Ephrin receptors (EphRs) comprise the largest subfamily of receptor tyrosine kinases (RTKs). They are subdivided into 2 groups, A and B type receptors, depending on their ligand ephrin-A or ephrin-B, respectively. In general, class EphA receptors bind GPI-anchored ephrin-A ligands. There are ten vertebrate EphA receptors (EphA1-10), which display promiscuous interactions with six ephrin-A ligands. Class EphB receptors bind to transmembrane ephrin-B ligands. There are six vertebrate EhpB receptors (EphB1-6), which display promiscuous interactions with three ephrin-B ligands. One exception is EphB2, which also interacts with ephrin A5. EphRs contain a ligand binding domain and two fibronectin repeats extracellularly, a transmembrane segment, and a cytoplasmic tyrosine 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, making it important in neural development and plasticity, cell morphogenesis, cell-fate determination, embryonic development, tissue patterning, and angiogenesis.
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