EphRs comprise the largest subfamily of receptor tyr kinases (RTKs). 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. EphA2 negatively regulates cell differentiation and has been shown to be overexpressed in tumor cells and tumor blood vessels in a variety of cancers including breast, prostate, lung, and colon. As a result, it is an attractive target for drug design since its inhibition could affect several aspects of tumor progression. 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.
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