Structure of the Extracellular Domain of Tie Receptor Tyrosine Kinases and Localization of the Angiopoietin-binding Epitope

Angiogenesis is essential for tissue repair and regeneration during wound healing but also plays important roles in many pathological processes including tumor growth and metastasis. The receptor protein tyrosine kinase Tie2 and its ligands, the angiopoietins, have important functions in the regulation of angiogenesis. Here, we report a detailed structural and functional characterization of the extracellular region of Tie2. Sequence analysis of the extracellular domain revealed an additional immunoglobulin-like domain resulting in a tandem repeat of immunoglobulin-like domains at the N terminus of the protein. The same domain organization was also found for the Tie1 receptor that shares a high degree of homology with Tie2. Based on structural similarities to other receptor tyrosine kinases and cell adhesion molecules, we demonstrate that the N-terminal two immunoglobulin-like domains of Tie2 harbor the angiopoietin-binding site. Using transmission electron microscopy we furthermore show that the extracellular domain of Tie receptors consists of a globular head domain and a short rod-like stalk that probably forms a spacer between the cell surface and the angiopoietin-binding site. Mutational analysis demonstrated that the head domain consists of the three immunoglobulin-like domains and the three epidermal growth factor-like modules and that the stalk is formed by the three fibronectin type III repeats. These findings might be of particular interest for drug development because Tie receptors are potential targets for treatment of angiogenesis-associated diseases. Angiogenesis is essential for tissue repair and regeneration during wound healing but also plays important roles in many pathological processes including tumor growth and metastasis. The receptor protein tyrosine kinase Tie2 and its ligands, the angiopoietins, have important functions in the regulation of angiogenesis. Here, we report a detailed structural and functional characterization of the extracellular region of Tie2. Sequence analysis of the extracellular domain revealed an additional immunoglobulin-like domain resulting in a tandem repeat of immunoglobulin-like domains at the N terminus of the protein. The same domain organization was also found for the Tie1 receptor that shares a high degree of homology with Tie2. Based on structural similarities to other receptor tyrosine kinases and cell adhesion molecules, we demonstrate that the N-terminal two immunoglobulin-like domains of Tie2 harbor the angiopoietin-binding site. Using transmission electron microscopy we furthermore show that the extracellular domain of Tie receptors consists of a globular head domain and a short rod-like stalk that probably forms a spacer between the cell surface and the angiopoietin-binding site. Mutational analysis demonstrated that the head domain consists of the three immunoglobulin-like domains and the three epidermal growth factor-like modules and that the stalk is formed by the three fibronectin type III repeats. These findings might be of particular interest for drug development because Tie receptors are potential targets for treatment of angiogenesis-associated diseases. Proper regulation of angiogenesis is required for the normal growth of embryonic and postnatal tissues as well as physiological processes in the adult such as the continuous remodeling of the female reproductive system and wound healing (1Carmeliet P. Jain R.K. Nature. 2000; 407: 249-257Crossref PubMed Scopus (7531) Google Scholar, 2Carmeliet P. Nature. 2005; 438: 932-936Crossref PubMed Scopus (2840) Google Scholar). In contrast, dysregulation of angiogenesis contributes to numerous malignant, ischemic, inflammatory, infectious, and immune disorders including conditions such as diabetic retinopathy and tumor growth and metastasis (1Carmeliet P. Jain R.K. Nature. 2000; 407: 249-257Crossref PubMed Scopus (7531) Google Scholar, 2Carmeliet P. Nature. 