JUXTAMEMBRANE TYROSINE RESIDUES COUPLE THE EPH FAMILY RECEPTOR EPHB2 NUK TO SPECIFIC SH2 DOMAIN PROTEINS IN NEURONAL CELLS/

Eph-related receptor tyrosine kinases have been implicated in the cont rol of axonal navigation and fasciculation. To investigate the biochem ical mechanisms underlying such functions, we have expressed the EphB2 receptor (formerly Nuk/Cek5/Sek3) in neuronal NG108-15 cells, and hav e observed the tyrosine phosphorylation of multiple cellular proteins upon activation of EphB2 by its ligand, ephrin-B1 (formerly Elk-L/Lerk 2). The activated EphB2 receptor induced the tyrosine phosphorylation of a 62-64 kDa protein (p62(dok)), which in turn formed a complex with the Ras GTPase-activating protein (RasGAP) and SH2/SH3 domain adaptor protein Nck, RasGAP also bound through its SH2 domains to tyrosine-ph osphorylated EphB2 in vitro, and complexed with activated EphB2 in viv o. We have localized an in vitro RasGAP-binding site to conserved tyro sine residues Y604 and Y610 in the juxtamembrane region of EphB2, and demonstrated that substitution of these amino acids abolishes ephrin-B 1-induced signalling events in EphB2-expressing NG108-15 cells. These tyrosine residues are followed by proline at the +3 position, consiste nt with the binding specificity of RasGAP SH2 domains determined using a degenerate phosphopeptide library. These results identify an EphB2- activated signalling cascade involving proteins that potentially play a role in axonal guidance and control of cytoskeletal architecture.