Catalytic domain of the Protein Tyrosine Kinases, Class EphB Ephrin Receptors
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Class EphB receptors bind to transmembrane ephrin-B ligands. There are six vertebrate EphB receptors (EphB1-6), which display promiscuous interactions with three ephrin-B ligands. One exception is EphB2, which also interacts with ephrin A5. EphB receptors play important roles in synapse formation and plasticity, spine morphogenesis, axon guidance, and angiogenesis. In the intestinal epithelium, EphBs are Wnt signaling target genes that control cell compartmentalization. They function as suppressors of colon cancer progression. EphRs comprise the largest subfamily of receptor PTKs (RTKs). They contain an ephrin-binding domain and two fibronectin repeats extracellularly, a transmembrane segment, and a cytoplasmic tyr kinase domain. Binding of the ephrin ligand to EphR requires cell-cell contact since both are anchored to the plasma membrane. The resulting downstream signals occur bidirectionally in both EphR-expressing cells (forward signaling) and ephrin-expressing cells (reverse signaling). Ephrin/EphR interaction mainly results in cell-cell repulsion or adhesion. The EphB subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.