2005; 438: 932-936Crossref PubMed Scopus (2840) Google Scholar). The angiopoietin (Ang) 3The abbreviations used are: Ang, angiopoietin; EBNA, Epstein-Barr virus nuclear antigen; ECD, extracellular domain; EGF, epidermal growth factor; ELISA, enzyme-inked immunosorbent assay; FNIII, fibronectin type III; HEK, human embryonic kidney; LE, laminin epidermal growth factor-like; Mat3, matrilin 3; RTK, receptor tyrosine kinase; TEM, transmission electron microscopy; Tie, tyrosine kinase with Ig-like and EGF-like domains./Tie signaling system plays important roles in the regulation of angiogenesis (3Yancopoulos G.D. Davis S. Gale N.W. Rudge J.S. Wiegand S.J. Holash J. Nature. 2000; 407: 242-248Crossref PubMed Scopus (3298) Google Scholar, 4Eklund L. Olsen B.R. Exp. Cell Res. 2006; 312: 630-641Crossref PubMed Scopus (260) Google Scholar). Tie1 and Tie2 constitute a family of vascular-specific receptor tyrosine kinases (RTKs) that are expressed mainly on endothelial cells but also in certain hematopoietic cells (5Thurston G. Cell Tissue Res. 2003; 314: 61-68Crossref PubMed Scopus (244) Google Scholar, 6Dumont D.J. Yamaguchi T.P. Conlon R.A. Rossant J. Breitman M.L. Oncogene. 1992; 7: 1471-1480PubMed Google Scholar, 7Partanen J. Armstrong E. Makela T.P. Korhonen J. Sandberg M. Renkonen R. Knuutila S. Huebner K. Alitalo K. Mol. Cell. Biol. 1992; 12: 1698-1707Crossref PubMed Scopus (298) Google Scholar). Angiopoietins are a family of four distinct growth factor ligands that bind to Tie2 but not Tie1 (8Davis S. Aldrich T.H. Jones P.F. Acheson A. Compton D.L. Jain V. Ryan T.E. Bruno J. Radziejewski C. Maisonpierre P.C. Yancopoulos G.D. Cell. 1996; 87: 1161-1169Abstract Full Text Full Text PDF PubMed Scopus (1698) Google Scholar, 9Maisonpierre P.C. Suri C. Jones P.F. Bartunkova S. Wiegand S.J. Radziejewski C. Compton D. McClain J. Aldrich T.H. Papadopoulos N. Daly T.J. Davis S. Sato T.N. Yancopoulos G.D. Science. 1997; 277: 55-60Crossref PubMed Scopus (2975) Google Scholar, 10Valenzuela D.M. Griffiths J.A. Rojas J. Aldrich T.H. Jones P.F. Zhou H. McClain J. Copeland N.G. Gilbert D.J. Jenkins N.A. Huang T. Papadopoulos N. Maisonpierre P.C. Davis S. Yancopoulos G.D. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 1904-1909Crossref PubMed Scopus (400) Google Scholar). They are unique and differ from other growth factors by having opposing effects on receptor phosphorylation. Overexpression and knock-out studies in mice have revealed that Ang1 is an agonist promoting stabilization of vessels by maximizing interactions between endothelial cells and their surrounding support cells and the extracellular matrix (11Suri C. Jones P.F. Patan S. Bartunkova S. Maisonpierre P.C. Davis S. Sato T.N. Yancopoulos G.D. Cell. 1996; 87: 1171-1180Abstract Full Text Full Text PDF PubMed Scopus (2406) Google Scholar, 12Suri C. McClain J. Thurston G. McDonald D.M. Zhou H. Oldmixon E.H. Sato T.N. Yancopoulos G.D. Science. 1998; 282: 468-471Crossref PubMed Scopus (699) Google Scholar). In contrast Ang2 was found to be a context-dependent antagonist that promotes angiogenic sprouting or vessel regression depending on the expression of vascular endothelial growth factor-A (13Gale N.W. Thurston G. Hackett S.F. Renard R. Wang Q. McClain J. Martin C. Witte C. Witte M.H. Jackson D. Suri C. Campochiaro P.A. Wiegand S.J. Yancopoulos G.D. Dev. Cell. 2002; 3: 411-423Abstract Full Text Full Text PDF PubMed Scopus (826) Google Scholar, 14Kim I. Kim J.H. Moon S.O. Kwak H.J. Kim N.G. Koh G.Y. Oncogene. 2000; 19: 4549-4552Crossref PubMed Scopus (269) Google Scholar). Ang3 and Ang4 also show context-dependent actions as antagonistic and agonistic ligands, respectively, and they are believed to represent widely diverged counterparts of the same gene in mouse ANGPT3 and human ANGPT4 (10Valenzuela D.M. Griffiths J.A. Rojas J. Aldrich T.H. Jones P.F. Zhou H. McClain J. Copeland N.G. Gilbert D.J. Jenkins N.A. Huang T. Papadopoulos N. Maisonpierre P.C. Davis S. Yancopoulos G.D. