErbB2. We at the ability of ErbB3 to tyrosine phosphorylation upon HRG cells low of ErbB2, which heteromeric complexes with ErbB3, with was not necessary in these that ErbB3 a increase in tyrosine phosphorylation HRG stimulation The at ErbB2 and ErbB3. In binds HRG and with ErbB2, it was not to activate the ErbB2 kinase. The results in that a or interaction between the intracellular domains of ErbB3 and ErbB2 was necessary for HRG-mediated activation of the intrinsic kinase activity of ErbB2. We questioned whether the intracellular domain of ErbB3 or only a segment of it was required for ErbB2 transactivation. this a series of ErbB3 mutants containing from the were are in in A. The receptor mutants were and to receptor activation analysis as described In these the phosphorylation signals were exclusively due to tyrosine phosphorylation of ErbB2 the mutants the tyrosine phosphorylation located on the of the results are also in A. not show a increase in ErbB2 phosphorylation upon HRG this we mutants to that may to ErbB2 transactivation. was to receptor deletion of in a phosphorylation and deletion of in stimulation of tyrosine phosphorylation we were to the region for the transactivation activity to an to amino acid This region is found at the C-terminal end of the impaired kinase as in C. of the ErbB3 transactivation region was by deletion and substitution mutants in the of the Although on ErbB3 has not been described as a phosphorylation we questioned whether play a role in the of by as a phosphorylation for kinase. We with in the ErbB3 of the substitution a phosphorylation indicating that not play a role in the transactivation A a series of deletion mutants that were designed to the region for ErbB2 transactivation. which the amino that the between the kinase domain and the C-terminal was of receptor phosphorylation activity. with were and for transactivation activity. of three amino acid and was to phosphorylation HRG stimulation In deletion of amino acid and and and not the transactivation It is that deletion of three amino at the of important structural on ErbB3 to on ErbB2. The of to activate the ErbB2 kinase have from a in the ErbB3 to ErbB2, deletion of a specific activation this we and with This for the of the A phosphorylation signal was not in cells expressing indicating that the sequence is required for transactivation. this a series of and substitution mutants were mutants are in in A with results of the phosphorylation analysis for in to cells expressing not ErbB2 upon HRG for this is in at in the was not to This was with The of with an in a in the phosphorylation indicating that a role in the transactivation. these we conclude that the region necessary for transactivation of the ErbB2 kinase upon HRG stimulation is the sequence at to play a role in this However, we the that to this may also to transactivation of ErbB2. of the tyrosine kinase domains of members of the ErbB receptor family that the segment is conserved in ErbB4 as as ErbB3 and EGFR We questioned whether a transactivation mechanism also between ErbB2 and ErbB2 transactivation in an we the intrinsic kinase activity of ErbB4 by an ErbB4 in which at the was with The lack of kinase activity in this was by phosphorylation analysis a human cell that is of all ErbB family members not Receptor activation analysis in cells expressing a phosphorylation signal upon HRG that a transactivation of ErbB2 also in the of the segment in a three-amino acid deletion residues and was in the of cells expressing increase in tyrosine phosphorylation signal upon HRG to the ErbB3 activation of ErbB2 kinase also requires the in phosphorylation analysis of ErbB4 ErbB4 mutants as described in the to The ErbB4 receptor with by is designated in to the amino acid and phosphorylation analysis of ErbB4 cells were with the and cells were to phosphorylation analysis as described in the to ErbB4 was a anti-ErbB4 It is that ErbB2 transactivation is not the of a interaction that between ErbB3 and ErbB2. is that a protein with tyrosine kinase activity is to the cytoplasmic domain of ErbB3 and that this kinase ErbB2. This to the transactivation mechanism in receptor and receptor signaling T. T. S. Cell. 1994; Full Text PDF PubMed Scopus Google Scholar, G. J. P. T. 1995; PubMed Scopus