EPHRIN-B3, A LIGAND FOR THE RECEPTOR EPHB3, EXPRESSED AT THE MIDLINE OF THE DEVELOPING NEURAL-TUBE

The ephrins are a family of ligands that bind to Eph family receptor t yrosine kinases, and have been implicated in axon guidance and other p atterning processes during vertebrate development. We describe here th e identification and characterization of murine ephrin-B3. The cDNA en codes a 340 amino acid transmembrane molecule, most closely related to the two other known transmembrane ligands, ephrin-B1 and ephrin-B2. I n addition to homology in their extracellular receptor binding domains , these transmembrane ligands share striking homology between their cy toplasmic domains, with 31 of the last 34 amino acids of ephrin-B3 bei ng identical to ephrin-B2, suggesting functional interactions of the c ytoplasmic tail. While most Eph family ligands are promiscuous in thei r interactions with Eph receptors, binding studies with the five recep tors known to bind other transmembrane ligands only revealed a high af finity interaction of ephrin-B3 with EphB3, with a dissociation consta nt of approximately 1 nM. In situ hybridization of mouse embryos showe d ephrin-B3 is expressed prominently at the dorsal and ventral midline of the neural tube, particularly in the floor plate, a structure with key functions in patterning the nervous system. The isolation of this ligand may help to elucidate the molecular basis of patterning activi ties at the neural tube midline.