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 1904-1909Crossref PubMed Scopus (400) Google Scholar, 15Lee H.J. Cho C.H. Hwang S.J. Choi H.H. Kim K.T. Ahn S.Y. Kim J.H. Oh J.L. Lee G.M. Koh G.Y. FASEB J. 2004; 18: 1200-1208Crossref PubMed Scopus (134) Google Scholar). Although Tie1 is considered an orphan receptor with no known ligands, it has recently been found that Ang1 and Ang4 can induce Tie1 phosphorylation (16Saharinen P. Kerkela K. Ekman N. Marron M. Brindle N. Lee G.M. Augustin H. Koh G.Y. Alitalo K. J. Cell Biol. 2005; 169: 239-243Crossref PubMed Scopus (172) Google Scholar). Furthermore, it has been shown that Tie1 and Tie2 form heteromeric complexes in cells that express both receptors, which is consistent with the observation that phosphorylation of Tie1 is significantly amplified through Tie2 (16Saharinen P. Kerkela K. Ekman N. Marron M. Brindle N. Lee G.M. Augustin H. Koh G.Y. Alitalo K. J. Cell Biol. 2005; 169: 239-243Crossref PubMed Scopus (172) Google Scholar, 17Marron M.B. Hughes D.P. McCarthy M.J. Beaumont E.R. Brindle N.P. Adv. Exp. Med. Biol. 2000; 476: 35-46Crossref PubMed Scopus (17) Google Scholar). However, the mechanistic details of how angiopoietins differentially regulate Tie receptor activity are currently not known. Mutations in the kinase domain of Tie2 resulting in ligand-independent activation are associated with vascular anomalies such as multiple cutaneous and mucosal venous malformations and intramuscular hemangiomas (18Vikkula M. Boon L.M. Carraway III, K.L. Calvert J.T. Diamonti A.J. Goumnerov B. Pasyk K.A. Marchuk D.A. Warman M.L. Cantley L.C. Mulliken J.B. Olsen B.R. Cell. 1996; 87: 1181-1190Abstract Full Text Full Text PDF PubMed Scopus (615) Google Scholar, 19Calvert J.T. Riney T.J. Kontos C.D. Cha E.H. Prieto V.G. Shea C.R. Berg J.N. Nevin N.C. Simpson S.A. Pasyk K.A. Speer M.C. Peters K.G. Marchuk D.A. Hum. Mol. Genet. 1999; 8: 1279-1289Crossref PubMed Scopus (109) Google Scholar, 20Wang H. Zhang Y. Toratani S. Okamoto T. Oncogene. 2004; 23: 8700-8704Crossref PubMed Scopus (19) Google Scholar). Furthermore, overexpression of Tie2 has been observed in a wide range of diseases including psoriasis (21Kuroda K. Sapadin A. Shoji T. Fleischmajer R. Lebwohl M. J. Investig. Dermatol. 2001; 116: 713-720Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar, 22Voskas D. Jones N. Van S.P. Sturk C. Chang W. Haninec A. Babichev Y.O. Tran J. Master Z. Chen S. Ward N. Cruz M. Jones J. Kerbel R.S. Jothy S. Dagnino L. Arbiser J. Klement G. Dumont D.J. Am. J. Pathol. 2005; 166: 843-855Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar), pulmonary hypertension (23Du L. Sullivan C.C. Chu D. Cho A.J. Kido M. Wolf P.L. Yuan J.X. Deutsch R. Jamieson S.W. Thistlethwaite P.A. N. Engl. J. Med. 2003; 348: 500-509Crossref PubMed Scopus (338) Google Scholar, 24Sullivan C.C. Du L. Chu D. Cho A.J. Kido M. Wolf P.L. Jamieson S.W. Thistlethwaite P.A. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 12331-12336Crossref PubMed Scopus (126) Google Scholar), infantile hemangiomas (25Yu Y. Varughese J. Brown L.F. Mulliken J.B. Bischoff J. Am. J. Pathol. 2001; 159: 2271-2280Abstract Full Text Full Text PDF PubMed Scopus (134) Google Scholar), and different cancers (26Tait C.R. Jones P.F. J. Pathol. 2004; 204: 1-10Crossref PubMed Scopus (201) Google Scholar). Ang/Tie2 signaling has been studied extensively and is known to activate several downstream pathways that in endothelial cell and cell L. Olsen B.R. Exp. Cell Res. 2006; 312: 630-641Crossref PubMed Scopus (260) Google Scholar). Although the of a has it to the of a receptor of the extracellular domain of receptor and the domain of Tie1 show that Tie1 activation can induce phosphorylation of C.D. Cha E.H. Peters K.G. Mol. Cell. Biol. 2002; PubMed Scopus Google Scholar). signaling is also of the pathways through Tie2 which functions of Tie1 and Tie2. with their expression in other cells such as in the Tie receptors are found to be required during postnatal M.C. A. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: PubMed Scopus Google Scholar). a of the signaling has also been demonstrated in hematopoietic cells in the in the A. M. Sato H. S. K. K. Koh G.Y. T. Cell. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). has been shown that activation of Tie2 on hematopoietic cells promotes adhesion of the cells to the resulting in their and Tie1 and Tie2 receptors a high degree of The extracellular of both receptors three EGF-like modules that are by two and three fibronectin type III D.J. Yamaguchi T.P. Conlon R.A. Rossant J. Breitman M.L. Oncogene. 1992; 7: 1471-1480PubMed Google Scholar, 7Partanen J. Armstrong E. Makela T.P. Korhonen J. Sandberg M. Renkonen R. Knuutila S. Huebner K. Alitalo K. Mol. Cell. Biol. 1992; 12: 1698-1707Crossref PubMed Scopus (298) Google Scholar). The Ig-like domain and the EGF-like modules of Tie2 are and to bind both Ang1 and that receptor is not to be for the different functions of Ang1 and Ang2 U. T. S. C. T. Deutsch U. G. D. Augustin J. Biol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). The tyrosine kinase domains including a of phosphorylation and protein L.M. B. Davis R. J.T. 2000; 8: Full Text Full Text PDF PubMed Scopus Google Scholar). the important functions of both receptors in a of we a detailed structural and functional characterization of the extracellular of Tie2 and amplified by and a that a for E. D. U. R. J. PubMed Scopus Google the domain of human Ang1 at its N terminus to the domain of human the extracellular domain of human Tie2 to the domain of human or a a Tie2 for the the two Ig-like domains to the or a the extracellular domain of Tie1 to the and the extracellular domain of the Tie1 the EGF-like to the The Tie2 was also the a of a and the Tie2 the at the N of the Tie protein used in is shown in of by expression in cells to the with cells for the of the from and the by on to the and Tie2 was expressed in and conditions as S. T. R. A. T. P. J. R.A. J. Biol. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). The protein was as V. S. S.J. D. 1997; PubMed Scopus Google Scholar). Sequence and for protein J.A. M. 1998; PubMed Scopus Google was The for protein by and with and potential M.J. J. Mol. Biol. 2000; PubMed Scopus Google was of the Tie2 to the N-terminal Ig-like domains of the receptor T.J. Res. PubMed Scopus Google Scholar). The structural was the T. J. N. M.C. Res. 2003; PubMed Scopus Google Scholar), the between Tie2 and N-terminal Ig-like domain as an and the of the Ig-like domain T. Y. W. A. R. E. J. 2006; PubMed Scopus Google as a was with the of protein in at and in a with at an of U. J. Res. PubMed Scopus Google Scholar). in a transmission electron at with a III at on a with a The was at a and in to by for the of with at and at with for at The ligands to well at and for at three with in and with a Cruz for at as and with for at The of to was by of The of Tie a Ig-like N-terminal of Tie2 that the EGF-like have been to a Ig-like domain D.J. Yamaguchi T.P. Conlon R.A. Rossant J. Breitman M.L. Oncogene. 1992; 7: 1471-1480PubMed Google Scholar, 7Partanen J. Armstrong E. Makela T.P. Korhonen J. Sandberg M. Renkonen R. Knuutila S. Huebner K. Alitalo K. Mol. Cell. Biol. 1992; 12: 1698-1707Crossref PubMed Scopus (298) Google Scholar). Ig-like domains of of the has no domain the N-terminal in by several analysis J.A. M. 1998; PubMed Scopus Google the to distinct not The Ig-like is by an observation that the of a Ig-like domain in the N terminus the EGF-like repeats. a between and domain is found for domains of multiple J. Mol. Biol. 1998; PubMed Scopus Google Scholar), the organization of the Tie2 gene was also The well with the and support the that the N terminus of Tie2 an Ig-like The of an additional Tie2 Ig-like domain is by the protein M.J. J. Mol. Biol. 2000; PubMed Scopus Google Scholar), which a high to the tandem Ig-like of the T. Y. W. A. R. E. J. 2006; PubMed Scopus Google Scholar, C. B. J. Biol. 2004; Full Text Full Text PDF PubMed Scopus Google and growth factor receptor J. M. Cell. 2000; Full Text Full Text PDF PubMed Scopus Google to the shown in