Google Scholar). the activity of a kinase was directly by ErbB3, the kinase activity of ErbB2 not required for receptor this we a kinase-inactive ErbB2 in which at the was with of this was in a cell that only ErbB3 a cytoplasmic kinase was for cells that ErbB3 and kinase-inactive ErbB2 show a phosphorylation signal upon HRG with or were with HRG and to with antibodies against ErbB2 or ErbB3. with demonstrated tyrosine phosphorylation on ErbB2 and ErbB3 ErbB2 was constitutively ErbB3 phosphorylation was In cells with or HRG stimulation on ErbB2 or ErbB3. The of ErbB2 and was by analysis that of the ErbB2 kinase the phosphorylation signal and that ErbB3 has intrinsic kinase activity. of the receptors was not due to a cytoplasmic kinase by binding to the transactivation segment of ErbB3, was the of the ErbB2 kinase. The oligomerization for EGFR by J. 1988; PubMed Scopus Google Scholar, J. Biochem. Sci. 1988; Full Text PDF PubMed Scopus Google Scholar) that ligand binding induces formation of receptor which the intracellular domains into and to ErbB3 intrinsic kinase activity and ErbB2 not bind HRG, this not the phosphorylation in the ErbB2-ErbB3 complex HRG It was also previously whether activation of the intrinsic kinase required specific cytoplasmic domain we that transactivation of the ErbB2 kinase by ErbB3 requires the presence of a structural within the ErbB3 cytoplasmic ErbB3 deletion and substitution we found a discrete sequence of three amino (LVI) at the of the kinase domain of ErbB3 that is necessary for ErbB2 transactivation. of to this region indicating that are not required for this activity. At we the that to this region may also to the transactivation of ErbB2. The results of suggest at least molecular interactions may in receptor transactivation. the J. 1988; PubMed Scopus Google J. Biochem. Sci. 1988; Full Text PDF PubMed Scopus Google Scholar) to the ErbB2-ErbB3 and are in In the activation of ErbB2 results from a interaction between the intracellular domains of the receptors. In this HRG binding to ErbB3 results in a in the extracellular domain of ErbB3 of ErbB2 and formation of the heterodimeric complex. The interaction between the extracellular domains of the receptors intracellular the of ErbB3 into with an as region of ErbB2. in leads to the activation of the ErbB2 kinase. An the of a heterodimerization of ErbB3 with ErbB2 an with specific for receptor to intracellular The protein binds to the sequence in ErbB3 and to an sequence in ErbB2. We that the interactions between cytoplasmic domains that to the activation of the ErbB2 in of the receptors. of the tyrosine kinase domains of members of the ErbB receptor family that the segment is conserved in EGFR as as ErbB3 and ErbB4 in ErbB2, the is by The of in the ErbB receptors that the transactivation mechanism seen in the ErbB2-ErbB3 and is not to these may a mechanism of transactivation in with a mechanism are from of R. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). ErbB2 is with the transactivation of ErbB2 upon EGF binding and only between amino and containing the are in a EGFR It is that the sequence in EGFR has been as part of the sequence that to the of EGFR Chang P.M. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar). This segment has also been as part of the binding of a isolated protein Science. 1996; PubMed Scopus Google Scholar). signaling from ErbB family members is in S.A. Growth Factor Res. Full Text PDF PubMed Scopus Google Scholar). of EGFR or ErbB2 with high and with H. C. J. 1994; Google Scholar, Ullrich A. Science. PubMed Scopus Google Scholar, N.E. Cancer Biol. 1993; Google Scholar). the ErbB signaling may an for an several have been to the signal A. A. Science. 1995; PubMed Scopus Google Scholar). antibodies that directly bind to the extracellular domain of ErbB2 or EGFR have been as J. J. 1994; PubMed Scopus Google Scholar, J. D. J. S. L. J. P.P. T. L. J. 1996; 14: PubMed Scopus Google Scholar, J. J. Cancer Res. 1994; PubMed Scopus Google Scholar). The of EGFR tyrosine kinase with also results in activity A.J. 1997; PubMed Scopus Google Scholar). The of the transactivation sequence in ErbB3 to signaling in a heteromeric complex. A that directly with the segment in ErbB3 specific receptor and all signaling We the group at for