Molecular mechanism of angiopoietin-1-mediated signaling

Angiopoietins are secreted glycoproteins that play crucial roles in both developmental and tumor-induced angiogenesis. They exert their biological effects through a transmembrane receptor tyrosine kinase Tie2. Ang1 is the primary physiological ligand for Tie2. It is essential for embryonic vessel assembly and maturation, and functions as a key regulator in maintaining the quiescence of adult blood vessels.;In an effort to gain a more accurate molecular understanding of Ang1-Tie2 interaction, we determined the structures of Ang1 receptor binding domain (Ang1-RBD) alone and the Ang1-Tie2 complex. The structure of Ang1-RBD shows a compact fibrinogen-like fold with a unique carboxy-terminal P domain and a conserved calcium binding site. Ang1 and the extracellular domain of Tie2 form a complex with a 1:1 stoichiometry. The structure of the Ang1-Tie2 complex reveals that Ang1 binds at the tip of the arrowhead-shaped Tie2, interacting only with the Ig2 domain of Tie2 via its carboxy-terminal P domain. Ang1 undergoes no significant structural changes upon binding to Tie2. The overall structure of Tie2 in the Ang1-Tie2 complex is similar to that of the unbound Tie2. No domain rearrangements or significant conformational changes are detected in Tie2 upon Ang1 binding. These observations suggest that the recognition by Tie2 of Ang1 utilizes a "lock and key" mechanism.;Comparison with the known structures of Ang2-RBD and the Ang2-Tie2 complex reveals that the fibrinogen-homology domains of Ang1 and Ang2 have similar structure patterns, and they bind to Tie2 in a structurally similar manner. However, Ang1 and Ang2 elicit distinct biological responses. We hypothesize that additional co-receptors exist to modulate the angiopoietin-Tie2 signaling. Integrin alpha5beta1 is a potential candidate. We show that Ang1, but not Ang2, enhances the association of integrin alpha5beta1 with Tie2 in 293 human embryonic kidney (HEK) cells expressing full-length Tie2. In vitro pull-down assays show that the extracellular domain of integrin alpha5beta1 binds to Ang1, but not to Ang2. These results suggest that integrin alpha5beta1 is a co-receptor for Ang1 and might modulate the different functions of Ang1 and Ang2.