the and domains of Tie2 are of and from its high degree of homology to the additional Ig-like domain was also for Tie1 not The of a tandem repeat of Ig-like domains in Tie receptors has important functional of Ig-like domains are found in other and cell adhesion they important roles for is the RTK, which plays important roles in and S. B. 2006; PubMed Scopus Google Scholar). The extracellular domain of consists of two Ig-like by two and has a domain organization to that of Tie the of the two Ig-like domains of the in with the has been T. Y. W. A. R. E. J. 2006; PubMed Scopus Google Scholar). two laminin but in the the domain two tandem Ig-like at distinct and how the receptor through to and are required for The structural to that the Ig-like domain of Tie2 the is by a the a different In an to the in U. T. S. C. T. Deutsch U. G. D. Augustin J. Biol. 2003; Full Text Full Text PDF PubMed Scopus Google expressed a of Tie2 as to a in the system and their to Ang1 and Ang2 by and with findings they that the Ig-like of Tie2 in with the EGF-like is required and to bind both Ang1 and that receptor is not to be for the different functions of Ang1 and In contrast, the Ig-like domain or the EGF-like are not of Ang1 and of the the Ig-like domain of revealed that of the Ig-like domain is In the the which of the Ig-like is that the domain a of the domain also in a structural of a at the of the domain a for the of the protein to with for the of the of Ig-like domains of Tie2 in is by the that knock-out mice of Tie2 express a Tie2 protein that the Ig-like domain U. T. S. C. T. Deutsch U. G. D. Augustin J. Biol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). The receptor the Ig-like domain is as shown by the of and receptor phosphorylation. findings demonstrate that both domains of the Ig-like tandem repeat of Tie2 are required to bind to of the EGF-like that domains are unique to the Tie2 the of the Tie EGF-like have the to the laminin epidermal growth factor-like which is found in the extracellular matrix and J. U. R. R. J. Mol. Biol. 1996; PubMed Scopus Google Scholar). domains mainly as but can also be in The and has and the three to with in the of of the of and the of in modules is for Tie EGF-like different in The of Tie a a a structural characterization of the extracellular domain of the Tie2 we expressed the of the human protein to the of human in cells for also of protein by is shown in of the Tie2 by and in which three globular are by short rod-like and The form as by conditions not the in the is to represent the of the protein that two The of the rod-like is consistent with the for three domains A. J.A. M.J. Jones J. 1999; 18: PubMed Google The are to to the Ig-like domains and the EGF-like which have a as the four domains of the we two additional Tie2 the and the of the receptor to the Ig-like domains and the EGF-like shown in with the observed by from the and a of globular was for the protein the electron of Tie2 and Tie1 of Tie2 high of Tie2 Tie1 Tie2 and also the of the of the Tie1 receptor by Although Tie1 can be by Ang1 and no with the ligands has been In endothelial cells that express both receptor activation of Tie1 is to through with Tie2. in that are from Tie2 for Tie1 that both receptors the same bind and we also the of the of Tie receptors in the of from observed for both receptors not that is not required for the of the globular domain of the Ig-like domains and the EGF-like repeats. In demonstrate that the of Tie a globular and a short rod-like The is formed by the three Ig-like domains and the three EGF-like and the stalk consists of the three repeats. The EGF-like of the globular head domain of Tie receptors to an important in protein are two known of Tie1 that the or the EGF-like repeat J. Armstrong E. Makela T.P. Korhonen J. Sandberg M. Renkonen R. Knuutila S. Huebner K. Alitalo K. Mol. Cell. Biol. 1992; 12: 1698-1707Crossref PubMed Scopus (298) Google Scholar). we to express the Tie1 we not to of from The on the of the Tie EGF-like was we to express the tandem