Fig. 1. Domain structure of Eph receptors and ephrins and their mode of engagement. (A) EphA and EphB receptors have a conserved domain structure (bottom). Ligands are attached to the cell membrane with a GPI (glycosylphosphatidylinositol) anchor (ephrin-A molecules) or are transmembrane proteins (ephrin-B molecules). (B) Ligand-receptor engagement induces bi-directional signaling. Both Eph receptors and ephrin-B ligands become tyrosine phosphorylated through autophosphorylation (receptors) or recruitment of a tyrosine kinase (ligand). (C) Dimerization and tetramerization interfaces of a ligand-receptor tetrameric complex.