Catalytic domain of the Protein Tyrosine Kinase, Tie2
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Tie2 is a receptor PTK (RTK) containing an extracellular region, a transmembrane segment, and an intracellular catalytic domain. The extracellular region contains an immunoglobulin (Ig)-like domain, three epidermal growth factor (EGF)-like domains, a second Ig-like domain, and three fibronectin type III repeats. Tie2 is expressed mainly in endothelial cells and hematopoietic stem cells. It is also found in a subset of tumor-associated monocytes and eosinophils. The angiopoietins (Ang-1 to Ang-4) serve as ligands for Tie2. The binding of Ang-1 to Tie2 leads to receptor autophosphorylation and activation, promoting cell migration and survival. In contrast, Ang-2 binding to Tie2 does not result in the same response, suggesting that Ang-2 may function as an antagonist. Tie2 signaling plays key regulatory roles in vascular integrity and quiescence, and in inflammation. The Tie2 subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.