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Tie2/Tek is an endothelial cell receptor tyrosine kinase that induces signal transduction pathways involved in cell migration upon angiopoietin-1 (Ang1) stimulation. To address the importance of the various tyrosine residues of Tie2 in signal transduction, we generated a series of Tie2 mutants and examined their signaling properties. Using this approach in conjunction with a phosphorylation state-specific antibody, we identified tyrosine residue 1106 on Tie2 as an Ang1-dependent autophosphorylation site that mediates binding and phosphorylation of the downstream-of-kinase-related (Dok-R) docking protein. This tyrosine residue is contained within a unique interaction motif for the phosphotyrosine binding domain of Dok-R, and the pleckstrin homology domain of Dok-R further contributes to Tie2 binding in a phosphatidylinositol 3'-kinase-dependent manner. Introduction of a Tie2 mutant lacking tyrosine residue 1106 into endothelial cells interferes with Dok-R phosphorylation in response to Ang1. Furthermore, this mutant is unable to restore the migration potential of endothelial cells derived from mice lacking Tie2. Together, these findings demonstrate that tyrosine residue 1106 on Tie2 is critical for coupling downstream cell migration signal transduction pathways with Ang1 stimulation in endothelial cells. [ABSTRACT FROM AUTHOR]

Tek/Tie-2 is an endothelial cell (EC)-specific receptor tyrosine kinase that plays a critical role in angiogenesis via its regulation by the angiopoietin family of growth factor ligands. Angiopoietin-1 (Angi) can promote EC migration; however, the signaling mechanisms underlying this process remain elusive. Here we demonstrate that Dok-R/Dok-2 can associate with Tek in ECs following Angi stimulation, resulting in tyrosine phosphorylation of Dok-R and the subsequent recruitment of Nck and the p21-activating kinase (Pak/Paki) to the activated receptor. Ang1-mediated migration is increased upon Dok-R overexpression and this requires a functional Nck binding site on Dok-R. Localization of this Dok-R-Nck-Pak complex to the activated Tek receptor at the cellular membrane is coincident with activation of Pak kinase. The ability of Dok-R to bind Nck is required for maximal activation of Pak and overexpression of Pak results in increased Ang1-mediated cell motility. Our study outlines a novel signaling pathway underlying Angi-driven cell migration that involves Dok-R and its recruitment of Nck and the subsequent activation of Pak. [ABSTRACT FROM AUTHOR]

Shc family proteins serve as phosphotyrosine adaptor molecules in various receptor-mediated signaling pathways. In mammals, three distinct Shc genes have been described that encode proteins characterized by two phosphotyrosine-interaction modules, an amino-terminal phosphotyrosine binding (PTB) domain and a carboxy-terminal Src homology 2 domain. Here, we report the analysis of an uncharacterized fourth Shc family protein, ShcD/Shc4, that is expressed in adult brain and skeletal muscle. Consistent with this expression pattern, we find that ShcD can associate via its PTB domain with the phosphorylated muscle-specific kinase (MuSK) receptor tyrosine kinase and undergo tyrosine phosphorylation downstream of activated MuSK. Interestingly, additional sites of tyrosine phosphorylation, including a novel Grb2 binding site, are present on ShcD that are not found in other Shc family proteins. Activation of MuSK upon agrin binding at the neuromuscular junction (NMJ) induces clustering and tyrosine phosphorylation of acetylcholine receptors (AChRs) required for synaptic transmission. ShcD is coexpressed with MuSK in the postsynaptic region of the NMJ, and in cultured myotubes stimulated with agrin, expression of ShcD appears to be important for early tyrosine phosphorylation of the AChR. Thus, we have characterized a new member of the Shc family of docking proteins, which may mediate a specific aspect of signaling downstream of the MuSK receptor. [ABSTRACT FROM AUTHOR]