repeat of Ig-like domains in the system in to demonstrate that the two domains the site. both shown in no protein expression was The to protein from three be by a to the Ig-like domain through interactions with the EGF-like Although a of the and the is the protein was in the expression system that the of the protein to the domain be U. T. S. C. T. Deutsch U. G. D. Augustin J. Biol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). is that expression in can in and J. C. M. Y. 2003; PubMed Scopus Google Scholar). the is that the protein can be in the but not the The of the head domain for protein because mouse endothelial cells have been shown to express a and form of the receptor that the Ig-like domain U. T. S. C. T. Deutsch U. G. D. Augustin J. Biol. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). The Ig-like of Tie2 the that the tandem repeat of Ig-like domains of Tie2 the we expressed the protein in The of Tie2 protein as an form in an observation that is Ig-like However, it has been shown that of Ig-like domains can be in from and that is not required for V. S. S.J. D. 1997; PubMed Scopus Google Scholar, C. B. J. Biol. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar, S. Martin A. PubMed Scopus Google Scholar). of Tie2 was by The of the protein was consistent with Tie2 a that is in The of the of Ig-like Tie2 domains was by and with of Tie2 Tie2 ECD, and In a form of was used in which the domains of Ang1 by the short domain of protein was recently shown to have and activation to but its is with Ang1 C.H. R.A. Lee H.J. Lee K. Kim K.T. Kim I. Choi H.H. Kim W. Kim Lee G.M. Koh G.Y. Proc. Natl. Acad. Sci. U. S. A. 2004; PubMed Scopus Google Scholar). shown in Tie2 to Ang1 with an to the that the Tie2 can be and it that high to the not a distinct of the Ig-like domain Tie2 to the Tie2 or can be by the protein as a of the of the Tie2 In contrast Tie1 not bind significantly to the Ang1 not which is consistent with findings C.H. R.A. Lee H.J. Lee K. Kim K.T. Kim I. Choi H.H. Kim W. Kim Lee G.M. Koh G.Y. Proc. Natl. Acad. Sci. U. S. A. 2004; PubMed Scopus Google Scholar). four potential two of which are found in the Ig-like of the has been observed for D.A. K. M. P.C. C. Mol. PubMed Scopus Google and other K.A. A. Choi I. J.A. Wolf M.C. L. Lee B. J. 2006; PubMed Scopus Google Scholar). of the protein in no in to that the are not in of the site. the of the that Tie receptors can form heteromeric complexes in endothelial cells that express both we also the of Tie1 to Tie2 and Tie2 between the extracellular domains of the two receptors was observed not that the of the two is through between the kinase which has been M.B. Hughes D.P. Brindle N.P. J. Biol. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). have shown in that Tie harbor a Ig-like domain in the N terminus of the in the domain organization for of the Based on the structural with other and cell adhesion molecules, the was to the N-terminal of Ig-like The of Tie receptors an N-terminal globular head and a short rod-like stalk that probably as a spacer between the cell and the site. The stalk domain is formed by the three and the head domain consists of the Ig-like domains and the EGF-like structural of the Tie receptors that through their kinase domains is shown in from their physiological functions Tie receptors important roles in processes such as tumor angiogenesis. represent an for with P. Nature. 2005; 438: 932-936Crossref PubMed Scopus (2840) Google Scholar). Tie2 or have potential in the treatment of and other angiogenesis-associated diseases. Furthermore, the domain a to the in the Tie2 we the of a between Tie2 and the domain of Ang2 been D. Mol. Biol. 2006; PubMed Scopus Google Scholar). The the of the globular head domain of Tie2 including the of the Ig-like with findings the Ig-like domain the site. a unique of the Tie2 EGF-like is in the as in structural is required to how the activity of the receptor is differentially by its B. for in the and also the for of the of for to the transmission electron